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2013 SOUTHEASTERN NATURALIST 12(Monograph 6):1–43
A Review of Common Bottlenose Dolphins (Tursiops
truncatus truncatus) in the Northern Gulf of Mexico:
Population Biology, Potential Threats, and Management
Nicole L. Vollmer1,2,3,* and Patricia E. Rosel2
Abstract - Common Bottlenose Dolphins (Tursiops truncatus truncatus) are one of the
most abundant marine mammal species in the northern Gulf of Mexico (GOMx), a region
where they are exposed to a variety of man-made and natural threats. In order to assess
and minimize the impacts of these threats, it is important to understand how Bottlenose
Dolphins live within and utilize their habitat in the GOMx. Much of what is understood
regarding the biology, ecology, genetic, and life-history characteristics of Bottlenose
Dolphins in the GOMx has been gathered from small-scale research projects conducted
within inshore waters, and much less is understood about dolphins inhabiting coastal
and offshore waters. Over thirty years ago, Shane et al. (1982) reviewed the literature
on Bottlenose Dolphin research in the GOMx. The 2010 Deepwater Horizon oil spill has
highlighted the environmental risks to wildlife in the GOMx and the need for an updated,
comprehensive review of the current knowledge base and status of Bottlenose Dolphins.
Here we summarize research conducted on Bottlenose Dolphins within US waters (inshore,
coastal, and offshore) of the GOMx, building on the work presented in Shane
et al. (1982) with work published since. We highlight what is currently known about
Bottlenose Dolphin biology, ecology, and demographics, emphasize where knowledge
is still lacking concerning Bottlenose Dolphins in this region, and summarize the major
stressors faced by populations in the GOMx. We hope this review will aid researchers as
they try to assess both the short- and long-term impacts from threats in the GOMx and
may help direct future avenues of research to ensure effective conservation of Bottlenose
Dolphins in this environmentally and economically important habitat.
Introduction
Tursiops truncatus truncatus (Montagu) (Common Bottlenose Dolphin) is
one of the best-known and well-recognized marine mammal species. The species
is found in temperate and tropical waters worldwide. However, despite its
familiarity, and with a few exceptions, a thorough understanding of the biology
and ecology of the species is lacking. A few populations have been well-studied
and have significantly advanced our understanding of many aspects of dolphin
biology (e.g., Sarasota Bay, FL: Irvine et al. 1981, Scott et al. 1990, Wells
2003; Shark Bay, Australia: Connor et al. 1992a, Randić et al. 2012), but just
how far results from these studies may be extrapolated to populations in other
1University of Louisiana at Lafayette, Department of Biology, PO Box 42451, Lafayette,
LA 70504. 2NOAA, National Marine Fisheries Service, Southeast Fisheries Science Center,
646 Cajundome Boulevard, Lafayette, LA 70506. 3Current address - NOAA, National
Marine Fisheries Service, Smithsonian Institution, PO Box 37012, National Museum of
Natural History, Room 59-WC, MRC 0153, Washington, DC 20013-7012. *Corresponding
author - vollmern@si.edu.
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areas is unknown. Extrapolation is a particular problem for Bottlenose Dolphins
because they occupy an extremely diverse set of habitats (e.g., shallow,
warm tropical bays; pelagic waters; cold water fjords; etc.). These disparate
habitats impose such different selective regimes that, not surprisingly, Bottlenose
Dolphin feeding ecology, social behavior, morphology, life history, and
biology in general, is extremely variable.
Accurately characterizing the ecological and habitat requirements of Bottlenose
Dolphins is critical for their successful conservation and management. In
areas such as the northern Gulf of Mexico (GOMx), the need for such information
has never been more urgent than now, as impacts related to the 2010 Deepwater
Horizon (Mississippi Canyon 252) oil spill are currently being assessed throughout
many areas of the GOMx. Despite this catastrophic event, exploitation of natural
resources in the GOMx is not likely to slow down in the foreseeable future because
these waters are one of the most economically important areas within the US. For
example, offshore areas in the GOMx are 2nd (Texas is 1st) in total energy production
(predominantly natural gas and crude oil) in the US (US Energy Information
Administration 2012), and over $1.0 billion in annual landed value is gained from
commercial fisheries alone in the GOMx (McCrea-Strub et al. 2011).
Studies of Bottlenose Dolphins in the GOMx have been undertaken for decades;
however, most have been limited to a few inshore and coastal areas and
focused on relatively small groups or populations. The most recent comprehensive
review of Bottlenose Dolphin research in the GOMx was published over
30 years ago by the US Fish and Wildlife Service (Shane et al. 1982), while the
National Marine Fisheries Service produces annual Stock Assessment Reports
for Bottlenose Dolphin populations in US waters that summarize information
relevant to assessing the status of each population stock (see Conservation and
Management; Waring et al. 2013).
This paper attempts to provide an up-to-date (through 2012) resource summarizing
the research and accumulated knowledge concerning Bottlenose Dolphins
in the US waters of the GOMx. We start by briefly describing the current understanding
of Bottlenose Dolphin taxonomy and summarizing the current state of
knowledge of their general biology (e.g., life history, reproductive strategy, ecological
characteristics) for animals that inhabit enclosed or semi-enclosed bay,
sound, and estuary (BSE, also referred herein as inshore), coastal, and offshore
areas. We then review genetic studies focused on identifying and understanding
relationships among genetically distinct populations of Bottlenose Dolphins in
the GOMx and the potential environmental and anthropogenic pressures that may
threaten them. Finally, we discuss the conservation and management procedures
and guidelines currently in place for Bottlenose Dolphins in the GOMx.
Taken together, this information may be used to interpret the work that has been
done as well as identify where gaps still exist in our knowledge of Bottlenose Dolphins
in this diverse and economically important environment. A synopsis of publications
used in this review is presented in Supplemental Table 1 (available online
at https://www.eaglehill.us/SENAonline/suppl-files/s12-Mon6-S1180-Vollmer-s1
and for BioOne subscribers, at http://dx.doi.org/10.1656/S1180.s1). This table is
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organized by geographic location and type of study/research, lists the resulting major
conclusion(s), and provides corresponding citations. It covers a wide range of
government documents (technical memoranda, reports, etc.), peer-reviewed publications,
book chapters, and academic theses and dissertations.
Taxonomy
Recognized species
More than 20 nominal species of Bottlenose Dolphins, Tursiops spp. (Cetartiodactyla:
Odontoceti), have been described throughout the world’s oceans
(Hershkovitz 1966, Mead and Potter 1990) but currently only two are commonly
accepted: T. truncatus (Montagu) (type specimen from Devonshire, UK)
(Common Bottlenose Dolphin), found in temperate and tropical waters around
the globe; and T. aduncus (Ehrenberg; type specimen from the Red Sea) (Indo-
Pacific Bottlenose Dolphin), found in the Indo-Pacific. These two species have
been identified based on both morphological and genetic characteristics (Hale
et al. 2000, LeDuc et al. 1999, Wang et al. 2000). Both species inhabit oceanic
and coastal waters as well as enclosed areas such as bays and estuaries (Reeves
et al. 2002). Recently, a third species, T. australis Charlton-Robb, Gershwin,
Thompson, Austin, Owen and McKechnie (Burrunan Dolphin), was described
from coastal waters of southern Australia. Genetic data from both nuclear (microsatellites)
and mitochondrial DNA (cytochrome b and control region sequences),
along with differences in foraging ecology (based on stable isotope analysis) and
morphology (Charlton-Robb et al. 2011, Möller et al. 2008, Owen et al. 2011),
separated this southern Australian Tursiops from other Tursiops in the area; however,
further work is necessary to place this species into worldwide context of the
genus Tursiops. Bottlenose Dolphins in the Black Sea have been recognized as a
separate subspecies, T. truncatus ponticus Barabash-Nikiforov. This subspecies
differs significantly in both morphology and genetics from T. truncatus truncatus
in the Mediterranean Sea and other worldwide locations (Natoli et al. 2005,
Viaud-Martinez et al. 2008). Evidence also exists for the possibility of another
Tursiops species, as dolphins from coastal waters of South Africa, currently
classified as T. aduncus, are highly genetically differentiated from T. aduncus
in Chinese waters as well as from T. truncatus worldwide (Natoli et al. 2004,
Särnblad et al. 2011). Overall, the taxonomy and systematics of the genus Tursiops
is in flux, and the term “Bottlenose Dolphin” can be applied loosely for all
above-mentioned Tursiops (although Charlton-Robb et al. [2011] proposed the
common name “Burrunan Dolphin” for T. australis). For the purposes of the present
review, “Bottlenose Dolphin” refers exclusively to the Common Bottlenose
Dolphin, T. truncatus truncatus, unless otherwise specified.
Intraspecific variation in US waters
Off both the Pacific and Atlantic coasts of the US, two different forms of
Bottlenose Dolphin have been identified: an offshore form and a coastal form.
These two forms have been described as coastal and offshore “morphotypes” and/
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or “ecotypes” due to the morphological and ecological differences between them
(described below). Because the morphology of the offshore and coastal forms
in the Atlantic and Pacific are reversed, i.e., the offshore form in the Atlantic is
larger than the coastal form in the Atlantic but the reverse is true in the Pacific,
the terms “offshore and coastal morphotype” do not imply the same morphology
in both places; therefore, for the current review, we use the terms coastal
and offshore ecotype rather than morphotype. The term ecotype has previously
been applied to Bottlenose Dolphins in US waters (e.g., Perrin et al. 2011), and
we recognize that little has been confirmed about the morphological differences
between offshore and coastal forms in the GOMx.
