TROPHIC NICHE OVERLAP OF ATLANTIC COD AND BLACK SEA BASS IN RELATION TO WARMING SOUTHERN NEW ENGLAND WATERS

In southern New England (SNE), a new ecological interaction is occurring as temperate black sea bass abundance increases in the home range of the iconic Atlantic cod as a result of climate-induced ocean warming. In this study, we examined the diet and trophic dynamics of both SNE Atlantic cod and black sea bass using a multi-tracer approach of stomach content analysis as well as bulk and compound-specific amino acid isotope analyses. We reveal a highly selective diet (0.01-0.43 Levin’s index) with decapod crustaceans occurring in 82-89% of stomachs and making up 72-79% of the total wet weight for both species, resulting in strong overlap in diet (0.34-0.99 Morisita-Horn index). The most prominent decapod crustacean seen in stomachs were juveniles of the Cancer genus, which has cascading implications for Jonah crab (Cancer borealis) population dynamics and the burgeoning SNE fishery they support. We also reveal high overlap (3085%) in isotopic niche between species. We show a seasonal shift in δ15N Phe, which reflects changes in regional productivity but also indicates that both species are foraging locally in relatively small ranges relative to cross shelf isotope gradients and in the same locations seasonally. Our trophic position estimations corroborate our diet information and further reveal that cod (3.5-4.1) and black sea bass (3.4-3.5) generally occupy the same trophic position in the food web with high diet selectivity for crustaceans. This work reveals the potential for future shifts in SNE food web dynamics and the fisheries they support, while serving as a case study for future studies in other regions of the world seeking to examine the impacts of climate-driven shifts in species distribution on food web structure and ecological integrity.


PUBLICATION STATUS
This manuscript is being prepared for submission to Marine Ecology Progress Series.

INTRODUCTION
Interspecific competition can have a variety of consequences for competing species including altering their access to prey and habitat, changes in body condition, and shifts in population abundance and distribution (Overholtz and Tyler 1986;Drinkwater 2005). For example, in the northwest Atlantic the decline of top predator gadid populations due, in large part, to overharvesting allowed for a competitive release of elasmobranchs, which increased in abundance with the decrease in competition for shared resources (Fogarty and Murawski 1998 between these species could alter the relationships they have within the ecosystem (Fogarty and Murawski 1998;Link et al. 2008;Steneck 2004). Furthermore, both the black sea bass and the Atlantic cod have valuable commercial and recreational fisheries that could be affected by new competitive interactions, which would need to be taken into consideration when developing effective management strategies (Shepherd 1991;NMFS 2018 (Myers et al 1996;Fogarty et al. 2008). Both species prefer similar habitats with hard-bottomed substrates and are often found on artificial reef structures (Reubens 2013;Steimle 1996). Additionally, the black sea bass is thought to inhabit shallow, coastal waters in summer, and move offshore to deeper water in the Mid-Atlantic Bight region while SNE cod seasonal movements are not well understood (Musick and Mercer 1977;BOEM 2020). With black sea bass becoming more abundant in SNE, there is growing concern about the potential ecological and economic impacts of competition between Atlantic cod and black sea bass given their increasing overlap, both spatially and temporally, in SNE marine ecosystems (NEFSC, 2020).
According to Link and Auster (2002), in order to infer that interspecific competition is occurring, there are distinct requirements that must be assessed, including: overlap both spatially and temporally between two potential competitors, overlapping resource utilization patterns for a shared resource, and resulting opposing population trajectories (Link and Auster, 2002). Though the cod and black sea bass interaction is a new developing one, we know that these species have the potential to shared similar realized ecological niches. They now overlap both spatially and temporally, utilizing the same demersal habitat, and are experiencing opposing populations trajectories in SNE.
However, it is unclear how much overlap there is in their resource utilization patterns and specifically their diet and trophic ecology. In SNE, Atlantic cod are upper trophic level predators that feed on cancer crabs, herring, and to a lesser extent, other fish species and the proportion of each of these diet items change seasonally (Link and Garrison, 2002 (Alley 1982;Chase and Leibold 2003). Specifically, the competitive exclusion principle suggests that overlap between the two species may exacerbate the population decline in cod as they compete for shared resources or will cause a diversification of the species' realized ecological niches (Hutchinson 1957;Hardin 1960). Assessing dietary and trophic niche overlap between Atlantic cod and black sea bass in SNE is critical to understanding the current overlap in resource utilization and thus the potential for competitive interaction between these two top predators under growing climate-change driven shifts in ecological interactions.
