2007 SOUTHEASTERN NATURALIST 6(4):705–714 Marine Fish Diversity and Composition in the Mid-Atlantic and South Atlantic Bights Joseph W. Love1,* and Peter D. Chase2 Abstract - We sampled fishes from nearshore, continental shelf (≈30 m) to shelfslope, deep-water habitats (≈100 m) in the Mid-Atlantic Bight (MAB) and South Atlantic Bight (SAB) during winter (2005) to explore compositional differences among temperatures and depths. Trawl surveys conducted by the National Marine Fisheries Service do not typically sample winter fish assemblages concurrently from both the MAB and SAB, although increased concern over changes in distribution of species such as Pterois volitans (Lionfish) may warrant such studies. We collected 41 families and 68 species of fish, and found that temperature and depth influenced their distribution. More species were collected in the SAB where temperature was 10 ºC higher. At nearshore sites of SAB, we collected reef fishes (Chaetodontidae; Fistulariidae) and Stenotomus chrysops (Scup). At deep water sites of SAB, we collected Ophichthidae, Acropomatidae, and Scorpaenidae. Assemblages of the MAB were dominated by Squalus acanthias (Spiny Dogfish), particularly at nearshore sites. Pomatomus saltatrix (Bluefish) and Scomber scombrus (Atlantic Mackerel) were also abundant in the MAB. Our results highlight distributions of some fish species during winter. However, more data are necessary for understanding macroecological patterns of marine fish distribution in the northwestern Atlantic Ocean, especially as they relate to the interactive effects of temperature and depth on populations. Introduction One of the most dramatic temporal changes in the North Atlantic has been the decline in the number of top predators over the past 100 years (Christensen et al. 2001), especially shark populations. This decline can result in changes in abundance and composition of lower trophic levels (Kvitek et al. 1992) or diminished diversity (Paine 1966). “Fishing down the food chain” may have affected population sizes and fishery interests for fish species occupying lower trophic levels (Pauly et al. 1998). To better protect and manage marine resources, fishery scientists are now exploring ecosystem management options (Zabel et al. 2003) and marine protected areas (Fogarty 2004). In addition to managing native populations (and communities), marine researchers may be required to consider managing exotic species that have recently invaded marine communities of the northwestern Atlantic Ocean (Meister et al. 2005). Changes in distribution or population size of higher trophic levels or invasive species introductions, as well as global climate change, may interactively affect broad-scale, macroecological 1University of Maryland Eastern Shore, Living Marine Resources Cooperative Science Center, Princess Anne, MD, 21853. 2National Marine Fisheries Service, Ecosystem Surveys Branch, Woods Hole, MA, 02543. *Corresponding author - jlove@ umes.edu. 706 Southeastern Naturalist Vol.6, No. 4 patterns of marine fish assemblages. Unfortunately, there are broad gaps in our understanding of marine fish distributions. Macroecological patterns of marine fishes in the North Atlantic need further elucidation (Floeter et al. 2004), but studies indicate that latitude and depth explain patterns of diversity for teleosts and elasmobranchs (Macpherson 2002, Macpherson and Duarte 1994) with the species-to-genus ratio usually increasing closer to the equator (Floeter et al. 2004). The pattern of diversity may be explained by greater habitat complexity, especially near reefs (Floeter et al. 2004) or rocky shores (Ferreira et al. 2001) and thermal refugia (Paull et al. 1984). The location of the gulf stream may also influence the biomass or diversity of fishes because of its higher sea-surface temperature that may provide thermal refugia or greater productivity for species. Our study differs from earlier work by examining how species composition differs between sites of the mid-Atlantic Bight (MAB) and the South Atlantic Bight (SAB) of the northwestern Atlantic Ocean, in shelf (≈30 m) and shelf-slope (≈100 m) habitats during winter. Our objectives were to provide information on species distributions for fishes collected during our winter survey (2005), and