In the northeastern Pacific, the two ecotypes (previously described as two separate
species: T. gillii Dall for the coastal ecotype and T. nuuanu Andrews for the
offshore ecotype) differ not only genetically but also in tooth size, parasite composition,
food habits, skull morphology, and overall size (Carretta et al. 2011, Curry
and Smith 1997, Perrin et al. 2011, Walker 1981). The offshore ecotype is typically
smaller and darker in coloration than the coastal ecotype (Curry and Smith 1997,
Walker 1981). Morphological differences in cranial characters suggest that these
coastal and offshore ecotypes are adapted to different environments, with the
coastal form likely feeding on larger prey (Perrin et al. 2011). In the northeastern
Pacific, it was initially thought that the coastal ecotype was found within 1.0 km of
shore and the offshore ecotype in waters >1.0 km from shore, with no overlap in
between (Carretta et al. 2011, Defran et al. 1999), but recent analysis has suggested
that spatial overlap does occur between the two forms, with coastal animals found
as far as 15.0 km from shore (Bearzi et al. 2009).
In the western North Atlantic (wNA, outside of the GOMx), both ecotypes
have always been referred to as T. truncatus, but they can be distinguished
on the basis of genetic data, skull morphology, overall size, hematological
data, parasite load, and prey differences (Curry and Smith 1997; Duffield et
al. 1983; Hersh and Duffield 1990; Hoelzel et al. 1998; Kingston and Rosel
2004; Kingston et al. 2009; Mead and Potter 1990, 1995; Rosel et al. 2009).
Color differentiation between the offshore and coastal ecotypes is similar to
the pattern seen in the Pacific; however, size variation is reversed: the wNA
coastal ecotype is smaller than the wNA offshore ecotype. In the wNA, genetic
analysis of tissue samples collected from New York to central Florida, coupled
with a spatial analysis, suggested that dolphins in inshore waters and out to approximately
7.5 km of shore are most likely of the coastal ecotype and animals
seaward of 34.0 km from shore are most likely of the offshore ecotype (Torres
et al. 2003). However, the offshore ecotype, determined genetically, has
been sampled as close as 7.3 km from shore and the coastal ecotype has been
sampled 75.0 km from shore (Garrison et al. 2003). Furthermore, Garrison et
al. (2003) found that although spatial separation is evident for the two ecotypes
north of Cape Lookout, NC, this separation breaks down south of this area in
the Atlantic, where coastal and offshore animals may overlap over the continental
shelf; however, the offshore ecotype has never been recorded alive in
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inshore, estuarine waters. Mounting evidence within the past few years, particularly
from morphology and genetics, suggests that the two ecotypes in the
wNA might appropriately be considered two different subspecies or even species
(Kingston and Rosel 2004, Rosel et al. 2009, Tezanos-Pinto et al. 2009),
although as of yet no taxonomic distinction has been applied.
In the GOMx, accounts of Bottlenose Dolphins, mostly from stranded animals,
suggest the presence of both offshore and coastal ecotypes (Gunter 1942,
Shane et al. 1982, Würsig et al. 2000), and current classification of T. truncatus
in the GOMx includes both the offshore and coastal ecotypes based on genetic
analyses, with the coastal ecotype found in BSE, coastal, and shelf waters and
the offshore ecotype confined primarily to deeper waters of the continental
shelf and slope (Curry 1997, Vollmer 2011, Waring et al. 2013). Furthermore,
research based on skull morphology suggests that the offshore ecotype is larger
than the coastal ecotype (Turner and Worthy 2003). These characteristics are
comparable to the differences seen in the coastal and offshore ecotypes in the
wNA. However, during both aerial and ship-based surveys, the two ecotypes
cannot be consistently distinguished visually in either the wNA or the GOMx
(Keith Mullin, National Oceanic and Atmospheric Administration, National
Marine Fisheries Service, Southeast Fisheries Science Center, Pascagoula, MS,
pers. comm.), and as a result, the exact ranges of the two ecotypes in the GOMx
have yet to be defined.
Biology of the Common Bottlenose Dolphin
The Common Bottlenose Dolphin is a cosmopolitan species that occurs in a
variety of habitats, from relatively shallow estuarine systems to deep oceanic
waters. Traditionally, most biological studies of Bottlenose Dolphins in US waters
(and worldwide) have occurred inshore in BSE and/or coastal areas close
to shore that are easily accessible to researchers. As a result, relatively little is
known about Bottlenose Dolphins that inhabit deeper waters further from shore
(particularly for dolphins in the GOMx).
Information on offshore animals is primarily gathered through visual observations
from ships and airplanes and occasionally from fisheries by-catch and
stranding events. However, animals from deeper waters over the continental
shelf and beyond rarely strand along the coast. For example, the probability that
a stranded Bottlenose Dolphin on the US Pacific coast is of the offshore ecotype
is only 14.0% (Perrin et al. 2011), and the average recovery rate of a cetacean
carcass in the GOMx was estimated to be extremely low at 2.0% (Williams et
al. 2011 [however, no Tursiops samples were included in their study]). Additionally,
the high air temperatures of the GOMx (compared to other parts of the
US) promote rapid decomposition, causing many stranded animals to be highly
decomposed if found. Because of the comparatively fewer offshore specimens
available for study, much less is known about the general biology and ecological
characteristics of this ecotype; therefore, much of the information presented below
may not be applicable to all T. truncatus within GOMx waters.
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Life history
Life span. Bottlenose Dolphins are long-lived animals typically having life
spans upwards of 20–30 years (Connor et al. 2000b). The longest temporal study
of wild Bottlenose Dolphins is in Sarasota Bay, FL, initiated in 1970 and still
ongoing (Scott et al. 1990; Wells 1991, 2009). There, females have been known
to live for more than 50 years and males for more than 40 years, based on age
analyses from teeth of live animals (Hohn et al. 1989, Wells and Scott 1999).
Based on similar tooth analysis from stranded animals, the oldest male and
female documented on the Texas coast were 33 and 41 years old, respectively
(Fernandez and Hohn 1998), and on the Mississippi coast were 27 and 30 years
old, respectively (Mattson et al. 2006).
Sexual dimorphism. Evidence of sexual dimorphism based on overall body
size has been documented for Bottlenose Dolphins in some GOMx BSE areas.
For example, males from Sarasota Bay have larger standard length, girth, and
mass compared to females (Read et al. 1993, Tolley et al. 1995). However,
sexual dimorphism in other morphological characteristics, particularly cranial
measurements, is not consistent for Bottlenose Dolphins across geographical
areas (Perrin et al. 2011). For instance, Turner and Worthy (2003) examined 35
cranial characteristics in male and female skulls from Texas and Florida and
found significant differences between the sexes from Texas, but not from Florida.
However, it should be noted that the sample size for adult males from Florida was
quite small in comparison to the other groups.
Body length. Asymptotic length has been estimated for Bottlenose Dolphins
from various BSE locations within the GOMx. In Sarasota Bay, Mississippi
Sound, and along the Texas coast, the length has been estimated to be ≈2.6 m for
adult males and ≈2.5 m for adult females (Fernandez and Hohn 1998, Mattson et
al. 2006, Read et al. 1993). In Sarasota Bay, females reach asymptotic length typically
by 10–12 years; however, males continue to grow, increasing both weight
and girth, until about 20 years old (Read et al. 1993, Wells and Scott 1999). Data
from Mississippi Sound indicate that both males and females experience a growth
spurt (in total body length) around the age of sexual maturity and that both sexes
reach asymptotic length starting around 10 years old (Mattson et al. 2006, McFee
et al. 2010).
Sexual maturity. Generally, females reach sexual maturity before males
(Mann et al. 2000, Wells et al. 1987). In Sarasota Bay, sexual maturity begins at
approximately 8–9 years old for males and between 6–7 years old for females,
but can also occur later for both sexes (Wells and Scott 1999, Wells et al. 1987).
Values from other locations range from 5–13 years for females (Mann et al.
2000, Mead and Potter 1990, Perrin and Reilly 1984, Sergeant et al. 1973) and
8–13 years old for males (Harrison and Ridgway 1971, Perrin and Reilly 1984,
Sergeant et al. 1973).
Reproduction. Bottlenose Dolphins have a 12-month gestation period
(Schroeder 1990) and give birth to a single offspring that remains with its
mother generally for at least the first three years of its life (Wells et al. 1987).
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Calves are usually weaned after their first year, but observations from Sarasota
Bay suggest that lactation can continue up to three or more years (Wells and
Scott 1999). The interbirth interval for females is most typically three years,
and females continue to reproduce throughout their adult lives (Connor et al.
2000b, Leatherwood 1977, Wells 2003, Wells and Scott 1999), although preliminary
data suggest that fewer calves are produced by females past the age of 30
(Wells 2000). Calves can be born during any time of the year; however, within
the GOMx, and similar to study sites in other areas, seasonal calving peaks occur.
For example, calf sightings are highest in the spring and summer months in
Sarasota Bay (Wells et al. 1987), Mississippi Sound (Miller et al. 2010, 2013),
San Luis Pass, TX (Henderson 2004), and Aransas Pass, TX (McHugh 1989),
all suggesting a peak in calving in these BSE areas during the warmer spring
and summer months.
