Swimming speed and foraging strategies of New Zealand sea lions (Phocarctos hookeri)

Lactating New Zealand sea lions (Phocarctos hookeri) have recently been rep orted to be the longest and deepest-diving otariid. An unusually large prop ortion of dives exceeded a theoretical aerobic dive limit, predicted from e stimated oxygen stores and measurements of diving metabolic rate. We invest igated swimming speed, a key variable in both the management of oxygen stor es and foraging strategies, and its relation to diving behaviour in New Zea land sea lions. Diving behaviour was nearly continuous with short inter-div e intervals, Mean diving swimming speeds ranged from 1.6 to 2.4 m/s. Mean s urface swimming speeds ranged from 0.9 to 1.8 m/s and were significantly lo wer than diving speeds in all subjects. New Zealand sea lions spend signifi cant but variable amounts of time resting at the surface. Diving and swimmi ng speed patterns were consistent with foraging on the benthos. Time in the foraging zone was maintained in deeper dives by increasing dive duration. This increased duration cannot be accounted for by a decreased metabolic ra te resulting from slower swimming speeds, as speeds increased with the maxi mum depth of dives. Patterns of swimming speed and acceleration suggest the use of a gliding phase during descent. For most females, the extended dura tion of deeper dives did not impact on surface times, suggesting the use of aerobic metabolism. Females exhibited significantly slower swim speeds dur ing the bottom segments of foraging dives than during descent or ascent. Th ese findings suggest that swimming behaviour should be considered a critica l component when modelling energetic costs for diving animals.