relate those distributions to depth and water temperature. Methods We conducted a fish survey of 7 stations representing depth and temperature variation from nearshore, continental shelf (≈30 m depth) to shelf-slope (≈100 m depth) habitats between Delaware Bay and South Carolina during winter (17 January–28 January 2005) aboard the NOAA vessel Albatross IV (Table 1; Fig. 1). At most stations, we conducted multiple tows (see Table 1), which were at least 1 km away from one other. In all, we conducted 14 tows (hereafter, sites), 8 in the SAB and 6 in the MAB. Sites were chosen based on a haphazard, random sampling design such that we sampled as many sites as possible (within 2 weeks), while compromising with weather and meeting other researchers’ needs aboard the vessel. Each site was sampled using a Yankee Otter trawl for 30 minutes at 4 knots. For each site, fishes were sorted, identified to species (Carpenter 2003, Hoese and Moore 1998, Murdy et al. 1997, Robins and Ray 1999), counted, measured for total length (TL), and weighed. Most were released alive, but voucher specimens for most species were preserved using 10% formaldehyde. In addition to sampling fishes, a conductivity-temperature-depth (CTD) instrument (Seabird SBD 19 CTD profiler) was used at each site to obtain a vertical profile of salinity and temperature, which was measured every second as the CTD was lowered. Surface temperature, average survey depth, minimum survey depth, and maximum survey depth were also recorded from the data logger of the Albatross IV (Table 1). All measures of depth were highly and positively correlated with one another (r > 0.90), and we therefore used average survey depth because of data availability. Depth was not well correlated with salinity or temperature (-0.15 < r < 0.08), and SAB habitats 2007 J.W. Love and P.D. Chase 707 Table 1. Environmental data collected in January (2005) for 14 tows of shelf and shelf-slope habitats in the Northwestern Atlantic Ocean for the Mid-Atlantic Bight (MAB) and South Atlantic Bight (SAB). Abbreviations are: LAT (latitude), SAL (average salinity, ppt), S_TEMP (surface temperature, °C), B_TEMP (bottom temperature, °C), V_TEMP (average vertical temperature, °C) and its standard deviation (SD), M_DEP (minimum depth, m), X_DEP (maximum depth. m), and DEP (average depth, m). NA = Not Available. TOW LAT SAL (SD) S_TEMP B_TEMP V_TEMP M_DEP X_DEP DEP SAB 10-1 3319.84 35.89 (0.02) 20.94 21.02 21.01 (0.06) 26.6 31.1 29.4 11-1 3314.40 35.86 (0.05) 22.20 21.90 22.11 (0.14) NA NA 39.0 11-2 3314.81 35.86 (0.03) 23.01 22.08 22.70 (0.50) 39.0 42.3 39.0 13-1 3255.99 35.82 (0.01) 23.23 21.37 22.63 (0.57) 122.5 125.4 123.9 13-2 3257.31 35.68 (0.77) 23.23 20.99 22.60 (0.72) 110.3 123.6 117.6 13-3 3256.43 35.82 (0.01) 23.23 21.70 22.64 (0.54) 114.8 121.4 118.1 16-1 3256.58 35.79 (0.08) 18.66 17.29 17.85 (0.37) 29.5 31.6 30.5 16-2 3254.67 35.84 (0.06) 19.80 19.08 19.62 (0.39) 33.4 36.8 35.0 MAB 27-1 3636.74 33.35 (0.40) 17.12 11.56 10.31 (0.61) 95.2 97.4 96.3 27-2 3641.01 33.59 (0.10) 16.00 11.05 10.70 (0.24) 96.8 100.9 98.4 32-1 3638.26 32.86 (0.04) 8.90 8.72 8.64 (0.08) 30.1 37.0 33.3 32-2 3641.46 NA 8.97 NA NA 33.9 44.5 39.6 34-1 3815.99 32.59 (0.01) 8.53 8.30 8.29 (0.01) 66.7 68.6 67.7 34-2 3816.65 NA 8.48 NA NA 63.8 68.6 66.2 Figure 1. Map of 14 sites within the Northwestern Atlantic Ocean during January (2005). For reference, C h e s a p e a k e Bay (CB) and Cape Hatteras, NC (CH) are labeled. were warmer and saltier than MAB habitats. Surface, bottom, and average vertical temperatures were highly correlated with one another (r > 0.93, for all), and because of data availability, we used surface temperature as a predictor in canonical correspondence analysis (CCA; ter Braak 1986). 