Mortality. Mortality rates are highest for Bottlenose Dolphin calves (compared
to other age classes), and an estimated 50% of calves in Sarasota Bay do
not survive past their first year (Wells et al. 2005), although mortality rates decline
after the first year to around 1–4% (Wells and Scott 1990). Contaminants
present in Bottlenose Dolphin tissues (i.e., blubber and milk) may contribute to
calf mortality and impact overall reproductive success. Examination of blubber
samples from both mothers and calves in Sarasota Bay has shown that polychlorinated
biphenyl compound (PCB) levels decrease in females around the age of
sexual maturation and lactation, timing that correlates to reproduction (Wells et
al. 2005). The authors speculate that females pass on these contaminants to their
offspring, most likely through nursing, and also found that firstborn calves are
exposed to higher amounts of contaminants from their mothers than subsequent
offspring. Although the contaminant levels passed on to calves from their mothers
likely are a contributor to the high calf mortality rate in this area, Wells et al.
(2005) suggest that predation and human interactions are likely contributors as
well. Also in Sarasota Bay, male mortality rates are higher than those of females,
resulting in a sex ratio that becomes more biased towards females as animals
age over time (Scott et al. 1990, Wells and Scott 1990). Causes of mortality for
Bottlenose Dolphins of all ages can be anthropogenically related (see below),
and natural causes most commonly include shark attacks, stingray barbs, biotoxin
exposure from harmful algal blooms (HABs), and disease (Connor et al. 2000b,
Reynolds et al. 2000).
Ecological characteristics
Prey preference. Bottlenose Dolphins are generally considered to be opportunistic
feeders (although see Berens McCabe et al. [2010] for an example
of selective foraging in the GOMx) whose prey preference often depends on
the environment in which they live. Examination of 76 stomachs from stranded
dolphins across the GOMx (and two locations off the eastern Florida coast)
identified at least 43 fish species in 39 genera (most commonly from the families
Mugilidae, Scombridae, and Sciaenidae; Barros and Odell 1990). Dunshea et al.
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(2013) similarly identified fish species from 16 different teleost families in fecal
and stomach samples collected from 18 Sarasota Bay resident dolphins. Invertebrates,
including multiple species of squid and shrimp, have also been found in
stomachs of stranded animals in Florida, Alabama, and Texas (Barros and Odell
1990), but they generally make up a smaller component of the overall diet of animals
examined to date. Prey preference of Bottlenose Dolphins in the GOMx has
been categorized based on direct observation of feeding events, but most often
from stomach content analysis of deceased animals.
Analysis of stomach contents from Bottlenose Dolphins of the coastal ecotype
has been conducted from multiple BSE and coastal areas of the GOMx. In
Sarasota Bay, Lagodon rhomboides (L.) (Pinfish), Mugil cephalus L. (Striped
Mullet), Orthopristis chrysoptera (L.) (Pigfish), and Leiostomus xanthurus
Lacépède (Spot) were most frequently found, and feeding behaviors were most
often observed near seagrasses (Barros and Wells 1998) where Pinfish are often
the dominant fish species (Stoner 1983). Opsanus beta (Goode & Beane) (Gulf
Toadfish) was also commonly found in stomach contents of stranded animals
known to be long-term residents of the Sarasota Bay area (Berens McCabe et al.
2010). This study also found that over 50% of total prey consumed were soniferous
fishes and hypothesized that dolphins might detect their prey by listening
for the sounds they produce (i.e., passive listening; Gannon et al. 2005). Some
Bottlenose Dolphins near Clearwater Harbor, FL did not preferentially forage
over seagrass habitats (Allen et al. 2001), as had been believed previously (Barros
and Wells 1998, Scott et al. 1996, Shane 1990). For these dolphins, Pinfish are
higher in abundance and of larger length in areas outside of seagrass bed habitats,
perhaps therefore causing dolphins to forage away from seagrass beds (Allen
et al. 2001). Within various Texas bays, there are early accounts of Bottlenose
Dolphins eating Scomberomorus maculatus (Mitchill) (Spanish Mackerel) and
Megalops atlanticus Valenciennes (Tarpon) directly from fishing lines, whereas
Mullet and Dorosoma cepedianum (Lesueur) (Gizzard Shad) were most often
encountered in stomach contents of dead dolphins (Gunter 1942, 1954).
The diet of the offshore ecotype of Bottlenose Dolphins in the GOMx has been
little studied. In the wNA, Mead and Potter (1995) found offshore specimens feed
primarily on myctophids (midwater fish) and pelagic squid. Upon examination
of stomach contents from 76 stranded animals from the GOMx and east coast of
Florida, a single offshore animal (based on skull morphometry) was identified and,
unlike most other stomachs, the contents for this animal were dominated by squid
(Barros and Odell 1990). Barros et al. (2010) were able to distinguish inshore,
coastal, and offshore dolphins in the eastern GOMx based on stable isotope ratios,
suggesting animals in these areas were exploiting different trophic levels.
Foraging strategies. In addition to the passive listening strategy, a number
of other feeding strategies have been observed, predominantly in BSE areas of
the GOMx, including “strand feeding” where dolphins chase fish completely
out of the water and onto the shore (Duffy-Echevarria et al. 2008, Leatherwood
1975, Würsig et al. 2000). Bottlenose Dolphins in the Florida Keys have been
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observed “mud-plume feeding” where an individual dolphin disturbs the underlying
sediment, thereby creating a linear or curvilinear mud plume, and then
lunges through the plume presumably to feed on fish concentrated in the area
(Lewis and Schroeder 2003). Another specialized feeding strategy called “mudring
feeding” is utilized by Bottlenose Dolphins in Florida Bay and has never
been observed in any other area worldwide (Torres and Read 2009). This strategy
involves multiple dolphins working together to corral fish (typically species
of mullet) by forming a ring-shaped mud plume. A variety of feeding behaviors
displayed by Bottlenose Dolphins around Sanibel Island, FL, have been described,
including “fish kicking” where dolphins use their tail flukes to kick
fish into the air, thereby stunning and/or injuring them, prior to consumption
(Shane 1987). In numerous inshore areas of western Florida, animals have also
been observed “kerplunking” where they use their flukes to slap the water to
presumably help locate and/or capture fish (Connor et al. 2000a). Evidence
suggests that Bottlenose Dolphins in Sarasota Bay transmit foraging strategies,
including kerplunking, within their community and particularly from mother to
calf (Weiss 2004).
There is also evidence that Bottlenose Dolphins in the GOMx can be flexible
in their foraging patterns. Juvenile Bottlenose Dolphins in Sarasota Bay were
found to expand their foraging range during HABs, likely in response to changes
in prey availability (McHugh et al. 2011), as abundances of common prey species
were significantly reduced during these times (Gannon et al. 2009). Observations
from the same animals prior to, during, and after red-tide events, suggested that
Bottlenose Dolphins can alter their foraging and ranging patterns in response to
intense environmental changes (McHugh et al. 2011).
Group density and size. Bottlenose Dolphin density in BSE and coastal areas
of the GOMx varies by season and location. For instance, dolphin densities
were highest in winter and slightly lower in summer months in Charlotte Harbor,
FL (Bassos-Hull and Wells 2007), while densities were highest in summer
months in Mississippi Sound (Hubard et al. 2004). In contrast, dolphin densities
were highest in the fall and winter months off the coast of Louisiana
(Mullin et al. 1990) and in Matagorda Bay, TX (Gruber 1981). Dolphins in the
inshore areas of the north central GOMx occur in higher densities in marshland
areas compared to the sounds (Leatherwood and Platter 1975). Around Galveston,
TX, densities were highest in nearshore coastal waters off of beaches
compared to within the bay (Jones 1988), and in Aransas Pass, TX, dolphin
densities were highest in the pass connecting the interior bay to the GOMx
compared to the inshore bays and channels (McHugh 1989). The underlying
causes of these different patterns are poorly understood, although one may
speculate that prey availability could play an important role. Comparisons of
sites with different patterns have yet to be made, but could shed considerable
light on habitat usage for the species.
Bottlenose Dolphins typically occur in groups that vary in size depending
on their habitat. For instance, within bays and relatively shallow waters, group
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sizes tend to be smaller compared to deep-water passes and areas offshore (e.g.,
Mobile Bay, AL [Goodwin 1985], north central GOMx [Leatherwood and Platter
1975], Barataria Bay, LA [Miller 2003]).
Residency and seasonal movements. Bottlenose Dolphins show varying
degrees of residency to a particular area. Long-term, year-round residency is
well documented for the Sarasota Bay population (e.g., Irving et al. 1981, Wells
2003), and has also been documented in several other BSE environments in the
GOMx where studies have been conducted (Waring et al. 2013), including other
areas of Florida (e.g., Balmer et al. 2008, Quintana-Rizzo and Wells 2001, Scott
et al. 1996, Shane 2004, Wells et al. 1996), Mississippi (e.g., Hubard et al. 2004,
Solangi and Dukes 1983), and Texas (e.g., Fertl 1994, Lynn 1995, Shane and
Schmidly 1978).
In BSE environments, where long-term, year-round residents are best documented,
there is also evidence for seasonal changes in abundance, most often
thought to result from movements of seasonal, short-term residents and/or
transients (very short-term visitors) to the area. It is likely these shorter-term
residents and visitors come from the adjacent coastal population, although
comprehensive, directed studies to address this question have yet to be
performed. In some cases, it could be possible the new animals come from adjacent
estuarine waters.