There are a number of useful analytical tools to assess dietary and trophic niche, from stomach-content analysis and bulk stable isotope analysis to recent developments in compound specific stable isotope analysis of amino acids (CSIA-AA), each with their own strengths and limitations. Stomach-content analysis has been used for centuries to understand species-level diet information (Hyslop 1980). While valuable for the level of taxonomic detail it provides and the ease of comparison between the species in this study, stomach-content analysis only offers a short snapshot of diet in time and is biased towards items that are slower to digest (Jackson and Ryan, 1986;Jackson, Duffy, and Jenkins 1987). It can also be challenging to put upper trophic level, generalist predator diet into the greater food web context without doing the same for all the prey items throughout the entire food web (Post, 2002).
Stable isotope analysis (SIA) complements stomach-content analysis by providing a temporally integrated signal of consumer trophic position and supporting baseline biogeochemical cycling within a food web (Michener and Kaufman 2007). Bulk tissue 13 C values change minimally as organic matter moves through a food web via trophic transfers and can be used to glean information about primary production at the base of the food web (Peterson and Fry, 1987). Meanwhile, 15 N values can provide information regarding changes in the sources and cycling of nitrogen in an ecosystem via baseline biogeochemical cycling and an organisms' trophic position (Peterson and Fry 1987;Cifuentes et al. 1988;Goering et al. 1990). Together, these metrics provide food web structure context to individual dietary data from stomach-content analysis. However, it can be difficult to tease apart the relative impact of changes in trophic dynamics versus changes at the base of the food web on consumer bulk tissue isotope values (Post, 2002), which paved the way for recent advances in the isotope analysis of individual compounds to study foraging ecology (Ohkouchi et al. 2017;Close 2019).
Compound-specific isotope analysis of amino acids (CSIA-AA) provides a greater level of detail about the metabolic history of how organisms acquire, modify, and allocate resources than traditional bulk tissue SIA. (Ohkouchi et al. 2017 acids provide a record of the nitrogen isotope baseline of the food web supporting that system, which can be used to examine baseline biogeochemical cycling (Sherwood et al. 2011;Vokhshoori and McCarthy 2014) and assess consumer foraging location across isoscapes (Dale et al. 2011;Popp et al. 2007). Another subset of amino acids, termed "trophic" amino acids (e.g., glutamic acid [Glu]), undergo significant isotope fractionation during trophic transfers. Together, trophic and source amino acid 15 N values can provide an estimate of both the baseline nitrogen isotope values of the food web and the target organism's trophic position that is internally indexed to the baseline (Chikaraishi et al. 2009;Nielsen et al. 2015;Ishikawa 2018). Characterizing consumer foraging ecology and trophic dynamics to better constrain isotopic niche is valuable for better characterizing consumer dietary and trophic niche within the context of potential interspecific competition.
Using a multi-metric approach of stomach-content analysis, bulk SIA and CSIA-AA, the goal of this study is to understand the seasonal diet and trophic ecology of Atlantic cod and black sea bass in SNE in order to quantify climate-induced resource use overlap. Given that black sea bass have only recently arrived in SNE and their populations are still growing, we hypothesize that Atlantic cod and black sea bass in SNE will strongly overlap in realized ecological niche: 1) consuming diets that strongly overlap, consisting predominately of crabs, shrimps, amphipods, and other epibenthic fauna consistent with diet profiles from their historic ranges, 2) sharing a similar trophic position and feed in food webs with the same baseline production, indicating potential ecological niche overlap, as determined by bulk SIA and CSIA-AA, and 3) showing seasonal shifts in baseline isotope values reflecting similar seasonal shifts in foraging ecology. However, if their ecological interactions are more established than we predict, we may see evidence of niche diversification in the form of divergence in diet and trophic dynamics away from dietary patterns in their historic ranges as a coexistence strategy under increasing competition (Hardin 1960). Understanding dynamics of this new ecological interaction will lead to a more holistic understanding of future ecological trajectories of both species and help better predict the top-down impacts that future competitive interactions between these species may have on ecosystem structure and function.

