2009 NORTHEASTERN NATURALIST 16(1):27–44
Large Nektonic Fishes in Marsh Creek Habitats in the
Delaware Bay Estuary
Kenneth W. Able1,*, K. Martha M. Jones2, and Dewayne A. Fox3
Abstract - Larger nektonic fishes, many of which are economically important, comprise
a large portion of the biomass in estuaries and may influence energy flow through
their migrations and feeding, yet we know relatively little of this faunal component.
To elucidate the patterns of species composition, distribution, and abundance in Delaware
Bay, we sampled (n = 2298 sets) nektonic fishes (n = 3693 individuals, mean
length = 261.4 mm, range = 53–600 mm) with multi-mesh gill nets in near-shore bay
and marsh creek habitats during the summer and fall (June–November 2001) when
fishes are more abundant in temperate estuaries. For the most abundant species, the
older and larger individuals (age 1+) often dominated the catches. Patterns of assemblage
structure were influenced by spatial gradients in salinity and dissolved oxygen
and temporal changes in temperature. Many of the large nektonic fishes that dominate
in Delaware Bay are also found in other temperate estuaries from the Gulf of Maine to
Chesapeake Bay, in part, because these species are highly migratory.
Introduction
Fishes make up 99% of estuarine nekton, i.e., actively swimming animals
that either swim in the water column (truly nektonic) or live near the bottom
but frequently feed in the water column (engybenthic) (deSylva 1975, 1985;
McHugh 1967). Of these, most are small because young-of-the-year (YOY)
fishes dominate in these important habitats (Haedrich 1983, Yáñez-Arancibia
1985) including temperate estuaries along the east coast of the US (Able 2005,
Able and Fahay 1998). Larger nekton are infrequently the focus of directed
sampling efforts, in part because they spend much of their time in the water
column, and most sampling programs are inappropriate for this habitat (de-
Sylva 1975, 1985). This lack of focus has occurred despite the fact that these
fishes may be a major component of estuarine fish biomass (Hartman and
Brandt 1995), important predators in these systems (Baker and Sheaves 2005,
Sheaves 2001), and economically important portions of the fauna that are
harvested in recreational and commercial fisheries (Haedrich and Hall 1976,
Nixon 1982). Further, the increasing realization that larger fishes are potentially
important contributors to conserving biodiversity (Steneck 2005), and to
enhanced fecundity and larval survival in populations (Birkeland and Dayton
2005) provide a reason for further attention.
In the Delaware Bay estuary, there has been a lot of emphasis on small,
benthic fishes in large-scale marsh restoration projects (Able and Fahay
1Rutgers Marine Field Station, 800 c/o 132 Great Bay Boulevard, Tuckerton NJ
08087-2004. 2Department of Biology, Cape Breton University, 1250 Grand Lake
Road, Sydney, NS B1P 6L2, Canada. 3Department of Agriculture and Natural Resources,
Delaware State University, 1200 North Dupont Highway, Dover DE 19901-
2277. *Corresponding author - able@marine.rutgers.edu.
28 Northeastern Naturalist Vol. 16, No. 1
1998; Able et al. 2000, 2001, 2004; Grothues and Able 2003a, b). The few
studies that have focused on large nektonic fishes are those for Morone saxatilis
Walbaum (Striped Bass) movements and diet (Nemerson and Able 2003,
Tupper and Able 2000), Carcharhinus plumbeus Nardo (Sandbar Shark)
movements and growth (McCandless et al. 2007, Merson and Pratt 2001,
Rechisky and Wetherbee 2003), reproduction in Cynoscion regalis Bloch
and Schneider (Weakfish) (Connaughton and Taylor 1995), and food habits
of these species and Paralichthys dentatus Linnaeus (Summer Flounder) and
Pomatomus saltatrix Linnaeus (Bluefish) (Taylor 1987).
The purpose of this paper is to provide an enhanced understanding of the
species composition, abundance, and distribution of larger nektonic fishes
across much of the estuarine salinity gradient of Delaware Bay during the
summer and fall, a period of typically high fish abundance for this and other
temperate estuaries. This information on large nektonic fishes was derived
from standardized gill-net collections.
