Evaluating the role of cephalopods within marine food webs and fisheries

In this dissertation, I examine critical gaps in our understanding of the role of cephalopods in marine ecosystems by evaluating their trophic ecology from a single species context to an ecosystems-level perspective. My work provides baseline data on the commander squid, Berryteuthis magister, inhabiting the eastern Bering Sea (EBS) continental slope ecosystem. I combine stomach content and stable isotope analyses to identify the feeding ecology of B. magister in this region. I also use novel methodology to elucidate potential finer scale variation in their trophic ecology by reconstructing feeding chronologies of individuals from concentric eye lens layers. This work is the first to identify the trophic ecology of B. magister within the EBS ecosystem and it reveals that B. magister is more likely to be a competitor of commercial fishes than to prey upon them. My dissertation also explores the role of cephalopods within an ecosystem context. I provide the first measure of the ecosystem services that cephalopods contribute to fisheries in 28 marine ecosystems, both as a commodity and an ecological support service. My findings indicate that cephalopod contributions to fisheries vary widely, but are substantial in many ecosystems. They also demonstrate the rising demand of cephalopod resources and the lack of a historical precedent of cephalopods withstanding heavy fishing pressure while simultaneously acting as an ecosytem support service. Further, I test the hypothesis that cephalopod abundance has increased over the past half-century in response to anthropogenic or natural forcing. I used the feeding habits of predator diets as a proxy for cephalopod availability in the eastern tropical Pacific Ocean (ETP). I found a notable increase in the frequency of cephalopods in yellowin tuna diets from the 1950s to the present; suggesting an increase in the availability of cephalopods in the ETP since the mid-20th century. Additional anlayses of the apex predator food habits data suggest that cephalopods and mesopredatory fishes are not the only marine fauna that might be regulated by top predators. My work indicates that sharks and billfishes could potentially regulate skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares) tuna populations in the ETP through intra-guild predation.