Seasonal changes in abundance have been documented in a variety of locations.
Near Sanibel Island, FL, abundances peaked during spring (Shane 2004).
At Sarasota Bay, Irving et al. (1981) found dolphins were concentrated at passes
and along the GOMx shoreline (within 1.0 km of shore) during the winter (November–
December), while numbers were highest within Sarasota and adjacent
bays in summer months. Irving et al. (1981) hypothesized that the seasonal movements
of Bottlenose Dolphins between Sarasota Bay, the passes, and adjacent
coastal waters were due to the seasonal movements of prey such as Striped Mullet.
Balmer et al. (2008) found evidence for seasonal changes in abundance in St.
Joseph Bay (northern Florida panhandle), where dolphins increased in abundance
during both spring and fall months. Animals seen in spring and fall had much
lower site-fidelity indices, and the authors suggest these seasonal visitors may
come from the adjacent coastal population (Balmer et al. 2008). Seasonal movements
have also been suggested in Mississippi Sound, with evidence supporting
that animals leave the Sound in winter months and temporarily reside outside of
the barrier islands (Hubard et al. 2004, Mullin 1988, Solangi and Dukes 1983).
Along the Texas coast, several different patterns have been documented: in San
Luis Pass, abundance patterns are similar to those observed in Sarasota Bay
(Maze and Würsig 1999); near Aransas Pass, dolphin abundance increased during
winter months and decreased during summer (Shane 1980); and in the Galveston
Bay ship channel, there were seasonal peaks in dolphin abundance in spring and
autumn (Fertl 1994). In contrast, no evidence for seasonal trends in abundance
has been found in BSE areas including Florida Bay and the Florida Keys (Mc-
Clellan et al. 2000).
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Seasonal movements have been suggested between coastal and estuarine areas
(based on abundance and distributional changes) for dolphins in the broader
northeastern GOMx (Lance Garrison, National Oceanic and Atmospheric Administration,
National Marine Fisheries Service, Southeast Fisheries Science Center,
Miami, FL, pers. comm.). It also has been suggested that dolphins along the coast
of Texas undergo seasonal migration along the coast, as higher abundances have
been observed in coastal waters of central and southern Texas during fall and winter
months and in northern Texas during summer months (Weller 1998). To date,
no further work has been carried out to examine this specific pattern; however,
in other areas along the west Texas coast, such as the outer edge of Galveston
Island, no evidence for peaks in occurrence of Bottlenose Dolphins was found
after over one year of continuous surveys (Beier 2001). In deeper waters, higher
abundance during colder months has been observed over the continental shelf and
in oceanic regions of the GOMx (Fritts et al. 1983); however, in contrast, Mullin
et al. (1994) observed that sighting rates in the north central GOMx increased in
summer and fall months in deeper waters and along the continental shelf. Overall,
much less is understood about dolphin movements in these offshore areas.
Social interactions
Association Patterns. Association patterns among Bottlenose Dolphins in
BSE areas have been well studied in several locations of the GOMx. In Sarasota
Bay, there is well-documented evidence of male-male relationships in the form of
strongly-bonded pairs observed together over a 10-year period (Owen et al. 2002,
Wells 1991, Wells et al. 1987), although the level of cooperation between male
pairs in Sarasota Bay is not as complex as it is in Shark Bay, Western Australia,
another location known for its long-term observations (since 1984) of Bottlenose
Dolphins (Tursiops sp.; Connor et al. 1992a, 1992b, 2000b; Randić et al. 2012).
Female-female associations are often in the form of female bands—groups of
females that tend to associate more with each other, sharing a similar home range
compared to other females in the same area (Wells 1991)—and have been well
documented in both Sarasota Bay and Shark Bay (Connor et al. 2000b). Research
conducted over three years in estuarine waters near Panama City, FL uncovered
strong associations between both pairs and trios of males (Bouveroux and Mallefet
2010), and both female bands and male pairs of Bottlenose Dolphins have
been documented in Cedar Keys, FL (Quintana-Rizzo and Wells 2001).
Communication. Some vocalization and acoustic research has been conducted
on Bottlenose Dolphins in the GOMx, primarily associated with animals from
Sarasota Bay. Individuals within Sarasota Bay have distinctive signature whistles
that are stable over time (Cook et al. 2004, Sayigh 1992, Sayigh et al. 2007). Bottlenose
Dolphins can recognize whistles from related conspecifics, and whistles
can convey information on individual identity (Janik et al. 2006, Sayigh et al.
1998). There is also evidence that certain characteristics in whistle production
(e.g., rate and duration) are used as indicators of stress in Bottlenose Dolphins
(Esch et al. 2009). For Bottlenose Dolphins in the Big Bend region of Florida
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(northeastern GOMx), whistle rates were highest in spring, though echolocation
rates were not found to differ seasonally (Rycyk 2007). Studies from Sarasota
Bay have also provided insights into how dolphins use vocalizations while foraging.
For example, single animals were much more vocal than those in groups
(Nowacek 2005), and the author suggests there are specific functions for different
sound types used during foraging activities. Vocalizations from Bottlenose Dolphins
between animals from the wNA and GOMx showed significant differences
in multiple whistle characteristics, thereby differentiating animals from the two
ocean basins (Baron et al. 2008). Comparisons of vocalizations from estuarine,
coastal, and offshore populations have yet to be made.
Population Structure of Bottlenose Dolphins
Successful conservation and management requires an accurate understanding
of the population structure of the species in question. However, the logistic difficulties
of studying Bottlenose Dolphins at sea, their highly mobile nature, and
the lack of obvious barriers to movement and genetic exchange make delimiting
population structure through direct observation quite difficult. The past 20
years have seen a vast increase in the application of molecular genetic methods
to questions of dolphin population structure and movements. These studies have
greatly enhanced understanding of how dolphins partition their environment. In
this section, we briefly describe the molecular markers most commonly used for
this type of work and then summarize studies of Bottlenose Dolphin population
structure in the GOMx.
Molecular markers commonly used
For management of dolphin populations in the GOMx to be successful, it is
important to identify populations of Bottlenose Dolphins that are genetically distinct
from one another and hence represent biological populations rather than just
management constructs (see Conservation and Management below for further
discussion). Multiple types of genetic markers are examined for population-level
research; one of the most commonly used is mitochondrial DNA (mtDNA). Mitochondrial
DNA is a circular, maternally inherited, haploid molecule (≈16,000
base pairs long in vertebrates; Brown 1983) that generally does not undergo
recombination. Because it is maternally inherited, this molecule provides information
on female-mediated genetic exchange among populations (Baker and
Palumbi 1997, Moritz et al. 1987). Numerous mtDNA studies have been conducted
to analyze Bottlenose Dolphin population dynamics (e.g., Curry 1997;
Duffield and Wells 1991, 2002; Krützen et al. 2004; Natoli et al. 2004; Rosel et
al. 2009; Sellas et al. 2005).
Microsatellites are a second widely used genetic marker for population studies.
They are short stretches of DNA composed of repeats of 2–6 base pairs (bp)
in length (Chambers and MacAvoy 2000, Selkoe and Toonen 2006) and are
bi-parentally inherited. Therefore, they provide information on both male- and
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female-mediated genetic exchange. As with mtDNA, microsatellites have been
used in numerous Bottlenose Dolphin population studies (e.g., Hoelzel et al.
1998, Krützen et al. 2004, Natoli et al. 2004, Rosel et al. 2009, Sellas et al. 2005,
Whitehead et al. 2000).
Single nucleotide polymorphisms (SNPs) are an additional genetic marker
that can be used for population history and structure analyses (Brumfield et al.
2003, Morin et al. 2004, Vignal et al. 2002). SNPs are single base-pair differences
that occur between alleles and are found throughout an organism’s genome.
Like microsatellites, SNPs are also bi-parentally inherited. Although more SNP
markers are often needed to assess genetic variation compared to microsatellites
(e.g., Haasl and Payseur 2011, Liu et al. 2005), they have technological and analytical
advantages over working with microsatellites (Helyar et al. 2011, Morin
et al. 2004, Vignal et al. 2002). SNPs, although popular with model organisms
such as humans and mice (Aitken et al. 2004, Lindblad-Toh et al. 2000, Wang et
al. 1998), are only very recently being used for genetic analyses for non-model
organisms including the Bottlenose Dolphin (see Garvin et al. 2010:Table 1,
Vollmer and Rosel 2012).
Previous genetics research
While numerous genetics-based research studies have been conducted on
Bottlenose Dolphins in the wNA and other areas around the US, relatively few
studies have been done examining population genetics of Bottlenose Dolphins
in the GOMx. One of the earliest studies analyzed samples collected during a
live capture of Bottlenose Dolphins in Mississippi Sound (Solangi and Dukes
1983) and found isoenzymes and plasma proteins to be variable among animals;
however, no overall comparisons were carried out within the Sound or with other
potential populations in the GOMx.
Using whole-chromosome staining profiles, Duffield and Wells (1991, 2002)
identified 38 variants within the T. truncatus karyotype using samples from BSE
areas on the west coast of Florida. A similar study in Matagorda Bay identified
48 variants (Gunter 1997). Between these two locales, a majority of variants
(68.0%) were found in both areas, indicating genetic similarity between the two
regions (Duffield and Wells 2002). It is likely these similarities are due to shared
ancestral states, rather than more recent interbreeding between locations, because
karyotype banding patterns are generally highly conserved among cetaceans (Árnason
1980, Ferguson-Smith and Trifonov 2007, Fredga 1977).
