Diving ontogeny and lunar responses in a highly migratory mammal, the northen fur seal Callorhinus ursinus

Diving ontogeny studies enable the examination of both the evolution of diving strategies and the physiological constraints and environmental factors determining foraging behaviour. Northern fur seal (NFS) Callorhinus ursinus pups that undertake far-ranging migrations in their first year are an ideal species for examining such factors. The diving behaviour of 64 NFS pups from 4 North American breeding sites was studied using satellite-dive recorders deployed on pups prior to weaning. Summarised diving activity (6 h histograms of dive depth and duration) was recorded during the pups’ first 8 mo at sea and transmitted via satellite. During the first month at sea, pups adopted the nocturnal diving patterns characteristic of adults, with average maximum nightly and crepuscular dive depths and durations exceeding daytime values by a factor of from 4 to 4.5. Diving capacity in terms of maximum depths (112 m) and durations (285 s) attained also increased linearly with age until ~8 to 10 mo of age. Overlaid on diving capability development was the significant influence of environmental cues, such as lunar phase, on migratory diving behaviour. During full moons, pups dived deeper and for longer periods than during other lunar phases, as pups likely mimicked the behaviour of their vertically migrating prey. These findings indicate that prey accessibility, particularly for younger pups with reduced diving capacity, may prove more challenging during higher lunar illumination periods.