Forage fish as a predator: summer and fall diet of Atlantic herring in Trinity Bay, Newfoundland

Abstract

Atlantic herring (Clupea harengus) is an important forage fish in the Northwest Atlantic. However, the diet of herring in eastern Newfoundland is unknown. I compared plankton assemblages to stomach content and stable isotope analyses of Trinity Bay adult herring collected in the late summer and fall (2017-2019) to characterize diet and selectivity. Amphipods (Themisto spp.) contributed most to herring diets in 2017 and August 2018, while calanoid copepods dominated the diet in late 2018 and 2019. Fish eggs and larvae were consistently observed in stomach contents and were most prevalent in the diet when peak larval densities were observed. Stable isotope analyses found that herring fed on prey at a higher trophic level than zooplankton, supporting the importance of piscivory in adult herring diets. As an adaptive predator, Newfoundland herring are resilient to bottom-up changes and may also exert top-down pressure on the early life stages of other fishes.