Evidence of likely foraging by pygmy blue whales in the Timor Trough during the late austral winter and early austral spring

Abstract

Understanding the behavioural context of wildlife movement patterns is imperative to the conservation of migratory species like cetaceans. The traditional model of baleen whale migration entails uninterrupted journeys performed throughout extended periods of fasting, during which individuals sustain the enormous costs of travelling from the poles to the tropics only from energy reserves acquired prior to departure. However, this ‘feast and famine’ paradigm is being challenged by increasing observations of supplemental feeding events along whale migratory routes. In this context, identifying the location of migratory stopovers is key to managing cetacean populations, particularly in data-poor ecosystems subject to changing ocean conditions. We report on likely foraging activity by migrant pygmy blue whales Balaenoptera musculus brevicauda in the Timor Trough (ca. 9.5° S, 126° E), a deep-water habitat south of the species’ presumed breeding grounds. Using photo-identification and generalised additive modelling, we analysed visual sightings collected aboard seismic vessels operating off Timor-Leste in 2007-2008 and demonstrate that (1) whales engage in surface behaviours suggestive of active feeding, (2) some individuals remain within the region for more than 1 d, and (3) whale presence is significantly associated with predictably high chlorophyll a concentrations. Despite previous efforts to examine pygmy blue whale movements at low latitudes using long-term satellite telemetry, knowledge of the species’ behavioural ecology in the tropics remains limited. Our results lend support to previously untested hypotheses about the possible use of the Timor Trough as a foraging site by eastern Indian Ocean pygmy blue whales during the late austral winter and early austral spring.