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Do fatter whales migrate earlier? Body condition and migratory timing of east Australian humpback whales

Grace Russell*, Andrew Colefax, Fredrik Christiansen, George Russell, Zoe Fowler, Daniele Cagnazzi

*Corresponding author:

ABSTRACT: In order to exploit seasonally favourable habitats for feeding and breeding, humpback whales (Megaptera novaeangliae) undertake one of the longest migrations in the animal kingdom. Stored energy is crucial for a successful migration, despite this few studies have investigated the relationship between migration timing and body condition in baleen whales. Using unmanned aerial vehicles, we quantified the body condition of east Australian humpback whales. We collected data on 513 individual whales (48 calves, 166 juveniles, 251 adults, and 48 lactating females) during their northbound and southbound migrations between June and October 2020. For adults and juveniles, we explored the loss of body condition between migration direction (north versus south) as well as the relationship of migration timing (day of year) and body condition. We found a significant loss in body condition between the northbound and southbound migrations for both adults (9.8%) and juveniles (18.3%). However, migration timing did not influence body condition for either reproductive class. Cow/calf pairs were analysed using relative calf length (percentage of mother’s length) as a proxy for days postpartum. We found a positive curvilinear relationship migration for timing on calf body condition. However, lactating females showed no relationship between migration timing and body condition. Whilst body condition is important for capital breeding whales, the lack of a correlation found for adults and juveniles suggests that body condition is not the main driver of migration timing from feeding or breeding grounds. However, calf body condition may be a significant factor for the migration timing of cow/calf pairs.