Predatory role of the commander squid Berryteuthis magister in the eastern Bering Sea: insights from stable isotopes and food habits

Squid are an important component of many marine food webs, and they can impact other species through predation and competition. However, quantifying their influence on other food web components requires knowledge of their trophic position and trophic ontogeny, which are unknown in many ecosystems. The eastern Bering Sea (EBS) is a highly productive region that supports large commercial fisheries, and a modicum of knowledge exists on the ecological role of squid in this region. We combined stomach content and stable isotope analyses of muscle tissue (delta N-15 and delta C-13) to identify the feeding ecology of the commander squid Berryteuthis magister in the EBS continental slope ecosystem. We also use a novel methodology to elucidate potential finer-scale variation in squid trophic ecology by reconstructing feeding chronologies of individual B. magister from concentric eye lens layers. Our analyses indicate that the position of B. magister in the EBS food web increases by approximately 1 trophic level between juvenile and adult stages. Also, in contrast to many squid species, we found that predation by B. magister is not constrained by prey body size and that B. magister are more likely to share prey resources with commercially valuable fishes, particularly walleye pollock Theragra chalcogramma, than to prey upon their juvenile stages. Further, the reconstructed feeding chronologies indicate substantial variability in squid feeding patterns that are not captured on the time scales of the conventional analyses. Together, the findings of this study contribute to a better understanding of the ecological role of B. magister and the trophic linkages and energy flow within the EBS food web.