Materials and Methods
Study sites
Delaware Bay, the drowned river valley of the Delaware River, has a
vertically homogeneous water column (Biggs 1978) with a tidal range of approximately
2 m and an annual temperature range of -2 °C to 28 °C (Sharp
1988). Almost the entire perimeter of the Bay is covered with extensive
salt marshes (Daiber and Roman 1988). However, vegetation type changes
markedly along the long axis of the Bay, with salt marshes dominated by
Spartina alterniflora Loiseleur (Smooth Cordgrass) and Spartina patens
(Aiton) Muhl. (Salt Hay Grass) in the lower, higher-salinity portion of the
bay, while Phragmites australis (Cav.) Trin. ex Steud. (Common Reed) is
much more abundant in the upper, lower-salinity portion of the Bay (Able et
al. 2001, Weinstein et al. 1997). Seven marsh creek sites, all located on the
New Jersey side of Delaware Bay (Fig. 1), were chosen as representative of
the range of salinity for the Bay (Table 1).
Table 1. Sampling and selected physical and fish fauna characteristics of Delaware Bay marsh
study sites. See Figure 1 for locations of individual sites. Values reported are mean and standard
error where applicable. GN = number of gill nets set, TF = total fish collected, CPUE = number
of fish per gill net set, and SR = species richness.
Surface Surface Surface
salinity DO temperature Depth Total
Site (ID No.) (‰) (mg/L) (°C) (m) GN TF CPUE SR
West Creek (1) 17.7 (0.2) 6.6 (0.2) 21.0 (0.6) 1.8 (0.1) 160 162 0.9 (0.2) 14
Riggins Ditch (2) 19.4 (0.2) 6.4 (0.2) 19.5 (0.6) 2.0 (0.1) 146 239 1.6 (0.2) 15
Dividing Creek (3) 18.5 (0.3) 6.6 (0.2) 22.7 (0.5) 2.2 (0.1) 146 348 2.0 (1.0) 13
Cohansey River (4) 12.1 (0.6) 6.9 (0.3) 23.6 (0.7) 2.1 (0.1) 136 235 1.6 (0.2) 11
Mad Horse Creek (5) 11.9 (0.3) 7.2 (0.2) 22.5 (0.6) 2.3 (0.1) 159 299 1.7 (0.2) 14
Alloway Creek (6) 5.6 (0.1) 7.2 (0.1) 21.2 (0.3) 1.8 (0.1) 1135 1718 1.5 (0.1) 16
Mill Creek (7) 3.9 (0.2) 8.0 (0.1) 24.9 (0.5) 1.6 (0.1) 256 648 2.3 (0.2) 14
2009 K.W. Able, K.M.M. Jones, and D.A. Fox 29
Sampling techniques
We sampled with anchored gill nets from late June to mid-November
2001 at seven sites in Delaware Bay, NJ (Fig. 1, Table 1). Each gill net
was 2.4 x 13.5 m with 5 panels of 5 mesh sizes (2.5, 3.8, 5.1, 6.4, or 7.6
cm bar). Gill nets were set at biweekly intervals throughout tidal cycles
in upper creek and creek-mouth habitats. For the most part, gill nets were
deployed at the surface, but nets were set on the bottom where depths
were >2.4 m. All nets were set during the day for approximately 60 min
and standardized to that time frame. Upon retrieval of each gill net, all
fishes were identified and counted, and the first 50 individuals were
measured (to nearest 1.0 mm fork length or total length); all Bluefish and
Morone americana Gmelin (White Perch) were measured.
Figure 1. Location of the seven sampling sites in Delaware Bay, NJ. Numbers indicate
distance (km) from the mouth of the Bay.
30 Northeastern Naturalist Vol. 16, No. 1
Physical and chemical parameters were measured at the beginning of
each gill-net deployment. Temperature (± 0.1 °C), dissolved oxygen concentration
(± 0.3 mg/L), and salinity (± 0.2‰) were measured just under the
water surface with a hand-held meter (YSI Model 85 or YSI model 600).
