Running on empty: capelin diet after stock collapse

Abstract

The Newfoundland capelin stock in Northwest Atlantic Fisheries Organization Divisions 2J3KL collapsed in 1991 with only a modest recovery over 30+ years. Resource limitation is hypothesized to be a driver of this slow stock recovery. Using detailed seasonal diet data (1999–2022), capelin diet was dominated by calanoid copepods in both spring and autumn, with hyperiid amphipods, euphausiids, and appendicularians increasing in importance with ontogeny. Euphausiids continue to be less important in the diet now compared to the 1970s and 1980s. Capelin spring diet changed around 2011 which corresponded with an increase in appendicularians in the diet, a lower quality prey taxa. Compared to other stocks worldwide, Newfoundland capelin have amongst the lowest ratios of stomach content weight to whole body weight suggesting they are experiencing resource limitations. Using a seasonal (spring and autumn) interval scale (0–4) of stomach fullness, we developed a stomach fullness index (ISFI) that covered 42 years (1980–2022) of capelin sampling. Five changepoints were detected during the “boom and bust” periods in the 1980s and early 1990s, with higher mean seasonal ISFI s during boom periods compared to the early 1990s bust period. Average ISFI from 1995 to 2022 is lower than the ISFI from the stock high of the late 1980s and has varied without trend since 1995. Our results provide support for the hypothesis that resource limitation, which could be due to prey availability and/or intra-specific competition, is impacting the recovery potential of Newfoundland capelin, supporting the hypothesis that this stock is bottom-up regulated.