Sample site and collections:
Atlantic cod and black sea bass samples were collected from Narragansett Bay Nearshore fish samples were collected off Block Island (BI), RI, including Cox Ledge (41˚2' N 71°5' W and 41˚4' N 71˚2' W), 14km offshore of Narragansett Bay (max water depth 45m) via rod and reel through partnerships with regional recreational fishing charters. Samples were collected during both winter and summer in order to characterize seasonal changes in diet that are thought to occur for SNE cod (Link 2002). Samples were collected from both NB and BI in order to characterize diet and trophic dynamics during potential inshore and offshore movements of black sea bass seasonally (Musick and Mercer 1977 were later thawed for tissue collection and stomach-content analysis.

Stomach-content analysis:
Stomach content analysis followed established RI-DEM protocols (Christine Denisovich, pers. comm.). Briefly, total length and sex of each specimen was recorded, intact stomachs were removed and weighed, and prey items in the stomachs were identified to the lowest possible taxonomic group and enumerated. Removed prey items were grouped by species and recorded as wet weight, along with empty stomachs. The carapace width of intact individuals of the dominant prey items (cancer crabs) was also recorded. Mean carapace width of cancer crab prey in the stomachs of Atlantic cod and black sea bass were compared with a Student's t-test.
The relative contribution of each prey item to the total diet as wet weight and the frequency of occurrence of that diet item were calculated in order to assess dietary overlap in prey composition between Atlantic cod and black sea bass. The percent weight vs. frequency of occurrence were plotted to illustrate the dominant prey items, with dietary items closer to 100% occurrence and 100% abundance considered dominant prey items (Costello 1990). Levins' standardized measure of dietary breadth (BA) was calculated as a means of understanding SNE Atlantic cod and black sea bass dietary niche diversity (Levins 1968): where BA is the Levin's standardized index of dietary breadth, Pi is the proportion of prey item "i" in the diet, and n is the total number of prey items (Levins, 1968). This standardized metric provides an index of dietary or niche breadth, ranging from 0 as a very narrow dietary niche and 1 as a broad dietary niche (Levins, 1968).
The Morisita-Horn index (Smith and Zaret, 1982), a measure of dietary overlap between Atlantic cod and black sea bass, was calculated in order to compare changes in dietary overlap. This metric provides a measure of dietary overlap ranging from 0 indicating minimal to no overlap and 1 indicating high overlap: In order to understand the isotopic niche width and overlap of SNE Atlantic cod and black sea bass, the standard ellipse area (SEA) of the bivariate plot of 13 C and 15 N values was calculated for each species using the R package SIBER (Jackson et al. 2011).
Bivariate isotope data have been described as indicating an organism's isotopic niche, where 13 C and 15 N values indicate both "bionomic -resource utilization" and "scenopoetic -bioclimatic" aspects of the niche (Hutchinson 1978;Newsome et al. 2007). Calculating the area of this "niche" provides a proxy for niche width and breadth and we can calculate the overlap of the niche space between our target species in order to understand overlap in resource utilization patterns (Jackson et al. 2011).

Compound-specific isotope analysis:
Approximately 5mg of freeze-dried, homogenized white muscle tissue for each sample was acid hydrolyzed in 0.5mL of 6N hydrochloric acid at 110°C for 20 hours to produce free amino acids (Gehrke et al., 1985). 5ul of Nor-leucine (with a known δ 15 N value) was added to each sample as an internal calibration standard prior to derivatization. The hydrolysates were evaporated to dryness under a gentle stream of nitrogen gas. The acid hydrolyzed samples, as well as a mixed amino acid standard of known δ 15 N value and a working cyanobacteria lab standard of assumed value from hundreds of repeat measurements in multiple independent laboratories, were derivatized to N-trifluoroacetic acid isopropyl esters according to Silfer et al. (1991)  Samples were analyzed in triplicate and the mixed amino acid standard and working lab standard were run every two samples to correct for potential kinetic isotope fractionation during the derivatization process (Yarnes and Herszage 2017). The variability reported for δ 15 N value of each AA measured (Table S3) therefore represents the analytical variation for n = 3 replicate GC-C-IRMS measurements. The long-term δ 15 N reproducibility of the working cyanobacterial laboratory standard, ±0.3‰, provides an estimate of full protocol reproducibility (replicate hydrolysis, wet chemistry, and analysis) (calculated as the long-term SD across >100 separate full analyses, averaged across all individual AAs). Measured amino acids were classified as trophic amino acids -Glu, aspartic acid (Asp), alanine (Ala), leucine (Leu), isoleucine (Ile), proline (Pro), and valine (Val), source amino acids -methionine (Met), lysine (Lys), and Phe, and metabolic amino acids -glycine (Gly), serine (Ser), and threonine (Thr) according to McMahon and McCarthy (2016). The δ 15 N Glu and δ 15 N Phe values (our representative trophic and source amino acids) compared between species for NB winter and BI summer samples using a two-way ANOVA and Tukey HSD post-hoc test.
