Behavioural Ecology of Bryde’s Whales

Abstract

Baleen whales are the largest mammals on earth, but many questions about their behaviour remain unanswered as they are submerged in the pelagic environment. They feed on small schooling fish and zooplankton aggregations, ephemerally dispersed throughout their habitat. The Bryde's whale (Balaneoptera edeni spp.) is a non-migrating species of baleen whale with a wide distribution; very little is known about the behaviour of these whales. There is a small (∼ 135) year-round population of Bryde's whales in the Hauraki Gulf, New Zealand, which are categorised as a Nationally Critical species. The aim of the present study was to investigate two critical behavioural states of Bryde's whales, resting and foraging. Data on the behaviour of Bryde's whales were collected by an archival tag (DTAG2), boat-based surveys, and drones. To identify rest, tag data were used to calculate root-meansquare (RMS) jerk and mean flow noise (as proxies for activity and speed, respectively), as well as changes in dive patterns, fluking and respiration r ates. Tag data were also used to detect lunges and investigate their kinematics, confirmed by visual observation data. These data were used to design head models and physical experiments to simulate head slaps, a behaviour which is performed when whales feed on zooplankton. The results were confirmed using mathematical models. Bryde's whales show strong diel patterns in behaviour with long periods of low activity consistent with resting occurring exclusively at night. The whales spend their days actively foraging, mostly at the surface using a stereotypical series of movements. Surface foraging tactics differ when whales were feeding on plankton or fish. Head slaps have biophysical applications and create water currents to increase the density of prey-laden water by 3-5 orders of magnitude. This pattern of intensive surface foraging during the day and rest at night indicates that either Bryde's whales rely on senses that are less effective in the dark to locate prey, or that prey aggregate less densely at night, making foraging less efficient. Also, dynamic behavioural tactics and diet plasticity seem to be the key for non-migrating Bryde's whales to meet their energetic demands in poorer quality waters. Plankton stratification is constrained by the shallow nature of the Hauraki Gulf, and Bryde's whales have evolved a unique be-haviour to increase the density of zooplankton, which is a remarkably successful strategy in maintaining their high energetic demands.