Evaluating Gain Functions in Foraging Bouts Using Vertical Excursions in Northern Elephant Seals

• Main Author: Ferraro, Michelle
• Format: Master Thesis
• Language: English
• Published: Sonoma State University 2016
• Subjects: Elephant Seal; Elephant Seals
• Online Access: http://hdl.handle.net/10211.3/175356

The marginal value theorem is used to model patch departure decisions in foragers using patchily distributed resources. A key component of these models is the decelerating energy gain function used to represent patch depletion. The form of within-patch gain functions have rarely been assessed in marine predators. We evaluated the gain functions in foraging bouts of northern elephant seals using a long-term dataset (2004-2012) that includes complete foraging trips from 205 individual female northern elephant seals on 303 migrations as revealed by TDRs and ARGOS satellite tags. Previous work has shown that 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 were fit in 83% of 77,820 bouts with the remainder showing accelerating functions. Despite wide variation for individual patches, mean deceleration constants did not vary with year or season. This suggests that average rates of patch depletion were relatively stable across the study period providing an opportunity for seals to develop and use optimal patch departure rules. The mean duration and number of dives in foraging bouts showed little annual or seasonal variation. However, the mean rate of vertical excursions during dives varied and predicted rates of energy gain across migrations. These results help to explain the relative consistency of individual diving behavior despite wide variation in geoposition and support the idea that the northern elephant seal foraging strategy buffers against short-term variation in prey abundance. These data suggest northern elephant seals are well-suited to provide a strong test of the marginal value theorem in predators foraging over a wide spatial and temporal scale. Ferraro, Michelle. 2016. Evaluating Gain Functions in Foraging Bouts ...