Size and body condition of sympatric killer whale ecotypes around the Antarctic Peninsula

Abstract

Killer whales Orcinus orca are apex predators, and their health can indicate trophic dynamics in ecosystems that support them. We used aerial photogrammetry to estimate body lengths, to better understand size differences and food requirements, and widths, to infer current nutritional condition, of killer whales in the rapidly warming waters around the Antarctic Peninsula. A remotely controlled hexacopter drone was used to collect aerial images of 242 killer whales of 3 sympatric ecotypes (Type A, n = 34; Type B1, n = 19; and Type B2, n = 189) in the austral summers between 2015/2016 and 2018/2019. Total length (TL) varied between ecotypes, with B2s being diminutive in size, indicating large differences in energy requirements. The mean length for adult females ranged from 5.82 m (B2s) to 6.93 m (B1s), and the mean for adult males ranged from 6.44 m (B2s) to 7.80 m (As). We also found significant differences in head width (HW, proxy for body condition), with B2s being significantly leaner. Although this variation may reflect natural shape differences, we also estimated divergent regression lines of HW~TL indicating that this difference was greater at larger body sizes, with some anomalously thin adult female B2s. We suggest that these dissimilarities may indicate a density-dependent response, with leaner body condition in adults with higher energetic requirements, as the abundance of B2s is almost an order of magnitude greater than that of B1s and As. We hypothesize that food limitation resulted from a decline in carrying capacity during recent reductions in sea ice and warmer ocean temperatures.