1 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. Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 2 2013 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 3 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 2013 Vol. 12, Monograph 6 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/ Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 4 2013 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 5 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 2013 Vol. 12, Monograph 6 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. Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 6 2013 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). 7 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 2013 Vol. 12, Monograph 6 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. Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 8 2013 (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 9 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 2013 Vol. 12, Monograph 6 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 Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 10 2013 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). 11 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 2013 Vol. 12, Monograph 6 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 Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 12 2013 (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 13 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 2013 Vol. 12, Monograph 6 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 Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 14 2013 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 15 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 2013 Vol. 12, Monograph 6 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 Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 16 2013 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 17 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 2013 Vol. 12, Monograph 6 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 Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 18 2013 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 19 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 2013 Vol. 12, Monograph 6 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 Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 20 2013 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. 21 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 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). Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 22 2013 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. 23 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 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). Southeastern Naturalist N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 24 2013 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.. 25 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 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 N.L. Vollmer and P.E. Rosel Vol. 12, Monograph 6 26 2013 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 27 Southeastern Naturalist N.L. Vollmer and P.E. Rosel 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. 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