Trophic positions of Atlantic cod and black sea bass were calculated using a TPCSIA approach that is internally indexed to the δ 15 N baseline: where δ 15 NGlu and δ 15 NPhe represent the nitrogen isotope ratios in fish tissue samples for Glu (a trophic amino acid) and Phe (a source amino acid), represents the difference in the Glu and Phe nitrogen isotope values in marine microalgae (2.9 +/-1. 9‰ Nielsen et al. 2015), and TDFGlu-Phe is the difference in trophic discrimination factors (6.3+/-0.35‰) for Glu and Phe (Nielsen et al. 2015). The error was propagated through the trophic position calculation using R package "propagate". TPCSIA were compared between species and seasons and with fish total length as a covariant using an ANCOVA and Tukey's HSD post-hoc test.

Stomach content analysis:
Decapod crustaceans were the dominant prey item, by frequency of occurrence and percent wet weight, in both Atlantic cod and black sea bass stomachs ( Fig. 2; Table   S1, S2). For Atlantic cod, decapod crustaceans occurred in ≥89% of stomachs across locations and seasons. Decapod crustaceans were similarly dominant by frequency of occurrence (≥82%) in black sea bass across seasons and locations (Table S1). Decapod crustaceans also made up the bulk of the total stomach wet weight for Atlantic cod and black sea bass in the study, making up ≥72% of Atlantic cod and ≥79% of black sea bass stomach wet weight across seasons and locations ( Fig. 2; Table S2).
Jonah crabs (Cancer borealis) and rock crabs (Cancer irroratus) (henceforth referred to as "cancer crabs") were the dominant type of decapod crustacean found in Atlantic cod and black sea bass stomachs occurring in ≥65% of cod and black sea bass stomachs across most locations and seasons, though only occurring in 18% of summer black sea bass samples from NB (Table S1). Cancer crabs also made up the bulk of the total stomach wet weight for Atlantic cod and black sea bass in the study, making up ≥72% of Atlantic cod stomach wet weight across most seasons and locations, though they only made up 13% of the wet weight for summer cod at NB ( Fig. 2; Table S2).
Meanwhile for black sea bass, cancer crabs made up ≥79% of the wet weight across most locations and seasons, but only 20% of the wet weight at NB in summer ( Fig. 2; Table   S2). Using the Costello (1990)

Bulk isotope analysis:
There were some significant differences in both δ 78 C and δ 79 N values between species and seasons ( Fig. 4; Table S3, S4). SEA-C niche breadth was larger for both Atlantic cod (1.42‰) and black sea bass (0.98‰) in winter at NB than it was for both Atlantic cod (0.67‰) and black sea bass (0.31‰) in summer at BI (Fig. 4). The Bayesian metric of Standard Ellipse Area overlap for these species indicates that roughly 30-80% of the niche area is overlapping at NB in winter compared to 50-85% of niche area overlap at BI in summer (Fig. 5, S1).

CSIA-AA:
Glu  Table S5). TPCSIA-AA of Atlantic cod (4.1 ± 0.09 SD) were significantly higher than black sea bass (3.5 ± 0.3 SD) in winter (ANCOVA p<0.05; Table 2) but not significantly different in summer (cod: 3.5 ± 0.3 SD, black sea bass 3.4 ± 0.9 SD) (Fig.   6). There was a significant effect of fish total length on trophic position for Atlantic cod (R 2 =0.5; ANCOVA p<0.05; Table 2) but not black sea bass (Fig. 7). South-Atlantic Bights (Sedbery 1988;Steimle 1996) as well as in our study region (Malek 2015). However, previous studies of Atlantic cod diet in SNE two decades ago indicated a more varied and higher trophic level diet, including herring and other fish (Link 2002) compared to the strong selection for cancer crabs that we found now. Studies of Atlantic cod diet in the Gulf of Maine did indicate that a diet of fish was less common for inshore cod in Maine bays and coves (Willis 2017). This could mean that cancer crabs are a more important resource for inshore cod, like the subpopulation in our study.

