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Large Nektonic Fishes in Marsh Creek Habitats in the Delaware Bay Estuary
Kenneth W. Able, K. Martha M. Jones, and Dewayne A. Fox

Northeastern Naturalist, Volume 16, Issue 1 (2009): 27–44

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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. 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