Résumé
The at-sea behaviour of marine predators is often described based on changes in behavioural states, such as transit, searching, and feeding. However, to distinguish between these behaviours, it is necessary to know the actual functions of the behaviours recorded. Specifically, to understand the foraging behaviour of marine predators, it is necessary to measure prey consumption. Therefore, the at-sea feeding behaviour of northern elephant seals (N = 13) was examined using satellite transmitters, time-depth recorders, and stomach temperature recorders. In addition, stomach temperature telemetry allowed for the validation of indirect measures of feeding behaviour used for marine predators, including decreases in transit rate and changes in dive shape.
Feeding data were recorded for the early phase of the migration (2.2-21 days). The first feeding events occurred shortly after animals departed (4.0 +/- 1.5 h) and close to the rookery (58.6 +/- 21.9 km), but these feedings were followed by extended periods without prey consumption (14.5 +/- 2.5 h). Continuous (bout) feeding did not occur until on average 7.5 +/- 1.8 days after the females left the rookery. Females showed significant differences in the feeding rate while feeding in a bout (1.3-2.1 feeding events hour(-1)).
There was a significant negative relationship between interpolated transit rate and feeding events (r(2) = 0.62, P < 0.01). Feeding, which was associated with all dive types, occurred most often during the foraging type dive shape (74.2%). Finally, successful feeding only occurred between 18-24% of the time when females displayed the foraging type dive shape suggesting that the use of dive shape alone, while indicative of behaviours associated with foraging (searching and catching prey) overestimates actual feeding behaviour.
This study showed females not only feed extensively during the early migration, but there was individual variation in both foraging locations and foraging success. In addition, by combining direct and indirect measures of feeding, this study has provided support for the use of foraging indicators in marine predators.