Our study found that Atlantic cod and black sea bass in SNE
Both Atlantic cod and black sea bass show more diverse diets in winter, with fish and other invertebrates making up 20-30% by wet weight of cod diet and 20% of black sea bass diet. This dietary expansion could reflect a shift to alternative diet sources as their primary prey, cancer crabs, exhibit seasonal movements offshore into deeper waters in winter (Truesdale 2019). Alternatively, this diversification of the diet in winter could also be due to a competitive release as many other piscivorous fishes move out of Narragansett Bay in winter, allowing both predators to increase foraging on fish at NB in the winter (Malek 2015). This seemingly broader dietary niche in winter could also be a limitation of the stomach content analysis technique, where selective digestion and rate of metabolism could play a role in which prey items we see seasonally (Hess and Rainwater 1939;Baldwin 1957). However, known shifts in prey movements, coupled with the isotopic niche measurements we observed for NB winter samples being broader than BI summer samples, support the hypothesis that the dietary niche of both species is in fact broader in winter. More research on the movement patterns of key prey items and competitors could help shed light on the relative importance of these potential drivers of winter dietary diversification.
Despite some seasonal variation in overall diet diversity, both Atlantic cod and black sea bass in SNE have highly selective diets focused on cancer crabs in the region.
Though both species are considered generalists in terms of their fundamental ecological niches, this high dietary selection indicates that in SNE, both species occupy similar narrow realized niche space (Hutchinson 1957;Vandermeer 1972). When coupled with the fact that we found relatively few empty stomachs (26% for Atlantic cod and 30% for black sea bass), it suggests that competition between these two species is likely low at the moment due to food not being a limited resource. However, this high diet selectivity for cancer crabs is important to note because it could have implications for how these species respond to future environmental changes that impact their prey sources. Ecological theory dictates that specialists are typically more sensitive to ecological and environmental change than organisms that have more generalist resource utilization patterns (Clavel et al. 2011;Moritz 2013;Menéndez et al. 2006;Lurgi et al. 2012). As such, it is likely that crab population dynamics in the region could play an important role in dictating resource utilization patterns between these species and potentially their population trajectories in SNE. Given their sensitivity to warming waters and increased hypoxia, both predicted to increase over the next century (Cheng et al. 2019), cancer crabs are likely to have high mortality rates as waters in SNE continue to warm (Frederich et al 2009;Deutsch 2015).
In order to assess the abundance of the dominant prey item, cancer crabs, and black sea bass through time in Narragansett Bay, count data for both species was queried from the URI-GSO fish trawl survey in Narragansett Bay from 1959 through 2019. The data for both species were plotted together to compare historical and modern population abundance in Narragansett Bay (Fig. 8). There is evidence of declining cancer crab abundance and increased black sea bass abundance since the early 2000s in Narragansett Bay recorded in the survey.
A significant decline in cancer crab abundance in the region, without a corresponding shift in Atlantic cod and black sea bass diet, could exacerbate interactions between these consumers through increased competition for limited resources, since the competitive exclusion principle says that two species cannot coexist when competing for limited shared resources (Hardin 1960 Though diet is a valuable metric of recent resource utilization patterns, the data can be challenging to interpret in a broader ecosystem or food web dynamic context. The standard ellipse area (SEA) analysis of bivariate isotope data allows us to estimate an organisms' isotopic niche breadth and can tell us about overall resource utilizationincluding diet and habitat -and thus gives us a more integrative understanding of how organisms fit into a food web (Jackson et al. 2011;Newsome et al. 2007). While black sea bass had a notably narrower isotopic niche than Atlantic cod, Bayesian estimates of SEA overlap indicate significant similarity between Atlantic cod and black sea bass isotopic niche at BI in summer and NB in winter as hypothesized (hypothesis 2 and 3) (Guzzo et al. 2013;Schoener 1968). For both species, the niche width expands in winter relative to summer, likely due to the diversification of diet in both Atlantic cod and black sea bass in winter, as shown in the stomach content data.
