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 (delta N-15; ratio of N-15 to N-14 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 d15N 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.