Anadromous fish as marine nutrient vectors

The tidal freshwater of Virginia supports anadromous herring (Alosa spp.) spawning runs in the spring; however, their importance as nutrient delivery vectors to the freshwater fish food web remains unknown. The stable isotope signatures of fishes from 21 species and four different guilds (predators, carnivores, generalists, and planktivores) were examined in this study to test the hypothesis that marine derived nutrients (MDNs) brought by anadromous fish would be traced into the guilds that incorporated them. Spawning anadromous fish were C-13 and S-34-enriched (delta C-13 and delta S-34 of approximately 18 parts per thousand and 17.7 parts per thousand, respectively) relative to resident freshwater fish. Of the guilds examined, only predators showed C-13 and S-34-enrichment similar to the anadromous fish, however, some generalist catfish also showed enriched signatures. Specific fatty acid delta C-13 signatures for gizzard shad (Dorosoma cepedianum), blue catfish (Ictalurus furcatus), and ale-wife (Alosa pseudoharengus), show a 10 parts per thousand range among fishes, clearly reflecting isotopically distinct dietary sources. The delta C-13 and delta S-34 distribution and range among the freshwater fishes suggest that both autochthonous and allochthonous (terrestrial C3 photosynthetic production and MDN) nutrient sources are important to the tidal freshwater fish community.