Migratory timing in southern right whales on their breeding grounds: What are the key factors stimulating migration?

Migrations are key events in the annual life cycle for many animal species, including baleen whales. Despite having some of the longest migrations among mammals, relatively little is known about what triggers migration behaviour in baleen whales, and how intrinsic and extrinsic factors influence the timing of migrations. This study investigated the timing of migration in southern right whales (Eubalaena australis) and whether the probability of departure of individuals from winter breeding and calving grounds is determined by intrinsic (maternal body condition and calf size) or extrinsic variables (day length, temperature). Unmanned aerial vehicles were used to record data on the body size (length and width) of right whale mother and calves between the June 24 and September 25, 2016, providing repeated measurements of 40 mother-calf pairs. I used the time interval between sightings of individuals to determine the departure date of whales from the breeding grounds. Generalised linear models were used to determine which variable (maternal body condition, calf size, day length and sea surface temperature) was the best predictor of the departure time of the whales from the breeding grounds. Diurnal period was found to be the best predictor of migration for southern right whales, followed by calf size (i.e. body volume and length), sea surface temperature and maternal body condition. Hence it seems like migration in this population of right whales is determined by calves reaching a large enough body size to facilitate migration, rather than mothers pushing themselves energetically to their lower limit. Apart from improving our understanding of migratory behaviour in large whales, the findings of this study will help determine residency times of southern right whales on their breeding and calving grounds, which can be used to identify key times for protection (e.g. temporary exclusion periods for fisheries) during the winter breeding and calving season.