Diets and Stable Isotope Derived Food Web Structure of Fishes from the Inshore Gulf of Maine
In the nearshore Gulf of Maine, a combination of factors (overfishing, ecosystem change, and ocean warming) is thought to govern groundfish recovery. We analyzed feeding habits of demersal predatory fish from Midcoast Maine (abundant river herring) and Passamaquoddy Bay (low river herring) in eastern Maine, using stomach content and stable isotope analyses, to determine the prevalence of river herring (alewife, Alosa pseudoharengus, and blueback herring, A. aestivalis) in nearshore (<4.5 km) groundfish diets. Invertebrates dominated all predator diets at all sites. At Midcoast sites, catch-per-unit-effort (CPUE) of Atlantic cod was higher, and fish predators showed a strong seasonal pattern in river herring consumption compared to Passamaqouddy Bay. Cod, pollock (Pollachius virens), and sculpins (Myoxocephalus octodecimspinosus and M. scorpius) from Midcoast sites tended to be enriched in δ15N relative to Passamaquoddy Bay. Contrasting fast vs. slow turnover tissue (fin vs. muscle) indicated that focal species migrated or food availability changed seasonally and Atlantic mackerel (Scomber scombrus) in Passamaquoddy Bay were assimilating into a trophically depleted food web. We posit that lack of forage fish in Passamaquoddy Bay contributed to conditions that encourage an invertebrate based diet. River herring are also an order of magnitude less abundant in Passamaquoddy Bay than at Midcoast sites, limiting the availability of this seasonal food source. River restoration may contribute to recovery of groundfish stocks nearshore by increasing the availability of high lipid, seasonally available prey.