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

Jw Durban,Ls Hickmott,A Paredes,Dj Leroi,H Fearnbach

doi:10.3354/meps13866

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.

Highlights

Killer whales Orcinus orca are apex marine predators known to have high caloric requirements (Noren 2011) and show population responses to changes in prey availability (Ford et al 2010)
Their physiology is adapting to the extreme cold in Antarctica (Foote et al 2011, 2016) enabling 2 divergent ecotypes (Durban et al 2017) to occur year-round in the coastal waters of the Antarctic Peninsula (AP): Type B1 specializes in hunting seals that haul out on pack ice (Pitman & Durban 2012), while individuals of Type B2 are generally found in more open water where they feed on penguins (Pitman & Durban 2010) and occasionally Weddell seals Leptonychotes weddellii
Data on body lengths are a key component for estimating the trophic requirements of killer whales and can help to fill key data gaps on apex predators by parameterizing energetic models of trophic dynamics (e.g. Guénette et al 2006)

Summary

Introduction

Killer whales Orcinus orca are apex marine predators known to have high caloric requirements (Noren 2011) and show population responses to changes in prey availability (Ford et al 2010). A growing body of data is defining 3 generally sympatric ecotypes of killer whales around the AP, which are distinct both morphologically (Pitman & Ensor 2003, Durban et al 2017) and genetically (Morin et al 2010, 2015, Foote et al 2016) These ecotypes have varying ecology and trophic interactions. Pitman unpubl. data) and regularly undertake prolonged bouts of deep diving (regular dives of 500−700 m, Pitman et al 2020), likely to feed on fish

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Conclusion