Depth was measured at each gill-net location using a hull-mounted depth
recorder. Latitude and longitude coordinates were determined for each gillnet
location with a global positioning system (GPS) unit.
Discrimination of age classes
We combined monthly collections across all sites to create
length-frequency distributions for the numerically dominant species. Based
on length-frequency distributions, six species were represented by multipleyear
classes: Brevoortia tyrannus Latrobe (Atlantic Menhaden), Weakfish,
Dorosoma cepedianum Lesueur (Gizzard Shad), White Perch, Striped Bass,
and Bluefish. In each of these species, multiple size modes were indicative of
the presence of multiple age classes. Within months, fish were divided into two
classes: YOY and age 1+ based on available monthly size estimates (Able and
Fahay 1998). In species where every fish was not measured, we determined the
proportions in each age class for measured fish and then applied these proportions
to unmeasured fish. YOY and age 1+ classes were treated as separate taxa
during all analyses for the six species with multiple year classes.
Statistical analysis
Canonical correspondence analysis (CCA; Ter Braak 1986), was performed
separately on the collections to determine assemblage structure.
Correlations between spatial patterns of large nektonic fishes of Delaware
Bay and physical and temporal parameters (i.e., dissolved oxygen, salinity,
temperature, and time of day) were determined using CCA. All analyses
were done using CANOCO software. CCA is a multivariate technique of
direct gradient analysis; it selects the linear combination of physical and
temporal variables that maximizes the dispersion of the species scores (Tabachnik
and Fidell 1989). It chooses the best weights for the physical and
temporal variables to construct the first CCA axis. The additional CCA axes
also select linear combinations of physical or temporal variables that maximize
the dispersion of the species scores, but are subject to the constraint
of being uncorrelated with previous CCA axes. CCA has been previously
used to correlate fish species distribution and abundance with environmental
factors (e.g., Adjeroud et al. 1998, Gomez et al. 1988, Tejerina-Garro et al.
1998). The statistical unit (or row) for the CCA was an individual collection
with each column representing separate physical and temporal parameters
and catch per unit effort (CPUE) by fish species.
From the CCA, factor loadings for physical and temporal variables greater
than 0.3 suggest that a variable explains a major portion of the relationship
(Jongman et al.1995, Tabachnik and Fidell 1989). These variables are represented
by vectors in the ordination plot derived from the first two roots of the
CCA. The longer the vector, the greater the relationship between that variable
and fish distribution and abundance. Species of fish positioned close to a physical
or temporal vector have a strong relationship with that variable.
2009 K.W. Able, K.M.M. Jones, and D.A. Fox 31
Results
Spatial variation in physical characteristics
The seven marsh creeks that were sampled covered a salinity gradient
ranging from mesohaline to oligohaline (Table 1). The three lowest sites
in the Bay (West Creek, Riggins Ditch, and Dividing Creek) had similar
mean (all months pooled) salinities, ranging from 17–19‰. The two middle
bay sites (Cohansey River and Mad Horse Creek) had lower mean annual
salinities that approximated 12‰. The remaining upper bay sites (Alloway
Creek and Mill Creek) had the lowest mean salinities, which ranged from
3–5‰. Dissolved oxygen concentrations (mg/l) followed a pattern opposite
to that found for salinities with higher values in the upper bay sites. Pooled
mean concentrations of dissolved oxygen were reduced in the lower bay and
followed a general trend of increasing concentration up the bay. Mean temperatures
at each creek site ranged from approximately 19–25 °C (Table 1).
Water depth at sampling locations varied among sites and ranged from 1.6
m at Mill Creek to 2.3 m at Mad Horse Creek.
