Marine nutrient contributions to freshwater apex predators

Recent investigations into the nutrient cycling of coastal ecosystems have suggested that migratory or anadromous fish could be important vectors of marine nutrients. Anadromous fish have assimilated marine nutrients that would contribute to the nutrient budgets of freshwater systems by excretion, gamete release, or the decay of post-reproductive carcasses. However, the extent to which freshwater predators utilize marine material is not well understood. In systems where anadromous fish temporarily constitute a major portion of the fish community, they may contribute substantially to the diet of piscivorous fish and other predators. Here we show the contribution of anadromous blueback herring, shad. and alewife (Alosa) to diets of large, non-indigenous piscivorous catfish (Ictalurus furcatus) using delta(34)S and delta(13)C. The spawning anadromous Alosa, captured in tidal freshwater, had enriched delta(34)S and delta(31)C values compared to resident, native freshwater species. As a result of consuming the anadromous Alosa, the I. furcatus isotope signature shifted towards the marine signal. The isotope analysis revealed that anadromous fish contribute substantially to the diet of most captured I. furcatus. The percentage of anadromous Alosa carbon and sulfur that was incorporated into I. furcatus (greater than or equal to 38 cm total length) ranged from 0 to 84% and 10 to 69%, and had means (+/-SD) of 42 (+/-24) and 43 (+/-16)%, respectively. Although the delta(15)N Signal of marine-derived biomass is enriched by approximately 3 parts per thousand relative to terrestrial or freshwater biomass, it was not as useful as delta(34)S and delta(13)C for nutrient source owing to trophic fractionation. This study demonstrates that anadromous fish may be a significant source of nutrients to tidal freshwater apex predators.