Duffield and Wells (1986, 1991) examined variability at five allozyme loci
among T. truncatus from Sarasota Bay and areas north (Tampa Bay) and south
(Charlotte Harbor and Pine Island). Significant differences were found in allele
frequencies between Sarasota Bay and the other two sampling areas for
three out of five red blood cell proteins. They also identified four different
mtDNA haplotypes (using restriction fragment length polymorphism analysis
[RFLP]) among the Sarasota Bay and Tampa Bay sampling locations, including
two that were shared between Sarasota Bay and Tampa Bay, one Sarasota
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Bay-specific haplotype, and one Tampa Bay-specific haplotype (Duffield and
Wells 1991). Samples were also examined from the east coast of Florida (Indian
River Lagoon) to the Florida Panhandle in the GOMx; five mtDNA haplotypes
(using the same procedures as the 1991 study) were found, and clinal variation
in haplotype frequencies from east to west was seen, with only one of the five
haplotypes being commonly found on both Florida coasts (Duffield and Wells
2002). In a similar study, Dowling and Brown (1993) also used RFLPs to examine
population structure of Bottlenose Dolphins with a small set of samples
taken from animals in the wNA and GOMx. Overall, six mtDNA haplotypes
were found and, contrary to the findings of Duffield and Wells (2002), none
were shared between the two regions (five from the GOMx and one from the
wNA). Within the GOMx, samples were collected from four different BSE locations:
Gulfport in Mississippi, and Destin, St. Petersburg, and Sarasota Bay
in Florida. Although sample sizes were quite low for the Florida locations (n =
2, 5, 1, respectively; but n = 22 for the Mississippi location), nearly all samples
had similar or identical RFLP patterns with <0.2% sequence divergence, and
the authors concluded that gene flow was occurring among these populations
(Dowling and Brown 1993). Interestingly, there was one haplotype found in
both the Destin and St. Petersburg samples that was as different from the other
GOMx haplotypes as the wNA haplotype was. While all other samples were
likely collected from animals of the coastal ecotype, the authors suggested that
animals of the offshore ecotype might have contributed the divergent haplotype
found in Destin and St. Petersburg. Overall, these studies suggest that while
some level of mixing is possible among populations within the GOMx, little to
no genetic exchange is occurring between populations in the GOMx and the Atlantic
coast of Florida.
A worldwide Bottlenose Dolphin population study that analyzed a 402-bp
portion of the mitochondrial control region (mtCR) included 55 animals from
the GOMx (13 from offshore waters, 20 from Matagorda Bay, TX, and the
remaining 22 scattered from inshore/coastal areas in Florida, Mississippi and
Texas; Curry 1997). The ecotype of each sample was inferred based on morphological
and/or ecological characteristics or geographic sampling location
prior to genetic analysis, and for the GOMx samples resulted in 13 offshore, 32
coastal and 10 unconfirmed ecotypes. There were no shared haplotypes found
between samples from the coastal wNA and coastal GOMx, and none of the 17
haplotypes identified in the GOMx samples were shared between coastal and
offshore ecotypes. The study found significant differentiation between GOMx
and wNA coastal samples and found a greater genetic distance between coastal
GOMx and wNA samples than between offshore animals from both regions.
Another worldwide survey of Bottlenose Dolphin population structure
included samples from the GOMx obtained from stranding events along the
coast of Texas (Natoli et al. 2004). The exact waters of origin for the stranded
animals (BSE, coastal, offshore) were unknown, and the authors assumed
that all GOMx samples were of the coastal ecotype. Twenty-two samples
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from the GOMx were genotyped at nine microsatellite loci, and 297 bp of the
5´ end of the mtCR were sequenced for 10 samples. No comparisons were
made between samples within the GOMx, but comparisons were made among
GOMx, coastal wNA, and offshore wNA (the latter two designated as coastal
and offshore ecotypes respectively; see Hoelzel et al. 1998) samples. For the
microsatellite results, significant differentiation was found among all three
regions using two estimators of genetic divergence, FST and RhoST, with both
estimators higher between the GOMx (all coastal) and wNA offshore than
between the GOMx and wNA coastal samples. For the mtDNA analyses, six
haplotypes were identified in the GOMx samples, including five that were
not shared with any other region (worldwide) and one that was shared with
samples collected from the nearshore Bahamas. Taken together, these results
suggest dolphins from coastal waters of the wNA and GOMx are genetically
differentiated from each other, but are more closely related to each other than
either is to dolphins from the offshore wNA.
Using mtDNA sequences and focusing on Bottlenose Dolphins around New
Zealand, Tezanos-Pinto et al. (2009) conducted another study of Bottlenose Dolphin
population structure and included a worldwide perspective incorporating the
same GOMx and coastal and offshore wNA samples used for mtDNA analysis
in Natoli et al. (2004). As expected, results using the GOMx and wNA data were
similar to the 2004 study, with samples believed to be of the coastal ecotype
from both basins clustering together (using a dendrogram reconstruction) and
separated from those from the offshore wNA (i.e., offshore ecotype).
Genetic differentiation among coastal and estuarine populations (all coastal
ecotype) along the US East Coast was recently investigated using 18 microsatellite
loci and a 354-bp fragment of the mtCR, and within this dataset were
77 samples of the coastal ecotype from one GOMx location (from the Florida
Panhandle) for comparison (Rosel et al. 2009). Of the 18 mtDNA haplotypes
identified in the study, four were found only in the GOMx and three were shared
between the GOMx and the wNA samples. Microsatellite and mtDNA divergence
estimates were highest between the GOMx population and wNA populations.
Phylogenetic analysis of the mtDNA data grouped the GOMx and coastal wNA
samples together to the exclusion of wNA offshore samples. Overall, both datasets
indicated that the coastal ecotype of Bottlenose Dolphins from the GOMx
and wNA are genetically distinct from each other. Furthermore, the mtDNA
data consistently separated coastal and offshore ecotype samples. This strong
differentiation between the two ecotypes is supported by other genetic studies
using genome-wide scans via amplified fragment length polymorphism analysis
(Kingston and Rosel 2004, Kingston et al. 2009).
The first study to thoroughly examine genetic differences among multiple
populations of Bottlenose Dolphins within the GOMx was conducted by Sellas
et al. (2005). They identified fine-scale population structure in the coastal
and inshore areas of the eastern and western GOMx based on analysis of nine
microsatellite loci and a 359-bp fragment of the mtCR. Using 223 samples, they
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sampled four populations present in BSE environments (defined as inshore dolphins)
and one population sampled within an area extending as far as 12.0 km
from shore along the west coast of Florida from outside Tampa Bay to the south
end of Lemon Bay (defined as coastal dolphins). Of the 11 mtDNA haplotypes
identified, three were unique to the coastal samples, two were unique to the
inshore population in the western GOMx, one was unique to Tampa Bay, two
were found in all five populations, and the remaining three were shared between
at least two areas from the eastern GOMx. Microsatellite analyses showed significant
differentiation for all pairwise population comparisons, the highest of
which were between the western population and all eastern populations. Overall,
significant genetic differentiation for both nuclear and mitochondrial DNA was
found among all four inshore populations and between the coastal and inshore
populations, indicating limited genetic exchange among these populations. Overall,
these studies indicate a remarkable level of fine-scale population structure
among the BSE populations of Bottlenose Dolphins in the GOMx, despite the
lack of obvious barriers to movement among these areas.
Potential Threats
Dolphins in the GOMx face threats similar to dolphins in other locations
around the world. The natural predators of Bottlenose Dolphins are sharks and
occasionally Orcinus orca (L.) (Killer Whale), although Bottlenose Dolphins are
not believed to be primary prey items for these predators (Shane et al. 1986, Wells
and Scott 1999). In contrast to natural predatory threats, many of the negative
impacts Bottlenose Dolphins face in their natural environment are humaninduced;
therefore, the following section will focus on the various risks humans
currently impose on Bottlenose Dolphins within the GOMx. This includes both
direct interactions (e.g., fisheries and research-related mortalities, entanglement,
provisioning) and indirect threats (e.g., habitat degradation, coastal development,
global climate change).
Direct interactions with humans in the GOMx
Interactions between Bottlenose Dolphins and both commercial and recreational
fisheries have been documented in the GOMx at least since the late 1930s
(Gunter 1938, 1942) and occur in inshore, coastal, and offshore waters. More
recent incidental mortalities have involved the pelagic longline, shark bottom
longline, longline swordfish/tuna, shrimp trawl, crab trap/pot, and menhaden
fisheries, as well as hook/line fishing gear (recreational) and sea turtle relocation
trawling activities (Waring et al. 2013). There have also been research-related
mortalities associated with gillnet and trawl entanglements (being used for
fisheries sampling), as well as incidental deaths that occurred during Bottlenose
Dolphin health-assessment projects (although the latter were not fisheries-related
incidents; Waring et al. 2013). It is not uncommon for Bottlenose Dolphins to
be found stranded on beaches showing evidence of human interaction as a contributing
cause of death, including signs of entanglement and ingestion of both
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commercial and recreational fishing gear (Gorzelany 1998; Powell and Wells
2011; Waring et al. 2013; Wells and Scott 1994; Wells et al. 1998, 2008). Overall,
effects of fisheries on Bottlenose Dolphins in the GOMx are largely unknown because
most of the fishing industry is not sufficiently monitored (Marine Mammal
Commission 2009).
