The marginal value theorem models patch departure decisions for foraging animals when resources are unevenly distributed. A key component of these models is the decelerating energy gain function used to represent patch depletion. However, the within-patch gain function has rarely been assessed in marine predators. We evaluated the gain functions in foraging bouts of northern elephant seals, Mirounga angustirostris, using a long-term data set (2004–2012) that included complete foraging trips from 205 individual female northern elephant seals on 303 migrations as revealed by time–depth recorders and satellite tags (Argos System Inc.). Since the majority of putative prey capture attempts are associated with vertical excursions at the bottom of dives, we used vertical excursions to evaluate patch depletion across foraging bouts as defined using dive shapes. Rates of energy gain were measured using changes in mass and body composition across trips. Decelerating gain functions occurred in 83% of 77820 foraging bouts, with the remainder showing accelerating functions. Rates of patch depletion strongly influenced patch residence times. Despite wide variation between individual patches, mean deceleration exponents did not vary with year or season, suggesting that average rates of patch depletion were relatively stable across the study period. The mean duration and number of dives in foraging bouts showed little annual or seasonal variation; however, the mean rate of vertical excursions during foraging dives varied and predicted rates of energy gain across migrations. The relative mean consistency of individual diving behaviour despite wide variation in geoposition supports the idea that northern elephant seals have evolved a foraging strategy that buffers against short-term variation in prey abundance and optimizes energy acquisition across the duration of the migration.
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