Fish composition and abundance
Based on collection of 3693 individuals, fifteen families comprising 22
species of large nektonic fishes were collected in 2298 gill net sets during
June–November 2001 (Table 2). The sciaenids (Weakfish, Leiostomus xanthurus
Lacepède [Spot], Micropogonias undulatus Linnaeus [Atlantic Croaker],
and Pogonias cromis Linnaeus [Black Drum]), and clupeids (Alosa aestivalis
Mitchill [Blueback Herring], Alosa mediocris Mitchill [Hickory Shad], Alosa
pseudoharengus Wilson [Alewife], Atlantic Menhaden, and Gizzard Shad)
were the most speciose families. These families were also among the most
abundant, comprising 4.5% and 42.9%, respectively, of the total number
of fishes collected. The other relatively speciose family was the ictalurids
(Ameiurus catus Linnaeus [White Catfish], Ameiurus nebulosus Lesueur
[Brown Bullhead], and Ictalurus punctatus Rafinesque [Channel Catfish]),
which accounted for 4.7% of the total number of fish collected. Of all species
collected, White Perch, (36.0%) made up the largest percentage of the
catch, while Atlantic Menhaden (20.4%), Bluefish, (7.5%), and Gizzard Shad
(22.1%) were also relatively large components of the fauna. Together, these
five species made up 85.6% of the total catch. These fishes were relatively
large, with most individuals >100 mm and some exceeding 600 mm (Fig. 2).
The overall average size of fish caught was 261.4 ± 1.8 mm (SE).
Of those species represented by multiple year classes, the older individuals
were typically more abundant (Table 2). Age 1+ White Perch were
the most abundant taxa/category (overall CPUE 0.87), but YOY individuals
of this species were much less abundant (less than 0.01 CPUE). The same was true
for the second most abundant species—Gizzard Shad (age 1+ CPUE 0.53,
YOY CPUE 0.01)—and another abundant species, Atlantic Menhaden (age
1+ CPUE 0.27, YOY CPUE 0.08). For Bluefish, another relatively abundant
species, the YOY individuals (0.13 CPUE) were more abundant than
the age 1+ individuals (0.06). The abundance of most other species was
low (less than 0.10 CPUE).
32 Northeastern Naturalist Vol. 16, No. 1
Table 2. Average abundance (CPUE, number of fish per gill net set) at each location in Delaware Bay, NJ. Locations are arranged (left to right) by increasing
river mile. Species and life stages are arranged from highest to lowest overall CPUE.
Mad
West Riggins Dividing Cohansey Horse Alloway Mill All sites
Species (age) Common name Species code Creek Ditch Creek River Creek Creek Creek combined
Morone americana (Gmelin) +1 White Perch Morame 0.08 0.36 0.11 1.16 0.57 1.14 1.29 0.87
Dorosoma cepedianum (Leseur) 1+ Gizzard Shad Dorcep 0.30 0.10 0.10 0.27 0.13 0.57 1.91 0.53
Brevoortia tyrannus (Latrobe) (1+) Atlantic Menhaden Bretyr 0.01 0.48 0.47 0.44 0.99 0.07 0.75 0.27
Pomatomus saltatrix (Linnaeus) (0) Bluefish Pomsal 0.10 0.38 0.13 0.09 0.53 0.07 0.03 0.13
Cyprinus carpio Linnaeus Common Carp Cypcar - - - 0.07 0.01 0.14 0.22 0.10
Brevoortia tyrannus (0) Atlantic Menhaden Bretry 0.02 0.06 0.92 - 0.06 <0.01 0.01 0.08
Ictalurus punctatus (Rafinesque) Catfish Ictpun - - - 0.02 - 0.11 0.13 0.07
Morone saxatilis (Waldbaum) (1+) Striped Bass Morsax 0.13 0.18 0.14 0.06 0.09 0.02 0.04 0.06
Pomatomus saltatrix (1+) Bluefish Pomsal 0.05 0.03 0.05 0.13 0.15 0.02 0.21 0.06
Pogonias cromis (Linnaeus) Black Drum Pogcro 0.01 0.04 0.06 0.12 0.06 0.05 0.01 0.05
Leiostomus xanthurus Lacepède) Spot Leixan 0.06 0.05 0.11 0.07 - 0.01 0.06 0.03
Ameiurus nebulosus (Leseur) Brown Bullhead Ameneb - - - 0.01 0.01 0.04 0.08 0.03
Cynoscion regalis (Bloch & Schneider) (1+) Weakfish Cynreg 0.06 0.16 0.05 - 0.05 - - 0.03
Ameiurus catus (Linnaeus ) White Catfish Amecat - - - - - 0.04 0.01 0.02
Dorosoma cepedianum (0) Gizzard Shad Dorcep - - - - - 0.02 0.01 0.01
Cynoscion regalis (0) Weakfish Cynreg 0.02 0.05 0.02 - - - - 0.01
2009 K.W. Able, K.M.M. Jones, and D.A. Fox 33
Table 2, continued.