Direct feeding also poses significant threats to Bottlenose Dolphins. Provisioning
has been documented in areas of Florida including Panama City (Samuels
and Bejder 2004, Samuels et al. 2000) and around Sarasota Bay (Cunningham-
Smith et al. 2006, Powell and Wells 2011), Key West, FL, and in areas of Texas
(Bryant 1994). Provisioning of wild dolphins can lead to changes in normal feeding
behavior as well as increase the chances that dolphins will come in contact
with potentially hazardous boats, propellers, jet skis, and fishing gear (Bryant
1994, Gorzelany 1998, Wells and Scott 1994, Wells et al. 1998). Dolphins may
also become habituated to being around humans, and some are known to steal
fish bait or catch from recreational fishermen (e.g., Powell and Wells 2011).
These interactions with humans further increase the potential for dolphins to be
entangled in fishing line and other gear and may disrupt normal feeding behavior.
Human interactions in the form of “swim-with” programs also have the potential
to modify dolphin behavior and bring them into unnecessary contact with anthropogenic
hazards (Samuels and Bejder 2004). There are currently swim-with
programs targeting wild Bottlenose Dolphins in the coastal waters near Panama
City, FL. Finally, vessel-based activities, including both commercial tourism and
recreational boating, can be a potential source of injury and disturbance for dolphins,
particularly in coastal and inshore areas (Nowacek et al. 2001).
Habitat degradation
Another source of threats to Bottlenose Dolphins in the GOMx comes from
oil and gas exploration and extraction. Twenty-one percent of the natural gas and
30% of the oil produced within the US comes from federal waters in the GOMx
(Marine Mammal Commission 2009). There are roughly 4000 active platforms,
located mostly on the continental shelf in the western and northern GOMx;
however, in the past several decades oil deposits have been discovered in deeper
offshore waters exceeding 1000 m (Crawford et al. 2009).
There are a variety of negative impacts related to these industries that threaten
Bottlenose Dolphins, including noise disturbances from seismic studies and removal
of platforms (the latter is typically performed using explosives). In a 2001
report, the Minerals Management Service estimated that about 186 platforms,
primarily those located in shallower waters, would be removed between 1999 and
2023, and an additional 142 new rigs would be installed annually (Pulsipher et al.
2001). Other potential risks from the oil and gas industries are leaks and spills,
which have been observed to affect marine mammal behavior (Scott 1991, Smultea
and Würsig 1995) and health (Peterson et al. 2003). Between 1996 and 2011,
there were 190 reported spills (≥2100 gallons each) of petroleum or other toxic
substances in the GOMx (BOEM 2012). Furthermore, in April 2010, the largest
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offshore marine oil spill in US history occurred off the Louisiana coast when the
Deepwater Horizon drilling rig exploded, releasing over 168 million gallons of
oil into the GOMx over a three-month period (Camilli et al. 2010, Deepwater Horizon
Unified Command 2010). As the immediate effects on marine mammals are
still being investigated (see Unusual mortality events below), future and longterm
effects on Bottlenose Dolphins, and the food webs on which they depend,
will have to be monitored and studied for many years to come to truly understand
the ramifications of this unprecedented event. For example, long-term effects on
marine life, including Enhydra lutris (L.) (Sea Otter) and Killer Whale populations,
in relation to the 1989 Exxon Valdez oil spill in Prince William Sound, AK,
are only now, over 20 years later, beginning to be fully understood (e.g., Bodkin
et al. 2012, Matkin et al. 2008).
The increasing number of residential, commercial, and industrial developments
located along coastal areas of the GOMx and inland rivers that drain into
the GOMx also have the potential to cause harmful repercussions on Bottlenose
Dolphins. For instance, Galveston Bay, TX is home to about one half of all
chemical production facilities in the US, and these facilities have the potential
to introduce pollution into the air and discharge toxic substances into the water
(Antrobus 2005). In other areas of the GOMx, exposure to PCBs in waters of
Matagorda Bay, TX and Sarasota Bay, FL have been associated with high risk of
reproductive failure in Bottlenose Dolphins (Schwacke et al. 2002). Additional
studies have found significant amounts of persistent organic pollutants (POPs) in
animals from Mississippi Sound, Tampa, and Sarasota Bay (Kucklick et al. 2011,
Yordy et al. 2010a), and that the concentrations and mixtures observed in Bottlenose
Dolphins (from the GOMx) are sufficient to cause estrogenic effects that can
negatively alter hormonal activities (Yordy et al. 2010b). Furthermore, nutrients
and pollutants that enter the Mississippi River through runoff from upstream
activities such as industrialization and agriculture have significant impacts in the
GOMx and are responsible for the production of one of the largest dead zones in
the world (Rabalais et al. 2002a, b). While the presence of a dead zone may not
have direct impacts on Bottlenose Dolphins, indirect effects may occur through
the food chain as habitat, mortality, migration patterns, and subsequent food
supply for fish and invertebrates can be negatively affected (Craig et al. 2001;
Rabalais et al. 2002a, b).
Coastal wetland loss in the northern GOMx is also a significant problem. The
US GOMx contains >40% of the nation’s coastal wetlands (Turner 1997). This
habitat is extremely productive due, in part, to nutrient input from the Mississippi
River. Many species of fish and invertebrates use these coastal marshes
as nursery grounds (Baltz et al. 1998, Grimes 2001, Zimmerman and Minello
1984), including species known to be important prey for Bottlenose Dolphins.
However, wetland areas in the GOMx are some of the world’s most threatened.
For example, wetland loss in Louisiana accounts for ≈90% of the total coastal
wetland loss in the entire continental US (Couvillion et al. 2011)—an estimated
rate of 100 km2 of coastal wetlands lost each year in Louisiana (Britsch and
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Dunbar 1993, Day et al. 2007). In addition to land subsidence and oil and gas exploration,
intense storms such as hurricanes (Barras et al. 2008, Couvillion et al.
2011) can further contribute to wetland loss both directly and indirectly, as they
can accelerate future wetland loss due to induced plant stress from elevated salinity
and sulfide levels (Steyer et al. 2010). Such rapid losses pose a severe threat
to both the quantity and quality of nursery grounds (Chesney et al. 2000), which
are important for numerous fish species that are part of the Bottlenose Dolphin
food chain. The possibility exists that continued loss of the wetland areas may
have significant effects on the amount and type of food available for dolphins in
coastal areas, and how this may affect dolphin distribution is currently unknown.
Unusual mortality events
Unusual mortality events (UMEs), in which 10s to 100s of dolphins die, are
a relatively common occurrence in the GOMx. Bottlenose Dolphin deaths associated
with mass die-offs of other marine species have been documented at
least as far back as 1946 (Gunter et al. 1948). There have been 12 documented
large-scale events in the GOMx since 1990, with upwards of 2000 marine mammals
(primarily Bottlenose Dolphins) having stranded dead during these events
(NOAA Fisheries Office of Protected Resources 2013, Waring et al. 2013).
Since February 2010, coincident with the Deepwater Horizon oil spill, a UME
has been ongoing in the northern GOMx along the Louisiana, Mississippi, Alabama,
and western Florida panhandle coastlines where over 1000 cetaceans, the
majority of them Bottlenose Dolphins, have stranded (NOAA Fisheries Office
of Protected Resources 2013).
According to the Marine Mammal Commission Annual Report (2009), between
1991 and 2007, 34% of UMEs affecting marine mammals in US waters
occurred in the GOMx. While the cause of some of these events remains unknown,
others have been attributed to morbillivirus (Duignan et al. 1996; Lipscomb et al.
1994, 1996; Worthy 1998) and outbreaks of the red tide dinoflagellate Karenia
brevis (C.C. Davis) G. Hansen & Ø. Moestrup (Flewelling et al. 2005, Waring et
al. 2013). The detrimental effects on marine mammals from harmful algal blooms
(HABs, e.g., red tides) are associated with the presence of toxins, including brevetoxin
and domoic acid, that are produced by blooming microorganisms (e.g.,
dinoflagellates and diatoms, respectively; Fire et al. 2007, 2008, 2011). These
toxins subsequently accumulate through the food chain. It was recently revealed
that dolphins in Sarasota Bay may be particularly vulnerable to such threats as
they are repeatedly exposed to the presence of HABs and the toxins they produce,
and therefore, there is the potential for chronic effects from toxin exposure
(Twiner et al. 2011). Furthermore, they may have increased susceptibility for
morbillivirus infection due to the presence of low levels of protective antibodies
(Rowles et al. 2011). Threats from HABs and viral outbreaks may differentially
impact Bottlenose Dolphin populations, particularly those with smaller numbers
of individuals, raising conservation concerns for small populations of dolphins
living in estuarine areas of the GOMx. An area of concern is around the Florida
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Panhandle in the northern GOMx, where over the past 12 years at least three
Bottlenose Dolphin UMEs directly attributed to biotoxins have occurred, causing
the death of hundreds of individuals (Gaydos 2006, NOAA Fisheries Office of
Protected Resources 2013, Schwacke et al. 2010, Twiner et al. 2012).
