A fish-mediated trophic cascade on freshwater calanoid copepod abundance is concealed by food web fatty acid availability, functional traits and population sex ratio

We applied essential fatty acids as a biomarker to experimentally test the effects of a fish-mediated trophic cascade on calanoid copepod nutritional state and functional traits in north-temperate freshwater ponds. A whole-pond experiment was conducted where young-of-the-year brook trout (Salvelinus fontinalis) were introduced into three natural, fishless ponds with no previous history of fish introduction, and compared with four similar naturally fishless ponds. We unexpectedly found no effect of the fish-mediated trophic cascade on copepod population nutritional state and abundance, despite enhanced phytoplankton resources in fish-stocked ponds. While salmonids increased individual female copepod fecundity, the fish also reduced gravid copepod female abundance and altered copepod population sex ratios in favour of males. Fish predation likely influenced the overall nutritional state of calanoid copepod populations by expenditure of key lipids in male copepods through increased activity linked to female mate limitation, and selective predation on gravid female copepods. Therefore, the positive effects of the fish in enhancing the essential fatty acid content of phytoplankton communities via nutrient additions were not sufficient to over-ride the negative effects of fish predation on copepod population nutritional state and abundance.