Trophic Ecology of Summer Flounder in Lower Chesapeake Bay Inferred from Stomach Content and Stable Isotope Analyses

Trophic studies of summer flounder Paralichthys dentatus have relied on traditional stomach content analyses to infer contributions of prey to species productivity. We applied both stable isotope and stomach content analyses to identify prey groups that are responsible for summer flounder productivity in lower Chesapeake Bay and to explore ontogenetic patterns in prey utilization. Summer flounder (total length = 138–682 mm; age = 0–11 years) were collected for stomach and tissue samples (liver, blood, and muscle) during spring–summer (May–July) and fall (November) of 2006 and 2007. Commonly consumed crustacean and fish prey species were also collected: mysid shrimp Neomysis americana, sevenspine bay shrimp Crangon septemspinosa, mantis shrimp Squilla empusa, bay anchovy Anchoa mitchilli, spotted hake Urophycis regia, and juvenile sciaenids. Analysis of the nitrogen stable isotope ratio (δ15N; ratio of 15N to 14N relative to a standard) revealed that crustaceans comprised the majority (72–100% on average) of the summer flounder diet except in spring 2006, when fish consumption was more dominant. Analysis of corresponding stomach contents indicated a lower contribution of crustacean prey. Based on isotopes, summer flounder tended to occupy the same trophic level as the prey fishes. The δ15N in all tissues exhibited a positive trend with body length, indicating that larger summer flounder fed at approximately one trophic level above smaller individuals; the positive trend also corresponded with increasing proportions of fish in summer flounder stomachs. Our stable isotope analysis indicates that growth and production of summer flounder in lower Chesapeake Bay are highly dependent on assimilation of mysid shrimp, sevenspine bay shrimp, and mantis shrimp—more so than previously expected based on stomach content research. Along the U.S. eastern seaboard, the Chesapeake Bay acts as a primary juvenile nursery and crucial foraging habitat for a broad fauna of coastal migratory fishes (Murdy et al. 1997), but the prey resources that support fishery production in this region have not been thoroughly examined with methods other than traditional stomach content analysis. The summer flounder Paralichthys dentatus is a seasonal inhabitant of Chesapeake Bay that resides in the estuary from spring to fall (Able et al. 1990; Szedlmayer et al. 1992; Bonzek et al. 2008) before migrating to spawning regions on the continental shelf (Packer et al. 1999; Terceiro 2002; Able and Fahay 2010). Owing to the economic and ecological importance of summer flounder along *Corresponding author: [email protected] Received October 4, 2010; accepted March 11, 2011 the U.S. Atlantic coast, this species has been strictly managed (Terceiro 2002). To support management needs and multispecies modeling efforts, summer flounder and other bay fishes have been monitored by the Chesapeake Bay Multispecies Monitoring and Assessment Program (ChesMMAP) since 2002 (Latour et al. 2003). To date, summer flounder diet assessments from this program (Latour et al. 2008) and trophic studies conducted in similar environments have relied solely on stomach content analyses (e.g., Burke 1995; Rountree and Able 1992; Link et al. 2002). These studies have documented that summer flounder primarily consume crustaceans (e.g., mysid shrimp Neomysis americana, sevenspine bay shrimp Crangon septemspinosa, and 1240 D ow nl oa de d by [ C ol le ge o f W ill ia m & M ar y] a t 0 6: 20 2 2 Se pt em be r 20 11 TROPHIC ECOLOGY OF SUMMER FLOUNDER 1241 mantis shrimp Squilla empusa) and small fishes (e.g., bay anchovy Anchoa mitchilli and sciaenids), but it remains unclear how each prey group contributes to actual somatic growth and production. Stable isotopes have emerged as a valuable complement to traditional stomach content analyses by contributing to the characterization of trophic relationships and aiding in food web modeling. Dietary studies involving stable isotope ratios (e.g., δ15N: the ratio of 15N to 14N relative to a standard) rely on the fact that an organism’s tissue is synthesized from assimilated organic matter and reflects the isotopic signatures of the consumed prey (Fry 2006). The consumer’s tissue will be a time-integrated dietary representation on a scale of weeks to months (e.g., Buchheister and Latour 2010), unlike stomach contents, which document feeding habits on the order of hours. Thus, stable isotopes can elucidate the prey groups that are directly responsible for driving tissue growth and production of consumer species (Fry 2006). This information is important for ecosystem modeling efforts requiring quantitative information on food web relationships over longer time periods that are more relevant to ecosystem processes. Stable isotope analyses do not attain the same level of taxonomic resolution afforded by stomach content analysis, but isotope techniques are useful for identifying broader sources of production and for differentiating between benthic and pelagic trophic pathways (Fry and Sherr 1989). The goal of this study was to use stable isotopes in multiple tissues of summer flounder to build upon previous ChesMMAP stomach content analyses by providing a broader understanding of the prey groups driving summer flounder production in lower Chesapeake Bay. The specific objectives were to (1) characterize and compare summer flounder food habits in the bay’s main stem by means of both stomach content analysis and stable isotope methods and (2) explore ontogenetic patterns in resource use. Diet analyses conducted by Latour et al. (2008) are expanded in the present paper by examining another year of ChesMMAP data, but here we focus primarily on stable isotope analyses of summer flounder diets. Despite the prominent role of Chesapeake Bay in the life histories of many fishes, few studies have applied stable isotope techniques to address diets of species at higher trophic levels in this estuary (Hoffman et al. 2007). To our knowledge, this is the first study to use stable isotopes from multiple tissues to assess summer flounder food habits. We demonstrate the utility of isotopic approaches while also highlighting the assumptions and complications associated with applying stable isotopes to migratory fish species that inhabit dynamic estuarine environments.