Mad
West Riggins Dividing Cohansey Horse Alloway Mill All sites
Species (age) Common name Species code Creek Ditch Creek River Creek Creek Creek combined
Caranx hippos (Linnaeus) Crevalle Jack Carhip - - - 0.01 - 0.01 0.03 0.01
Micropogonias undulatus (Linnaeus) Atlantic Croaker Micund 0.01 0.02 0.01 - - - 0.01 <0.01
Alosa pseudoharengus (Wilson) Alewife Alopse 0.01 - - - 0.03 <0.01 - <0.01
Morone americana (0) White Perch Morame - - - - - - 0.03 <0.01
Paralichthys dentatus (Linnaeus) Summer Flounder Parden 0.01 0.01 - - 0.01 - - <0.01
Rhinoptera bonasus (Mitchill) Cownose Ray Rhibon 0.01 - - - 0.01 - - <0.01
Mugil cephalus Linnaeus Striped Mullet Mugcep - 0.01 0.01 - - - - <0.01
Mugil curema Valenciennes White Mullet Mugcur - - - - - <0.01 - <0.01
Alosa mediocris (Mitchill) Hickory Shad Alomed - 0.01 - - 0.01 - - <0.01
Alosa aestivalis (Mitchill) Blueback Herring Aloaes 0.01 - 0.01 - - - - <0.01
Morone saxatilis (0) Striped Bass Morsax - - - 0.01 - - - <0.01
Trinectes maculatus (Bloch & Schneider) Hogshcoker Trimac - - - - 0.01 - - <0.01
Trachinotus falcatus (Linnaeus) Permit Trafal - 0.01 - - - - - <0.01
Mustelus canis (Mitchill) Smooth Dogfish Muscan - 0.01 - - - - - <0.01
Perca flavescens (Mitchill) Yellow Perch Perfla - - - - - <0.01 - <0.01
Strongylura marina (Waldbaum) Atlantic Needlefish Strmar - - 0.01 - - - - <0.01
Urophycis regia (Waldbaum) Spotted Hake Uroreg 0.01 - - - - - - <0.01
All species combined 0.94 1.55 2.03 1.63 1.74 1.54 2.31 1.64
34 Northeastern Naturalist Vol. 16, No. 1
The peaks in fish abundance were reasonably similar across locations
(Fig. 3). The highest overall catch per unit effort occurred at Mill Creek
(CPUE = 2.3 fish/net set), while the lowest values occurred at West
Creek (CPUE = 0.9 fish/net set). At most sites, abundance peaked in September.
This trend was most obvious at Riggins Ditch, Cohansey River,
Mad Horse Creek, and Mill Creek (Fig. 3). Possible exceptions included
Dividing Creek, where average catches were high in August. The November
collections, where they occurred, typically had some of the lowest
estimates of abundance.
Assemblage structure
The first two axes from canonical correspondence analysis (CCA)
explained 42.2% and 6.5% of the variance of weighted averages of species
with respect to each of the environmental variables (Fig. 4). From
the species-environment biplot (Fig. 4a), several species appeared to be
strongly associated with salinity (e.g., YOY Weakfish and age 1+ Atlantic
Croaker), whereas other species appeared to be more strongly associated
with dissolved oxygen (e.g., YOY White Perch and YOY Gizzard Shad). In
the samples-environment biplot (Fig. 4b), the sites separated along the first
axis, which corresponds with the increasing salinity gradient from upper
Delaware Bay sites (e.g., Mill Creek and Alloway Creek) to lower sites (e.g.,
Riggins Ditch and West Creek). There is separation of months along the
second CCA axis, which suggests a temporal shift within sites with seasonal
changes in temperature and dissolved oxygen.