708 Southeastern Naturalist Vol.6, No. 4 We used CCA to explore species’ distributions along environmental gradients of average depth and surface-water temperature, which differed by about 10 °C between SAB and MAB habitats (Table 1). Axes were scaled to optimize the representation of species scores in ordination space, and site scores were re-scaled with mean = 0 and variance = 1. Thus, the distances among species scores best approximates the relationships among each other and to the environmental predictors plotted in the ordination diagram (McCune and Grace 2002). The eigenvalues for each axis were tested for significance using a Monte Carlo permutation procedure that generated a null distribution of eigenvalues by randomizing the relationship between the species and environmental datasets for 1000 times. For CCA, we used PC-ORD (McCune and Mefford 1999). Results We collected and identified 41 families represented by 68 species of fish, 7 of which were cartilaginous (Table 2). Fourteen abundant species (>1% of catch) represented 89% of the total biomass (Table 3), which was 2664.53 kg. Squalus acanthias Linnaeus (Spiny Dogfish) was the most abundant species (28.6% of sample). Many Spiny Dogfish were bearing and birthing pups. Individuals ranged in size from 25 to 98 cm TL and 40 to 4200 g. Most were adults with a mean size of 76 cm TL (± 7.4 SD) and mean mass of Table 2. Fish families collected aboard the R/V Albatross IV by percent of catch (of 2742 fish) during January 2005 from shelf and shelf-slope habitats of the Mid-Atlantic and South Atlantic Bights. Numbers in parentheses are the number of identified species for each family. Family % catch Family % catch Acanthuridae (1) 0.11% Ophidiidae (2) 0.15% Acropomatidae (1) 0.40% Ostraciidae (2) 1.35% Balistidae (1) 0.84% Paralichthyidae (2) 1.20% Batrachoididae (1) 0.04% Phycidae (2) 0.22% Blenniidae (1) 0.04% Scorpaenidae (1) 0.07% Bothidae (2) 0.22% Pomacanthidae (1) 0.69% Carangidae (5) 0.40% Pomacentridae (1) 0.04% Carcharhinidae (1) 0.15% Pomatomidae (1) 0.55% Chaetodontidae (2) 1.06% Priacanthidae (2) 0.15% Dasyatidae (1) 0.55% Rajidae (2) 0.80% Fistulariidae (2) 0.22% Sciaenidae (1) 0.22% Gobiesocidae (1) 0.04% Scombridae (1) 2.04% Gymnuridae (1) 0.04% Serranidae (4) 1.53% Haemulidae (3) 13.82% Sparidae (5) 28.08% Holocentridae (1) 0.15% Squalidae (1) 28.56% Labridae (1) 0.51% Stromateidae (1) 2.44% Lutjanidae (1) 7.70% Synodontidae (3) 2.48% Monacanthidae (3) 1.57% Tetraodontidae (1) 0.22% Myliobatidae (1) 0.07% Triglidae (2) 1.17% Ogcocephalidae (1) 0.04% Uranoscopidae (1) 0.04% Ophichthidae (1) 0.04% 2007 J.W. Love and P.D. Chase 709 1700 g (± 653.1 SD). Only 2 of these individuals were under 30 cm TL and may have been birthed in the trawl. Cumulatively, 35% of the potential variance in the community data set was explained by two axes, with the first axis explaining more (Eigenvalue = 0.86, p = 0.001, % variance = 22.0) than the second (Eigenvalue = 0.49, p = 0.001, % variance = 12.6). Because these two axes represented 1/3 of the variance in the original data set, and to ease interpretations of the CCA, we utilized only the first two axes for evaluating how species were associated with environmental variables. Temperature was highly correlated with axis 1 (r = 0.92) and depth was highly correlated with axis 2 (r = -0.86). Correlations between species scores and those scores constrained by environmental variables were high (Axis 1: r = 0.98; p < 0.01; Axis 2: r = 0.95; p < 0.01). We interpreted the CCA plot, which ordinated species’ scores based on predicted, optimal abundances across habitat gradients of depth and temperature. From this plot, we grouped four, distinct winter fish assemblages: 1) shallow, warm-water (SAB) assemblages; 2) deep, warm-water (SAB) assemblages; 3) shallow, cold-water (MAB) assemblages; and 4) deep, cold-water (MAB) assemblages. Assemblages differed markedly in composition and diversity between colder water of the MAB and warmer water of the SAB. Species exclusive to our samples from colder water of the MAB included: Spiny Dogfish, Raja Table 3. Percentage of catch, average total length (TL) (± standard deviation), and average mass per individual (± standard deviation) for fishes collected on R/V Albatross IV during January 2005 from shelf and shelf-slope habitats of the Mid-Atlantic and South Atlantic Bights. Species listed are >1% of total catch and are given in descending order of % total catch. Species % catch N TL (cm) Mass (g) Spiny Dogfish 28.6% 291A 76.1 ± 7.4 1704.2 ± 653.1 Scup 18.3% 497 18.1 ± 2.3 103.2 ± 30.8 Tomtate, Haemulon aurolineatum Cuvier 13.4% 382 11.1 ± 4.4 28.4 ± 41.6 and Valenciennes Vermillion Snapper, Rhomboplites 7.7% 211 23.5 ± 6.7 184.7 ± 111.6 aurorubens (Cuvier and Valenciennes) Knobbed Porgy, Calamus nodosus 5.3% 144 