Diel vertical migration and feeding dynamics of capelin and cod in an epipelagic subarctic ecosystem in Greenland

Abstract This study investigates diel vertical movements and trophic interactions within a subarctic epipelagic community dominated by capelin ( Mallotus villosus ), Atlantic cod ( Gadus morhua ), and macrozooplankton, with implications for ecosystem modeling and acoustic surveys. Intensive diel sampling—combining acoustic observations with stomach content analysis of 2173 capelin and 353 cod—revealed pronounced diel and ontogenetic feeding patterns. Capelin primarily occupied shallow depths (< 60 m), with smaller individuals occasionally descending during daylight. They feed at night on Euphausiacea, coinciding with upward prey migration, whereas Hyperiidae and Copepoda were most important during daylight. Daytime cannibalism by larger capelin on conspecific juveniles was documented for the first time. Cod diets were dominated by Euphausiacea during night and capelin in the day, reflecting vertical overlap with prey. Similar capelin size distributions in trawl catches and cod stomachs suggest minimal size selectivity, supporting the reliability of pelagic trawls for surveys. Comparisons between diel versus daytime-only sampling revealed significant biases, underscoring the need to account for diel dynamics in trophic studies. These results highlight critical methodological considerations for improving trophodynamic modeling, and, ultimately, the accuracy of stock assessments and management advice for forage species such as capelin and shrimp where predation rates are key model inputs.