Figure 2. Length-frequency distribution of fishes collected by gill nets in Delaware
Bay during June–November 2001.
2009 K.W. Able, K.M.M. Jones, and D.A. Fox 35
Discussion
Limitations of this study
Most approaches to sampling fishes are influenced to a high degree by the
sampling gear, especially if it is based on a single gear type (e.g., Able 1999).
For example, it is clear that gill nets are selective (Hamley 1975, Porch et
al. 2002). Even multi-mesh nets, which are designed to catch a wider array
Figure 3. Temporal variation in sampling effort (number of gill nets set per month;
dashed lines) and abundance (CPUE, number of fish per gill net set, values are mean
± SE; solid lines) at seven sampling sites in Delaware Bay during 2001.
36 Northeastern Naturalist Vol. 16, No. 1
Figure 4. Fish associations (A) with environmental variables as sampled with gill
nets. CCA ordination diagram with fish species () and environmental variables (arrows).
Site associations (B) with environmental variables from gill-net samples.
2009 K.W. Able, K.M.M. Jones, and D.A. Fox 37
of species and sizes, are still selective (Finstad et al. 2000). Further, the high
proportion of fast swimming pelagic fishes (e.g., Gizzard Shad, Atlantic
Menhaden, Bluefish) collected in this study may well be due to the fact that
gill nets are more effective when fish are moving. While we recognize these
issues, we do not believe them to be a major problem because our objectives,
in large part, were focused on a general characterization of larger nektonic
fishes (species composition, abundance, size composition). Further, the gill
nets used in this study collected a larger size range (and older age range) than
other gears frequently used in estuaries (see below).
Additional limitations of this study were common to other studies. Most
of these pertain to shortcomings in the spatial and temporal breadth of the
sampling. Our observations were limited to a single year and were conducted
only during the day. The former problem may be important in estuaries of
the Middle Atlantic Bight where the highly migratory, seasonal fish fauna
(Able 1999, 2005; Miller et al. 1985; Musick et al. 1985) can be influenced
annually by events beyond the sampling area. The issue of day/night sampling
may be especially important because of the likelihood of enhanced
gear avoidance during the day with gill nets and other gears (Hagan and
Able 2008; Rountree and Able 1993, Sogard and Able 1994). This limitation
might not be as much of an issue in Delaware Bay, which is a very turbid
system, and as a result, may reduce gear avoidance. Another consideration is
that a limited number of relatively shallow habitats (marsh creeks, nearshore
bay waters) were the focus of this study. This focus may account, in part,
for the absence of some larger species that have been collected commonly in
other sampling in Delaware Bay, including Sandbar Shark (Merson and Pratt
2001), Weakfish (Connaughton and Taylor 1995), and Summer Flounder
(Taylor 1987).
Comparisons within Delaware Bay
The size composition for fishes collected in this study are generally larger
than that for other studies in this estuary (Able et al. 2000, 2001, 2004).
Some of this difference is due to the fact that these other studies sampled
with a variety of gears (see Fig. 3.1 in Able and Fahay 1998), which were
mainly directed at catching YOY fishes. In the latter summary, for example,
none of the fishes caught approached the average size of fishes caught in the
gill net (mean = 261.4 mm) (Fig. 2). Further, a prior otter trawl sampling
program in the same general areas in Delaware Bay in May–November
1996 collected smaller fishes (mean = 68 mm), with the majority <100 mm
(Able et al. 2001); thus, the gill nets did target larger fishes than previous
studies. However, some of the relatively abundant fishes in gill nets were
YOY (Bluefish, Atlantic Menhaden, Gizzard Shad, Weakfish, Menticirrhus
americanus Linnaeus [Southern Kingfish], Striped Bass). On the other hand,
larger, older fish (age 1+) were represented by the same species (Table 2);
thus, for these species the use of estuarine habitats extends beyond YOY. Another
recently completed analysis of Delaware Bay fishes (Able et al. 2007),
as collected by a larger otter trawl in deeper waters (6–20 m), also collected
larger fishes (range = 15–999, mean = 234 mm TL), thus overlapping in size
38 Northeastern Naturalist Vol. 16, No. 1
with those collected with gill nets in this study (Fig. 2). These results confirm that larger fish occur in the bay and should receive additional attention
if we are to understand the bay ecosystem, especially with regard to trophic
transfers via large nektonic fishes. Unfortunately, the Able et al. (2007) study
focused on species that need marshes and thus it did not provide a comprehensive
examination of the large nektonic fishes. Those species collected in
the large otter trawl that did overlap with the assemblage collected in gill
nets included Blueback Herring, Alewife, Atlantic Menhaden, Weakfish,
White Perch, Striped Bass, and Black Drum.
