Night-life of Bryde's whales: ecological implications of resting in a baleen whale

摘要

Many animals require intervals of rest or sleep in which their vigilance level is reduced. For marine fauna, including large baleen whales, resting potentially increases the risk of predation and vessel-strike. However, there is scarce information about how, and how often, whales rest which makes it difficult to assess the severity of this risk. Here we examine resting patterns of Bryde's whales (Baleaenoptera edeni/brydei), using data collected by sound and movement archival tags (DTAGs) deployed on four whales in the Hauraki Gulf, New Zealand. To identify low activity levels associated with resting, we used RMS jerk and mean flow noise (as proxies for activity and speed, respectively), as well as changes in dive patterns (dive depth and shape), fluking, and respiration rates. The tagged whales showed strong diel differences in behavior with long periods of low activity consistent with resting occurring exclusively during the night. This pattern indicates that either (i) Bryde's whales rely on senses that are less effective in the dark to locate prey, or (ii) that prey aggregate less densely at night, making foraging less efficient. Thus, Bryde's whales conserve energy through rest during times when the net benefit of foraging effort is low. However, by reducing their interaction level with their environment, night-time resting also makes Bryde's whales more vulnerable to vessel strikes, an important source of mortality for cetaceans.