Southern right whale (Eubalaena australis) migratory and foraging behaviour in a changing climate

Abstract

The name “right” whale reflects the violent history of southern right whales (SRW, Eubalaena australis) since their sluggish, predictable coastal behaviour and the tendency to float when dead led to the reputation of being the “right” whales to hunt. Now, the characteristics that contributed to their near demise carry a less sinister connotation, that of being the “right” climate sentinel. Their life histories and wide-ranging migratory behaviour make SRWs ideal candidates for monitoring environmental change. SRWs are capital breeders with discrete periods in their life histories dedicated to foraging and reproduction. Therefore, females have a strong impetus to accumulate sufficient energy reserves during their foraging period to sustain them through pregnancy and calf nursing. Thus, a strong connection exists between foraging efficiency and reproductive success. Foraging efficiency is largely dictated by the environment’s ability to generate productive feeding grounds. Sea ice, for example, has a fundamental role in the life history of Antarctic krill (Euphausia superba), a key prey species for SRWs. Changes in reproductive success in SRWs may indicate climate-induced alterations in prey, such as reductions in sea ice, in their offshore feeding grounds. Following a steady period of recovery from commercial whaling, SRW populations recently began displaying signs of reduced population growth rates and reproductive failure. This prompted an in-depth assessment into potential drivers behind these changes, and their contemporary migratory and foraging behaviour. This thesis is focused on the South African population, which through annual aerial surveys has been extensively monitored since 1969. The resultant time-series of population demographic data has revealed drastic reductions in reproductive success after 2009, together with a northward shift in foraging location as indicated by stable isotopes, and a decline in maternal body condition. To better understand these changes, an assessment into patterns of satellite-derived chlorophyll and sea ice as proxies for foraging habitat quality was performed over the period of observed population-level reductions in reproductive success. This revealed substantial declines in sea ice over historically important foraging grounds for SRWs, which likely contributed to reduced availability of their main prey at high-latitudes; Antarctic krill. The thesis reports on the use of animal-borne Conductivity, Temperature and Depth instruments and dive depth recording tags in better understanding the environmental characteristics of their mid-latitude foraging grounds. Results highlighted the importance of ocean fronts, temperature and salinity in determining favourable foraging grounds. Finally, through the deployment of 26 location-only satellite tags, a comprehensive investigation into the contemporary offshore migratory and foraging behaviour of South African SRWs was performed. From these, areas of high use were identified, and the presence of foraging whales was predicted from eight environmental covariates using Random forest. This significantly increased our knowledge of SRW contemporary habitat use. The findings of this thesis highlight the strong connections between SRWs and their environment, and the vulnerability of the species to climate change and variability. This also reaffirms the utility of SRWs as the “right” sentinels for environmental change.