The composition of fishes along the Delaware Bay shores in this study
was similar to a prior examination that sampled with an otter trawl (Able
et al. 2001) in that each was dominated by the same families of fishes:
sciaenids, clupeids, and moronids (Table 2). This similarity in composition
is supported by earlier and subsequent sampling with the same gear in
the lower (Able et al. 2000, 2004) and upper (Grothues and Able 2003a, b;
Nemerson and Able 2004; Smith 1971) bay.
The more detailed analysis presented here is based on sampling with
gill nets in 2001, which indicated that selected abiotic variables influenced
the Delaware Bay fish assemblages during 2001. The responses of several
species were clearly related to spatial variation in salinity, an unsurprising
finding for this (Able et al. 2001, Bulger et al. 1993) and other (Günter 1956,
Kinne 1967, Martino and Able 2003, Wagner 1999, Weinstein et al. 1980)
estuarine fish assemblages.
The assemblage, especially White Perch and Gizzard Shad, also responded
to the dissolved oxygen gradient. Interestingly, the gradient in dissolved
oxygen, while not pronounced, was in the opposite direction as that
reported for shallower marsh systems across the same locations. In intertidal
and shallow (1.4–2.8 m at high tide) subtidal marsh creeks, the highest dissolved
oxygen was in the lowest bay station, and the lowest values were in
the upper bay (Able et al. 2001), while the opposite is true for our sampling
sites (1.8–2.3 m) (Table 1). The abundance of White Perch and Gizzard
Shad, which are typically most abundant in the upper bay (Able et al. 2001),
largely corresponds to the gradients observed in this study. Dissolved oxygen
is an important contributor among the suite of abiotic variables that can
influence fish distribution and abundance (Breitburg et al. 2001, Rabalais
et al. 2001) and its influence in Delaware Bay is obvious as well. However,
it might be more instructive to know how fish assemblages are structured
at night when dissolved oxygen is often lower in estuarine systems (Kemp
and Boynton 1980, Lingman and Ruardij 1981) and especially in marshes
(Cochran and Burnett 1996, Smith and Able 2003, Szedlmayer and Able
1993). Both salinity and dissolved oxygen have seasonal components in
many estuaries, and thus it is not surprising that there are seasonal components
to the distribution and abundance of the assemblage. However, if this
study had been conducted through the winter, even more marked differences
would have likely been apparent in the abundance and assemblage structure
2009 K.W. Able, K.M.M. Jones, and D.A. Fox 39
of fishes and the role of temperature. This winter effect is especially likely
given that this assemblage, and that of other estuaries in the region, have a
large component of transients that spend much of the year in warmer ocean
waters offshore or to the south (Able 2005, Able and Brown 2005, Able and
Fahay 1998, Hagan and Able 2003).
