How elephant seals (Mirounga leonina) adjust their fine scale horizontal movement and diving behaviour in relation to prey encounter rate
Résumé
Understanding the diving behaviour of diving predators in relation to concomitant prey distribution
could have major practical applications in conservation biology by allowing the
assessment of how changes in fine scale prey distribution impact foraging efficiency and
ultimately population dynamics. The southern elephant seal (Mirounga leonina, hereafter
SES), the largest phocid, is a major predator of the southern ocean feeding on myctophids
and cephalopods. Because of its large size it can carry bio-loggers with minimal disturbance.
Moreover, it has great diving abilities and a wide foraging habitat. Thus, the SES is a well
suited model species to study predator diving behaviour and the distribution of ecologically
important prey species in the Southern Ocean. In this study, we examined how SESs adjust
their diving behaviour and horizontal movements in response to fine scale prey encounter
densities using high resolution accelerometers, magnetometers, pressure sensors and
GPS loggers. When high prey encounter rates were encountered, animals responded by (1)
diving and returning to the surface with steeper angles, reducing the duration of transit dive
phases (thus improving dive efficiency), and (2) exhibiting more horizontally and vertically
sinuous bottom phases. In these cases, the distance travelled horizontally at the surface
was reduced. This behaviour is likely to counteract horizontal displacement from water currents,
as they try to remain within favourable prey patches. The prey encounter rate at the
bottom of dives decreased with increasing diving depth, suggesting a combined effect of
decreased accessibility and prey density with increasing depth. Prey encounter rate also
decreased when the bottom phases of dives were spread across larger vertical extents of
the water column. This result suggests that the vertical aggregation of prey can regulate
prey density, and as a consequence impact the foraging success of SESs. To our knowledge,
this is one of only a handful of studies showing how the vertical distributions and structure
of prey fields influence the prey encounter rates of a diving predator.