Pygmy blue whale feeding drives greater nocturnal than diurnal song production

Effective management of threatened and migratory species requires monitoring of critical behaviours, such as foraging. Passive acoustic monitoring can provide cost-effective, long-term observations on the relative abundance of vocal animals across large spatial scales. However, linking specific sounds to behavioural contexts can be challenging, especially for cryptic and aquatic species. This study combined passive acoustic and biologging data to explore whether observed periods of greater nocturnal than diurnal song production were linked to feeding by eastern Indian Ocean pygmy blue whales, Balaenoptera musculus brevicauda. The proportion of time available for singing was modelled as a function of geographical region, diel period and the presence or absence of feeding dives with generalized linear mixed models, based on 130.1 days of diving data. Top-ranked models indicated less time was available for singing during diurnal feeding periods compared to nocturnal feeding periods and non-feeding periods during the day and night. These findings suggest that diurnal feeding by pygmy blue whales reduces the time available for song production. Therefore, greater nocturnal than diurnal song production may provide an acoustic indicator of population-level feeding activity for pygmy blue whales between the Perth Canyon and offshore of the northwest shelf of Western Australia.