Comparisons with other estuaries
An examination of fishes collected with gill nets in Delaware Bay with
fish collected in other temperate east coast estuaries reveals latitudinal
changes in species composition. For example, the northernmost gill-net sampling
effort in the US that we are aware of, from the Sheepscot River-Back
River estuary in Maine (Recksiek and McCleave 1973), shared the large
number of clupeids with our study. However, it differed in that Alewife and
Blueback Herring made up a much larger portion of the catch, while Atlantic
Menhaden were much less abundant and Clupea harengus Linnaeus (Atlantic
Herring), which were nonexistent in our collections in Delaware Bay,
were one of the dominant forms. The higher salinity in this Maine estuary
(primarily between 20–30‰) relative to our Delaware Bay sites (4–19‰)
may explain the greater abundance of marine forms there such as Squalus
acanthias Linnaeus (Spiny Dogfish), Scomber scombrus Linnaeus (Atlantic
Mackerel), Peprilus triacanthus Peck (Butterfish), and Pollachius virens
Linnaeus (Pollock). Another abundant species in Maine gill net samples,
Osmerus mordax Mitchill (Rainbow Smelt), does not even occur as far south
as Delaware Bay (Able and Fahay 1998). Another, geographically closer,
study in the Navesink River and Sandy Hook Bay (Manderson et al. 2006) in
northern New Jersey found a similar fauna, with the exception that Prionotus
evolans Linnaeus (Striped Searobin) was well represented there but not in
Delaware Bay, at least in the marsh creek collections. Also, in the polyhaline
marsh creeks in the Great Bay estuary in southern New Jersey, Rountree
and Able (1997) found a similar fauna to that from Delaware Bay, with the
exception that Mustelus canis Mitchill (Smooth Dogfish) and Hickory Shad
made up a large proportion of the catch in the former estuary. Some of the
dominant fishes in gill nets in New Jersey are also dominant farther south in
the Maryland waters of Chesapeake Bay, including Striped Bass, Bluefish,
and Weakfish (Hartman and Brandt 1995). In the York River estuary in the
Virginia portion of Chesapeake Bay, gill net catches were dominated by
the same species (Weinstein 1985) as in our study.
While our emphasis here is on Delaware Bay, there is accumulating
evidence that large nektonic fishes are common but relatively unquantified components of estuaries along the east coast of the US (e.g., Gardinier
and Hoff 1982, Hartman and Brandt 1995, Manooch 1973, Merriner 1975,
Rountree and Able 1997), in Mexico (Yañez-Arancibia 1985), South Africa
(Day 1981, Whitfield 1998), Australia (Lenanton and Potter 1987, Potter et
al. 1990), Europe (Elliott and Dewailly 1995, Elliott and Hemingway 2002)
and in the Indo-West Pacific (Blaber 2000); thus, our finding here may have
broader implications.
40 Northeastern Naturalist Vol. 16, No. 1
In summary, large (mean = 261.4 mm, range = 53–600 mm) and older
(age 1+ for abundant species) nektonic fishes are distributed throughout
the marshes and margins of Delaware Bay in response to salinity, dissolved
oxygen, and temperature. These assemblages, while relatively infrequently
sampled in this and other estuaries, undoubtedly play an important role in
the trophic transfers from marshes to the bay and ocean, as has already been
demonstrated for selected species such as Striped Bass (Nemerson and Able
2003, Tupper and Able 2000) and other piscivores (Nemerson and Able 2004).
These large nektonic fishes deserve further attention, especially in light of
the presumption that estuaries serve as refugia from predation, an idea that is
now being questioned (Baker and Sheaves 2005, Paterson and Whitfield 2000,
Sheaves 2001). Many of the species that dominate the large nektonic fishes
in Delaware Bay are shared with other estuaries from the Gulf of Maine to
Chesapeake Bay, in part because they are highly migratory. Certainly, much of
the interest in large nektonic fishes in Delaware Bay and other estuaries occurs
because of their economic importance in commercial and recreational fishes.
This consideration is yet another reason that there should be continued focus
on this component of estuary faunas.
Acknowledgments
We would like to thank numerous field technicians, namely S. Brown, M. Greaney,
K. Henson, A. Macan, P. McGrath, K. Mulligan, R. Nichols, and L. Parillo, for
assistance with field sampling. T.M. Grothues provided assistance with data analysis
and also provided comments on an earlier draft. Funding for this project was from
Public Service Enterprise Group’s Estuarine Enhancement Program, and Rutgers
University - National Marine Fisheries Service Bluefish-Striped Bass Dynamics Research
Program. This paper is Contribution 2008-9 of the Rutgers University Institute
of Marine and Coastal Sciences.
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