Prey consumption of bowhead whales in West Greenland estimated from drone measurements of body size and condition

Abstract

Understanding the energy requirement and prey consumption of Arctic predators is crucial to assess their vulnerability to climate change. The bowhead whale (Balaena mysticetus) is the largest predator of the Arctic, with Disko Bay in Greenland constituting a major feeding ground for a segment (predominantly larger juveniles and adult non-lactating females) of the Eastern Canada-West Greenland population. We used unmanned aerial vehicle photogrammetry to measure the body size and condition of bowhead whales in Disko Bay during the spring (March 26–June 1, 2022), from which we quantified their energy requirements and prey consumption. Measurements of body length, width and height were used to estimate the body volume and condition of juveniles (50 measurements from 39 individuals) and adults (232 measurements from 154 individuals). The body condition of adults increased at a rate of 0.112 percentage points day⁻¹, or 44.1–87.8 L of blubber day⁻¹ (for lengths 13.0–17.0 m). Using published data from harvested whales, the estimated blubber mass gain was 37.1–73.9 kg day⁻¹, which equals an energy deposition rate of 1.01–2.01 GJ day⁻¹. Body maintenance costs were predicted from allometric models, while activity costs and the heat increment of feeding were derived from published archival tag data. Reported length-at-age curves were used to estimate somatic growth costs. The energy requirements for juveniles and adults were 0.621–0.778 GJ day⁻¹ (for lengths 11.0–12.9 m) and 3.662–7.826 GJ day⁻¹ (for lengths 13.0–17.0 m), respectively, which equals a prey consumption rate of 17–22 kg of prey day⁻¹ (0.1% of body mass) and 102–218 kg day⁻¹ (0.3% of body mass).