30.9 ± 5.4 631.3 ± 315.7 Randall and Caldwell Whitebone Porgy 2.8% 76 28.8 ± 3.1 534.1 ± 168.2 Butterfish 2.4% 67 15.0 ± 3.0 51.4 ± 35.9 Atlantic Mackerel 2.0% 56 29.4 ± 2.5 170.2 ± 44.3 Inshore Lizardfish, Synodus foetens (L.) 1.7% 46 17.2 ± 10.7 66.9 ± 5.9 Red Porgy, Pagrus pagrus (L.) 1.6% 44 30.8 ± 6.0 435.0 ± 237.1 Orange Filefish, Aluters schoepfi(Walbaum) 1.5% 40 39.4 ± 11.8 711.5 ± 357.1 Scrawled Cowfish, Acanthostracion 1.2% 34 29.1 ± 2.1 411.9 ± 84.6 quadricornis (L.) Summer Flounder 1.1% 33 43.3 ± 9.1 918.9 ± 543.7 Northern Searobin 1.1% 31 23.9 ± 1.8 127.1 ± 31.6 AValue reflects a subsample (of 789) that was measured and weighed. 710 Southeastern Naturalist Vol.6, No. 4 eglanteria Bosc (Clearnose Skate), Leucoraja erinacea (Mitchill) (Little Skate), Prionotus carolinus (Linnaeus) (Northern Searobin), Pomatomus saltatrix (Linnaeus) (Bluefish), Scomber scombrus Linnaeus (Atlantic Mackerel), Peprilus triacanthus (Peck) (Butterfish), Hippoglossina oblonga (Mitchill) (American Fourspot Flounder), and Paralichthys dentatus (Linnaeus) (Summer Flounder). In contrast, assemblages from warmer water were more diverse (see list in Fig. 2) and consisted of many reef-associated fish families (e.g., Chaetodontidae, Pomacanthidae, Balistidae) and porgies (Sparidae), including Stenotomus chrysops (Linnaeus) (Scup), our second most abundant species (Table 3). Assemblages differed by depth, with a fairly distinct shallow, coldwater assemblage (Fig. 2) that included benthopelagic Spiny Dogfish, other pelagic species such as Peprilus triacanthus (Peck) (Butterfish), Atlantic Mackerel, and 4 demersal species. In shallow, warm water, we collected pelagic and reef-associated species that included: Diplectrum formosum (Linnaeus) (Sand Perch), Hypsoblennius hentz (Lesueur) (Feather Blenny), and Acanthostracion polygonius Poey (Honeycomb Cowfish). In deeper water, we mainly collected reef-associated and demersal species that included: Clearnose Skate, Ophichthus puncticeps (Kaup) (Palespotted Eel), Scorpaena brasiliensis Cuvier (Barbfish), Centropristis striata (Linnaeus) (Black Seabass), Synagrops bellus (Goode and Bean) (Blackmouth Bass), and Summer Flounder. Figure 2. Species associations when constrained by water temperature and depth gradients for fish assemblages sampled in the northwestern Atlantic Ocean (January 2005), as inferred from canonical correspondence analysis. Different symbols represent different assemblages: shallow, warm water (empty circle); deep, warm water (empty diamond); shallow, cold water (filled circle); deep, cold water (filled diamond). Species associated with each group are labeled near groups. 2007 J.W. Love and P.D. Chase 711 Discussion We found more species and a different composition of fishes in the SAB than the MAB during our winter survey. The latitudinal gradient of species diversity is an extremely well-known ecological pattern (Rosenzweig 1995), and our data reflect a pattern expected for fish and invertebrate communities of the Atlantic Ocean (Floeter et al. 2004, MacPherson 2002, MacPherson and Duarte 1994). Underlying processes explaining the patterns are less understood. MacPherson (2002) suggested that patterns of sea-surface temperature and nitrate load accounted for patterns of diversity for benthic and pelagic communities, respectively. In the SAB, the circulation of the gulf stream serves to warm continental-shelf and shelf-slope waters more than continental-shelf waters of the MAB. Higher endogenous sources of energy (i.e., more stable and high temperatures) may be associated with higher rates of species diversification, thereby explaining higher diversity (Hunt et al. 2005, Kaspari et al. 2004, Wright 1983). The greater abundance of reefs, or habitat complexity, may also explain greater species diversity (Floeter et al. 2004). In addition, diversity may be explained by dispersal, disturbance, and time since habitat isolation (Loreau and Mouquet 1999, Love and Taylor 2004, McCabe and Gotelli 2000), but such patterns depend on taxonomic, temporal, and spatial scales (Lyons and Willig 2002, Willig et al. 2003). Our survey was limited to marine fishes at 14 sites during one season of 2005, and much remains to be learned about patterns of marine fish diversity as they relate to