In situ measures of foraging success and prey encounter reveal marine habitat-dependent search strategies
Thums, M and Bradshaw, CJA and Hindell, MA, In situ measures of foraging success and prey encounter reveal marine habitat-dependent search strategies, Ecology, 92, (6) pp. 1258-1270. ISSN 0012-9658 (2011) [Refereed Article]
Predators are thought to reduce travel speed and increase turning rate in areas
where resources are relatively more abundant, a behavior termed "area-restricted search."
However, evidence for this is rare, and few empirical data exist for large predators. Animals
exhibiting foraging site fidelity could also be spatially aware of suitable feeding areas based on
prior experience; changes in movement patterns might therefore arise from the anticipation of
higher prey density. We tested the hypothesis that regions of area-restricted search were
associated with a higher number of daily speed spikes (a proxy for potential prey encounter
rate) and foraging success in southern elephant seals (Mirounga leonina), a species exhibiting
both area-restricted searches and high interannual foraging site fidelity. We used onshore
morphological measurements and diving data from archival tags deployed during winter
foraging trips. Foraging success was inferred from in situ changes in relative lipid content
derived from measured changes in buoyancy, and first-passage time analysis was used to
identify area-restricted search behavior. Seals exhibited relatively direct southerly movement
on average, with intensive search behavior predominantly located at the distal end of tracks.
The probability of being in search mode was positively related to changes in relative lipid
content; thus, intensively searched areas were associated with the highest foraging success.
However, there was high foraging success during the outward transit even though seals moved
through quickly without slowing down and increasing turning rate to exploit these areas. In
addition, the probability of being in search mode was negatively related to the number of daily
speed spikes. These results suggest that movement patterns represent a response to prior
expectation of the location of predictable and profitable resources. Shelf habitat was 49 times
more profitable than the other habitats, emphasizing the importance of the East Antarctic
shelf for this and other predators in the region. We have provided rare empirical data with
which to investigate the relationship between predator foraging strategy and prey encounter/
foraging success, underlining the importance of inferring the timing and spatial arrangement
of successful food acquisition for interpreting foraging strategies correctly.
area-restricted search, first-passage time, foraging success, Macquarie Island, Australia, Mirounga leonina, movement patterns, predictability, profitability, southern elephant seal