Climate change
Global climate change is also of concern for Bottlenose Dolphins in the
GOMx. Some estimates predict that global surface temperature will increase
between 1.1 and 6.4 °C, with a sea-level rise of 30–100 cm, by the end of the
21st century (IPCC 2007). For the GOMx, sea-level rise has been predicted to
be around 0.5–1.0 m by 2100 (Davis 2011). For marine mammals, changes in
distribution and geographic range are the most likely responses to environmental
change, and it is predicted that 88% of cetacean species’ ranges, including Bottlenose
Dolphins’, will be altered as a consequence of rising water temperature
(Kaschner et al. 2011, MacLeod 2009, Wiens and Graham 2005). However, it is
unclear how Bottlenose Dolphin populations restricted to BSE areas of the GOMx
could change their ranges. Furthermore, it has been predicted that changes in biodiversity
over the next century, particularly a decrease of biodiversity in tropical
waters, will occur as a consequence of increasing ocean temperatures (Whitehead
et al. 2008). These predictions, coupled with the findings from numerous studies
showing that low genetic and species diversities can increase the vulnerability of
an ecosystem to climate change (Reusch et al. 2005, Whitehead et al. 2008, Worm
et al. 2006), implies that even a wide-ranging species like T. truncatus will likely
be impacted by global warming.
Increases in water temperature also have been attributed to decreases in
the availability of prey items, which has subsequently affected calf survival
in some marine mammals (Greene and Pershing 2004). Changes in distribution,
abundance, and migration of prey may also be expected (Learmonth et al.
2006, Simmonds and Eliott 2009). Temperature increases are predicted to raise
susceptibility to disease and contaminants through increased physiological
stress or movement of animals into new areas where they would be exposed to
pathogens not previously encountered or where they could introduce pathogens
into naïve populations (Simmonds and Eliott 2009, Van Bressem et al. 2009).
Increases in water temperature, combined with nutrient input from sources such
as the Mississippi River, have the potential to increase both the frequency and
severity of HAB outbreaks (Learmonth et al. 2006). Sea-level rise also has the
potential to substantially change and/or increase loss of coastal wetlands that
are already threatened by current human activities (Nicholls 2004). Furthermore,
the presence of destructive storms such as hurricanes can have severe
impacts on wetlands and submerged aquatic vegetation (Barras et al. 2008,
Couvillion et al. 2011, Edmiston et al. 2008). Multiple studies predict that increased
water temperature from global warming will increase the intensity of
hurricanes in the wNA (Bender et al. 2010, Knutson et al. 2010) and potentially
the GOMx, presenting significant threats to the habitats that are important nursery
areas for dolphin prey.
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2013 Vol. 12, Monograph 6
The occurrence of hurricanes has also been suspected to alter both social
and reproductive behavior of Bottlenose Dolphins. In the Bahamas, significant
differences were observed in dolphin social structure before and after two
major hurricanes impacted the area within a span of three weeks (Elliser and
Herzing 2011). Approximately 30% of the population left the study area and
were replaced by a similar number of immigrant dolphins that became assimilated
into the remaining communities. In Mississippi Sound, a slight increase
in juvenile and calf strandings of Bottlenose Dolphins was observed during
winter and spring seasons following Hurricane Katrina (Miller et al. 2010).
The same study found an increase in calf encounter rates two years after the
hurricane, suggesting that female dolphins had increased reproduction due to
indirect effects of the storm, including possible increases in available food and
decreases in anthropogenic-induced stress. Finally, storm surge associated with
hurricanes has also been attributed to displacement of Bottlenose Dolphins (in
the GOMx) into inland areas not part of their usual habitat, where they become
confined to shallow, low salinity, and potentially hazardous areas (Rosel and
Watts 2008).
Conservation and Management
The Marine Mammal Protection Act (MMPA) of 1972, which aims to protect,
restore, and sustain all populations of marine mammal species found in US
waters, directs all management and conservation of Bottlenose Dolphins in the
GOMx. Under the MMPA, the unit of management is the “population stock”
which is defined as “a group of marine mammals of the same species or smaller
taxa in a common spatial arrangement, that interbreed when mature.” This wording
is equivalent to the definition of a biological population. The main goals of
the MMPA are to prevent stocks from diminishing to the point where they no
longer remain a functional component of their ecosystem and to maintain stocks
at or above the optimum sustainable population (OSP) level. If a stock diminishes
below its OSP, measures should be taken to restore that stock. The MMPA defines
OSP as the number of animals that “result in the maximum productivity of the
population or species, keeping in mind the carrying capacity of the habitat and
health of the ecosystem of which they form a constituent elemen t.”
In order to fulfill the goals of the MMPA, stocks need to be delimited and
then evaluated as to whether or not they should be considered depleted or strategic
based on population estimates and trends (e.g., abundance), productivity
rates, fishery mortality, and other sources of anthropogenic mortality. A stock is
considered strategic if (1) it is deemed threatened or endangered (according to
the Endangered Species Act), (2) it is determined to be declining and likely to be
listed as threatened or endangered, (3) it is considered depleted under the MMPA,
or (4) the level of direct human-caused mortality exceeds the maximum allowable
annual removal limit, otherwise known as the potential biological removal
(PBR) level. Currently in the GOMx, no Bottlenose Dolphin stocks are considered
depleted; however, 34 of the 37 stocks are considered strategic (see below).
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The MMPA mandates that all stocks are protected, and managers strive to delimit
and protect stocks that represent demographically independent populations.
Such demographic independence exists when the impacts of internal dynamics
(e.g., birth and death rates) on a population outweigh the influence of external
dynamics (e.g., immigration and emigration) (Wade and Angliss 1997). It should
be noted that while these are the goals of the MMPA, a lack of information on
basic biological parameters, movement patterns, and/or reproductive characteristics
for many marine mammal species often means that some stocks do not
represent biological populations, but instead are management constructs to aid
in protection and mitigation of anthropogenic impacts until enough data become
available to delimit the true biological population(s) represented by that stock.
In most cases, if significant genetic differentiation is seen among populations,
they are also likely demographically independent. If populations are distinct both
genetically and demographically, they should be managed as separate stocks.
However, it is possible that populations may be demographically independent but
not genetically differentiated. In this case, genetic differentiation has perhaps not
had enough time to accrue among populations that experience little or no genetic
exchange, or dispersal rates are just high enough to counter the microevolutionary
processes of genetic drift and mutation that would promote differentiation,
but too small to replenish one population from another were it to be extirpated
(Wade and Angliss 1997).
Ideally, managers try to incorporate various lines of evidence, such as distribution,
geographical, morphological, and molecular information, to help
determine stock structure (Wade and Angliss 1997). For example, in US waters
of the wNA, Bottlenose Dolphins in multiple coastal stocks have been delimited
based on differences in geographical location, seasonal distribution and
movements, and genetics (Waring et al. 2013). However, when various lines of
evidence are integrated to delimit stocks, there is inherently more confidence in
the delimitation.
In addition to accurate delimitation of stock structure, another important
component to successful conservation and management is accurate assessment of
anthropogenic impacts on each stock and whether some stocks may be impacted
more severely than others. Bottlenose Dolphins live in habitats that expose them
to a variety of threats, including pollution, boat traffic, marine noise, industrial
development (e.g., oil and gas), marine debris impacts, illegal fishing and harassment
activities, and both commercial and recreational fisheries (Garrison et
al. 2003, Waring et al. 2013, Wells and Scott 1997). The variety of ranges and
locations of Bottlenose Dolphin stocks in the GOMx (Figs. 1, 2) mean different
populations will encounter different types and magnitudes of anthropogenic
stressors. For example, animals residing in waters over the continental shelf in
the western GOMx likely have a greater chance of being affected by oil and gas
development compared to animals that reside over the shelf in the eastern GOMx.
Therefore, it is important to accurately identify all stocks in a given area so that
potential threats can accurately be assessed and, if necessary, mitigated.
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2013 Vol. 12, Monograph 6
Figure 1. Map of northern Gulf of Mexico depicting the approximate boundaries of Bottlenose
Dolphin stocks. The 32 currently recognized bay, sound, and estuary Bottlenose
Dolphin stocks. 1 = Laguna Madre; 2 = Corpus Christi Bay and surrounding bays; 3 =
Espiritu Santo Bay to Redfish Bay; 4 = Matagorda Bay and surrounding bays; 5 = West
Bay; 6 = Galveston Bay and surrounding bays; 7 = Sabine Lake; 8 = Calcasieu Lake; 9 =
Vermilion Bay to Atchafalaya Bay; 10 = Terrebonne and Timbalier Bays; 11 = Barataria
Bay; 12 = Mississippi River Delta; 13 = Mississippi Sound, Lake Borgne; Bay Boudreau;
14 = Mobile and Bonsecour Bays; 15 = Perdido Bay; 16 = Pensacola and East Bays; 17 =
Choctawhatchee Bay; 18 = St. Andrew Bay; 19 = St. Joseph Bay; 20 = St. Vincent Sound,
Apalachicola Bay, St. George Sound; 21 = Apalachee Bay; 22 = Waccasassa Bay to Crystal
Bay; 23 = St. Joseph Sound and Clearwater Harbor; 24 = Tampa Bay; 25 = Sarasota
Bay and Little Sarasota Bay; 26 = Lemon Bay; 27 = Charlotte Harbor, Gasparilla Sound
and Pine Sound; 28 = Caloosahatchee River; 29 = Estero Bay; 30 = Chokoloskee Bay to
Gullivan Bay; 31 = Whitewater Bay; 32 = Florida Keys.