depth, latitude, and their interaction. Seasonal changes in marine fish assemblages are well-documented (Desmond et al. 2002, Murphy and Secor 2006, Witting et al. 1999), and several migratory species were collected north of Cape Hatteras (e.g., Spiny Dogfish, Summer Flounder). The most abundant species encountered during this study—Spiny Dogfish—occurred north of Cape Hatteras . This species seasonally migrates south from Nova Scotia to Cape Hatteras as water temperatures cool to 7.2–12.8 °C (ASMFC 2002), which overlaps the observed water temperature during our collection. During winter, Spiny Dogfish typically overwinters in deeper water (McMillan and Morse 1999), but we also found notable biomass in shallow water (≈30 m). One interesting finding was that females prematurely birthed in sorting bins while being measured and weighed, which may be related to handling stress and have implications on estimating by-catch mortality for this species. Other marine fishes that inhabit waters less than 30 m in the MAB during summer (e.g., Summer Flounder) also retreat during winter to deeper water, which may be a thermal refugia due to the influence of the gulf stream. Bottom temperatures at deeper areas (edge of the continental shelf) of the MAB were 3 °C warmer than at shallower, shelf sites. Several benthic fishes, such as Lophius americanus Valenciennes (Goosefish), were not collected, though they occurred in the area (NOAA 2005), which likely was a result of gear bias. Our survey of 2 northern latitude zones in January 2006 yielded similar species to those collected in 2005, with the exceptional catch of a goosefish. 712 Southeastern Naturalist Vol.6, No. 4 Another interesting find in our study was the collection of Scup in the SAB. Generally, Scup is regarded as rare in the SAB (Carpenter 2003), but marine regions of MAB and SAB may be more connected by dispersal than previously thought (Jones and Quattro 1999), leading to an exchange of fauna. While Scup is replaced by S. caprinus Jordan and Gilbert (Longspine Porgy) south of Cape Hatteras, we did not collect Longspine Porgy, which is abundant south of the habitats we were surveying (Carpenter 2003). Recent investigations into morphological differences between SAB and MAB populations of Scup have shown that northern populations are typically larger, have higher scale counts, and slightly different body shapes (J.W. Love, unpubl. data), but more comprehensive studies are underway. While the taxonomy of southern and northern populations of Scup is unclear (Johnson 1978, Robins et al. 1991), this uncertainty should not have affected interpretations of our results. Learning how, when, and why species are distributed in the manner that they are is becoming increasingly valuable as the development of marine protected areas (MPAs) becomes a viable and effective option for protecting marine resources. Here, we publish a descriptive and exploratory account of composition and diversity of winter fish assemblages in the South and Mid-Atlantic Bights, but more work should be done to survey both regions thoroughly. The widely accepted process of global climate change (Oreskes 2004) deserves greater attention and has yet unappreciated consequences on continental-shelf or shelf-slope ecosystems. As temperatures within marine environments progressively change, primary productivity may change and affect species distributions, thereby altering the face of food-web dynamics and ecosystem function. While the influence of climatic phenomenon on trophic interactions is not well-known, it may have implications on ecosystem-based management for MPAs or other coastal habitats (Preston 2004), especially if species’ distributions (and thus, species’ interactions) are strongly affected by regional or global climatic phenomena (e.g., Gulf Stream or North Atlantic Oscillation). Acknowledgements We would like to thank participating students and faculty of the NOAA Living Marine Resources Cooperative Science Center for their work during collections of species. Specifically, we thank Drs. Eric May and Andrea Johnson for their logistical support and assistance with research activities. We thank NOAA for the opportunity to work aboard the NOAA R/V Albatross IV. 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