In order to better understand the patterns of resource utilization leading to the strong isotope niche overlap, we can explore potential underlying drivers of the isotope variability in Atlantic cod and black sea bass in SNE. For example, there is a seasonal difference in bulk δ 15 N and δ 13 C values in SNE Atlantic cod bulk isotope values. One explanation for this change in isotope value is a difference in trophic position for Atlantic cod and black sea bass seasonally, which can create a difference of up to ~3-4‰ in 15 N value and 0-1‰ in 13 C value with each additional trophic transfer (Peterson and Fry, 1987). Alternatively, this seasonal change in isotope values could reflect changes in baseline biogeochemical cycling and phytoplankton production dynamics that can alter the baseline 15 N and δ 13 C pool that are then subsequently passed through the food web to upper trophic level consumers (Sigman et al. 2009;Sigman et al. 1999;Fry 2002).
With bulk SIA, it can be difficult to tease apart the relative impact of changes in trophic dynamics versus changes at the base of the food web on consumer isotope values (Post, 2002). Thus, we used CSIA-AA δ 15 N values of trophic and source amino acids to tease apart trophic and baseline variation in our consumer tissues (McMahon and McCarthy, 2016).
Both Atlantic cod and black sea bass had similar δ 15 N values of Phe, a source amino acid that serves as a proxy for the nitrogen isotope baseline, indicating that the two species are likely foraging in food webs with similar baselines in both winter and summer. When put in the context of the strongly overlapping crustacean diets, these results suggest that both Atlantic cod and black sea bass are foraging on the same prey in the same food web, which supports our second and third hypotheses. However, there was a significant shift in both bulk and Phe δ 15 N value for both species seasonally, with higher δ 15 N values in summer compared to winter. Oczkowski et al. (2017) showed a similar summer δ 15 N enrichment and winter δ 15 N depletion in Narragansett Bay that was hypothesized to be related to changes in production and recycled particulate nitrogen.
Thus, it is likely that the observed differences in seasonal and NB/BI δ 15 N signals in this study are also related to the changes in mixing and subsequent nutrient inputs and availability for Narragansett Bay and Rhode Island Sound. Narragansett Bay and Rhode Island Sound are more well-mixed leading to similar nutrient availability between both locations in winter as opposed to summer where there is a stronger nutrient gradient (with higher levels of nitrate inshore) and less mixing (Chaves 2004;Oczkowski 2017;Rosa 2020 Hesler et al. 1995). Meanwhile, winter black sea bass consume more lower trophic level invertebrates in addition to cancer crabs and thus their trophic position remains relatively constant. These TPCSIA data suggest that the dietary patterns observed in the stomachcontent analyses were longer term, seasonal patterns and not biased by the short snapshot nature of the stomach-content analysis approach. It is also worth noting that our Atlantic cod samples in winter were larger (by total length) than those caught in summer, which has implications for the size of diet items that can potentially be consumed and thus could be the reason for the increased the trophic position of larger Atlantic cod (Cohen et al. 1993;France et al. 1998;Woodward and Hildrew 2002). Our study also focused on adult sizes of both species so juvenile diet has not been taken into account, thus future work could be done to understand the overlap in resource utilization patterns between juvenile cod and black sea bass in SNE.
This multi-metric approach provides a compelling example of how climate change can shape species interactions through species range shifts, changes in population sizes, and shifting prey fields and prey availability. The results of this study indicate a strong overlap in diet, dietary and trophic niche, trophic position, and foraging location between Atlantic cod and black sea bass in SNE waters. This dietary overlap between these two predators has a variety of potential implications for the top-down control on the structure and function of SNE ecosystems as well as the management of fisheries that depend on them. The strong selection of Atlantic cod and black sea bass for cancer crab coupled with the rapidly growing black sea bass population could have ecological and fisheries management implications for the burgeoning Jonah crab fishery. A decline in cancer crab abundance could have implications for Atlantic cod and black sea bass, which this study shows to have highly selective crustacean diets. Future research could be  Table S1. Frequency of occurrence (%) of prey items in diet of Atlantic cod and black sea bass in winter and summer in Narragansett Bay (NB) and Block Island (BI).     Figure 3. Predator feeding strategy and prey importance for Atlantic cod and black sea bass (BSB) at Block Island (BI) and Narragansett Bay (NB) represented as the frequency of prey occurrence and the proportion of the total wet weight made of up that prey item.