Modeling the field metabolic rate and prey consumption rate of Greenland sharks (Somniosus microcephalus) using archival biologgers

Eric Ste-Marie, Integrative Biology, University of Windsor, Windsor, ON, Canada, Yuuki Y. Watanabe, National Institute of Polar Research, Tachikawa, Japan; Department of Polar Science, Sokendai, Tachikawa, Japan, Jayson M. Semmens, Fisheries and Aquaculture Centre, Institute for Marine and Antarctic Studies, Taroona, TAS, Australia, Marianne Marcoux, Arctic and Aquatic Research Division, Fisheries and Oceans Canada, Winnipeg, MB, Canada and Nigel E. Hussey, Biological Sciences, University of Windsor, Windsor, ON, Canada

Background/Question/Methods
Despite being the largest ectotherm inhabiting the cold waters of the Arctic, very little is currently known regarding the ecological role of the Greenland shark (Somniosus microcephalus). Frequently caught as bycatch in rapidly expanding commercial fisheries for Greenland halibut and Northern prawn, there is a pressing need to bridge this knowledge gap, especially considering the late sexual maturation and k-selected life history of these long-lived sharks. Studying the energetics of organisms in their natural environment provides an important approach to achieve this goal. Given recently published baseline estimates of resting and active metabolic rate for Greenland sharks, we modeled field metabolic rate (FMR) for individual sharks tagged in Tremblay Sound and Scott Inlet (Nunavut) with either biologgers recording triaxial acceleration and temperature for several days (n=30), or pop-off archival satellite tags (PSATs) recording depth and temperature for an entire year (n=6). Combining these derived FMR estimates with literature data pertaining to their diet, we then estimated consumption rates of three culturally and economically valuable prey species: ringed seal, narwhal and Greenland halibut. Prey consumption rates were assessed at the individual-level for all tagged sharks, and at the local population-level for sharks in Scott Inlet during their summer residency.
Results/Conclusions
We estimated a mean mass adjusted FMR of only 21.67 ± 2.30 mgO2h-1kg-0.84 across all accelerometer equipped Greenland sharks, and 25.48 ± 0.47 mgO2h-1kg-0.84 across all PSAT tagged sharks. The latter slightly elevated FMRs resulted from higher water temperatures experienced while overwintering outside the cold-water coastal systems of Scott Inlet and Tremblay Sound. Unsurprisingly, FMR estimates for Greenland sharks are much lower than those published for other more active, warm-water shark species. Assuming incomplete assimilation of consumed prey and caloric densities for each prey species, we estimated that an average Greenland shark (224 kg) need only consume 61-71 g of ringed seal, 86-101 g of narwhal, or 164-192 g of Greenland halibut daily. Across Scott Inlet during their three-month summer residency, and accounting for proportional contributions of each prey species, the local Greenland shark population may collectively consume ~4.5 tonnes of ringed seal, 426 kg of narwhal (i.e. less than half an adult whale), and ~21.5 tonnes of Greenland halibut. Overall, the low predicted energy requirements of Greenland sharks, combined with their tendency to feed on energy rich prey, suggests they may be able to survive extended periods of weeks or months following the consumption of a single meal.