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MEPS prepress abstract   -  DOI:

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

J. W. Durban*, H. Fearnbach, A. Paredes, L. S. Hickmott, D. J. LeRoi

*Corresponding author:

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 three 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 between 5.83 m in B2s to 6.93 m in B1s, and the mean for adult males ranged between 6.44 m in B2s and 7.80 m in As. There were also significant differences in head width (HW, proxy for body condition), with B2s measuring significantly leaner. Although this 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 this 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 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.