Management in the US Gulf of Mexico
Bottlenose Dolphins are found throughout the GOMx, except in the deep central
basin (Figs. 1, 2, 3). There are currently 37 recognized stocks of Bottlenose
Dolphins in the GOMx (Waring et al. 2013). Thirty-two of these stocks have
been delimited among the bay, sound, and estuary environments (Fig. 1). Some
of these stocks have been identified based on residency patterns determined from
photo-identification and tagging studies (e.g., Balmer et al. 2008, Conn et al.
2011, Irvine et al. 1981, and see Waring et al. 2013 for additional list of publications
by area), ecological studies (Barros and Wells 1998), and/or genetic studies
(Duffield and Wells 1991, 2002; Sellas et al. 2005). All 32 of the BSE stocks
are considered to be strategic under the MMPA because most stock sizes are unknown
but thought to be small enough that just a few human-induced mortalities
could put them over PBR (Waring et al. 2013).
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Three of the 37 Bottlenose Dolphin stocks have been delimited as coastal
stocks, occurring in waters between the shoreline/barrier islands and the 20-m
isobath: the eastern coastal, northern coastal, and western coastal stocks (Fig. 2).
Figure 3. Map depicting large vessel-based survey effort conducted by NOAA Fisheries
in coastal and offshore waters of the GOMx between 1992–2009: all on-effort tracklines
from the ship surveys (gray lines) and the locations off all recorded sightings of T. truncatus
observed from ship surveys (black dots). The EEZ and 20-m, 200-m, and 2000-m
isobaths are shown.
Figure 2. Boundaries of the five currently recognized coastal, continental shelf, and oceanic
stocks. The 20-m isobath, 200-m isobath, and US Exclusive Economic Zone (EEZ)
are shown..
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2013 Vol. 12, Monograph 6
The separation between the eastern and northern coastal stocks lies at 84°W
longitude, while the break between the northern and western coastal stocks is
defined as the western edge of the Mississippi River Delta (Waring et al. 2013).
The longitudinal breaks between these three stocks separate areas with different
shoreline characteristics and amounts of freshwater input (Mullin et al. 2007).
For example, the Mississippi and Atchafalaya River systems in the north drain
approximately two-thirds of the US mainland (Davis et al. 2002) and deliver
sediments, nutrients, and pollutants into the GOMx. In part due to currents that
run along the shore from east to west in the northern GOMx, shelf sediments to
the west of the Mississippi River Delta are strongly influenced by river runoff
and are composed of silt and clay, while calcareous and sandy sediments are more
common to the east of the Delta (Rabalais et al. 1999), creating different benthic
communities there. Currently, the eastern coastal stock is not considered depleted
or strategic; however, the northern and western coastal stocks are considered to
be strategic due to a lack of systematic monitoring of the fisheries that may interact
with these stocks as well as the potential impacts of the ongoing 2010–2013
Northern Gulf of Mexico Cetacean UME. Furthermore, for the western coastal
stock, the population size is currently considered unknown because the last
abundance survey was conducted more than eight years ago, and there have been
numerous human-related mortalities that have impacted dolphins in this area
(Waring et al. 2013).
A separate continental shelf stock has been delimited for waters between
the 20-m and 200-m isobaths in the northern GOMx from the Florida Keys to
the US–Mexico border (Fig. 2). The 20-m isobath was chosen as a boundary
for these stocks because Bottlenose Dolphin densities are higher and there is a
greater concentration of human activities that pose risks to dolphins closer to
shore compared to continental shelf waters (Mullin et al. 2007). Finally, an oceanic
stock was delimited and extends from the 200-m isobath seaward to the extent
of the US Exclusive Economic Zone (Fig. 2). Neither the continental shelf
nor the oceanic stock are currently considered depleted or strategic (Waring et
al. 2013).
These delimited GOMx Bottlenose Dolphin stocks are currently the best
available information for management purposes. However, outside of the few
well-studied BSE environments, it remains unclear whether these stock delimitations
represent the true underlying biological populations of Bottlenose
Dolphins that exist in the GOMx. This uncertainty is largely due to lack of genetic
sampling (needed to determine presence/absence of genetic differentiation
and degree of interbreeding between regions) and individual animal tracking
(e.g., radio- and/or satellite-tagging, to determine demographics) from a large
range of individuals within and across each potential stock. More work is
necessary to accurately delimit all GOMx stocks and quantify the threats (especially
from human-induced mortality) each faces so priorities can be made for
conservation and management actions.
Southeastern Naturalist
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Vol. 12, Monograph 6
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Future Research and Conclusions
Taken together, although substantial research on Bottlenose Dolphins in
the GOMx has been performed, much more work needs to be done. Currently,
the best option for making inferences about how Bottlenose Dolphins utilize
the GOMx habitat on a large scale is to piece together data from the numerous
smaller-scale projects that have been conducted throughout the GOMx. The
information gathered together in this review provides an updated starting point
towards better understanding Bottlenose Dolphin populations in the inshore,
coastal, and offshore waters of the GOMx. Overall, the more that is understood
about how these animals live in their environment and how humans impact
Bottlenose Dolphins and their habitats, the better we will be able to protect and
conserve the biological populations that exist and prepare for the potential challenges
the future holds.
The greatest shortcomings in information for conservation of Bottlenose Dolphins
in the GOMx involve an incomplete understanding of stock structure, lack
of up-to-date abundance estimates and fishery mortality estimates for most stocks,
and limited understanding of the effect of the many other anthropogenic impacts on
the sustainability of the populations. Genetic analyses of population structure have
been conducted for only a few of the 32 BSE areas. While studies of movements
of animals or differences in stable isotope signatures and contaminant loads may
help identify groups of animals with different habitat preferences, only genetic
analyses may indicate whether two groups are demographically independent from
one another. Past (Sellas et al. 2005) and ongoing genetic analyses of BSE populations
have continued to support the original hypothesis that different BSE habitats
harbor differentiated populations. However, from a management standpoint,
ultimately all 32 stocks need to be tested. The current Bottlenose Dolphin stock
delimitations in the GOMx outside of BSE areas are based solely on suspected
physical environmental differences and may not accurately reflect true biological
population structure. A comprehensive molecular study of Bottlenose Dolphin
samples from coastal, continental shelf, and offshore areas of the GOMx is currently
being conducted (N.L. Vollmer and P.E. Rosel, unpubl. data). This invaluable
information concerning Bottlenose Dolphins in coastal and offshore waters will
help researchers and managers better understand habitat requirements, geographic
range, and evolutionary histories of both coastal and offshore ecotypes, as well as
population structure between and within each of the ecotypes. In order to manage
Bottlenose Dolphin stocks effectively and to prevent the detrimental results of
combining multiple biologically differentiated stocks into one, or the reverse—
splitting a stock into more than one—it is imperative to determine the degree of
genetic exchange occurring among populations. This information will in turn
provide biological evidence to identify and differentiate populations of Bottlenose
Dolphins in the GOMx and present managers essential information upon which accurate
stock delimitations can be developed.
Studies focused primarily on Bottlenose Dolphins in BSE areas (cited
herein) have provided most of the evidence in the GOMx for seasonal changes
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2013 Vol. 12, Monograph 6
in abundance. However, more research is needed within coastal and offshore
waters of the GOMx. Knowing whether the animals are moving among BSE
areas or perhaps out into deeper shelf waters could have implications for how
management guidelines are implemented and regulated. For instance, if a stock
diminishes below its OSP and is known to migrate into a certain area at a given
time of the year, adjustment of fishing and/or industrial development regulations
may be warranted in that area during that time to help prevent incidental mortalities.
More research involving the use of satellite and/or radio tags of individual
animals, as has been done for areas around Tampa, Sarasota, and St. Joseph
Bay (e.g., Balmer et al. 2008, 2010; Mate et al. 1995; Wells et al. 1999), would
provide invaluable data towards a better understanding of possible migration patterns
of Bottlenose Dolphins in other parts of the GOMx.
Accurate estimation of the impact of human-caused mortality on Bottlenose
Dolphin stocks is anchored in accurate stock delimitations, as described above,
as well as in accurate estimates of abundance and non-natural mortality. With
appropriate resources, good estimates of abundance and of mortality from commercial
fishery interactions can be obtained using standard scientific protocols
and, in tandem, these estimates can be used to assess the impact of those fishery
interactions on the appropriate stocks. However, dolphins in the GOMx are
impacted by more than just commercial fishery interactions. Methods to assess,
understand, and incorporate the affects of other anthropogenic stressors such as
contaminants, ocean noise, and habitat degradation are much less developed, but
no less important, and are critical to successful protection and conservation of
Bottlenose Dolphins in the GOMx.
Acknowledgments
We sincerely thank all who helped to track down literary resources, including
L. Broussard (USGS, Lafayette, LA), D. Fertl (Ziphius EcoServices), K. Urian (Duke
University), C. Fairfield (NMFS, Miami, FL), C. Sinclair (NMFS, Pascagoula, MS), and
E. Zolman (NOS, Charleston, SC). We also thank K. Mullin (NMFS, Pascagoula, MS)
and L. Garrison (NMFS, Miami, FL) for their help in obtaining survey cruise and sighting
data and J. Bracewell (NPS, Lafayette, LA) for help with ArcGIS. We would also like to
thank two anonymous reviewers who provided helpful comments on this manuscript.
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