Variability in the trophic position of larval fish in a coastal pelagic ecosystem based on stable isotope analysis

We used stable isotopes of carbon (C) and nitrogen (N) to investigate the trophic position of six species of larval fish in the pelagic ecosystem of coastal Newfoundland. Isotope profiles from phytoplankton, net plankton and macrozooplankton were consistent with previous studies. All species of larval fish showed a length-dependent shift in δ13C that indicates a move to a pelagic diet from the combined pelagic and demersal eating habits of the adult spawners. The trophic position of four larval fish species (American plaice, yellowtail flounder, cunner, radiated shanny) was consistent with them feeding primarily on copepods, as expected from stomach content analysis. The δ15N-based trophic position of larval witch flounder and capelin indicates that they feed significantly on phytoplankton and heterotrophic protists from the microbial loop, respectively; this evidence contrasts with stomach content analysis. Although links between larval fish and the microbial loop are not considered as common as is the link with crustacean zooplankton, this and other studies challenge the long-held belief that marine fish larvae feed effectively exclusively on copepods.