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, 7 (5), e36728

Foraging Behavior and Success of a Mesopelagic Predator in the Northeast Pacific Ocean: Insights From a Data-Rich Species, the Northern Elephant Seal


Foraging Behavior and Success of a Mesopelagic Predator in the Northeast Pacific Ocean: Insights From a Data-Rich Species, the Northern Elephant Seal

Patrick W Robinson et al. PLoS One.


The mesopelagic zone of the northeast Pacific Ocean is an important foraging habitat for many predators, yet few studies have addressed the factors driving basin-scale predator distributions or inter-annual variability in foraging and breeding success. Understanding these processes is critical to reveal how conditions at sea cascade to population-level effects. To begin addressing these challenging questions, we collected diving, tracking, foraging success, and natality data for 297 adult female northern elephant seal migrations from 2004 to 2010. During the longer post-molting migration, individual energy gain rates were significant predictors of pregnancy. At sea, seals focused their foraging effort along a narrow band corresponding to the boundary between the sub-arctic and sub-tropical gyres. In contrast to shallow-diving predators, elephant seals target the gyre-gyre boundary throughout the year rather than follow the southward winter migration of surface features, such as the Transition Zone Chlorophyll Front. We also assessed the impact of added transit costs by studying seals at a colony near the southern extent of the species' range, 1,150 km to the south. A much larger proportion of seals foraged locally, implying plasticity in foraging strategies and possibly prey type. While these findings are derived from a single species, the results may provide insight to the foraging patterns of many other meso-pelagic predators in the northeast Pacific Ocean.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.


Figure 1
Figure 1. Tracking data from 209 female northern elephant seals from 2004-2010.
The map includes 195 tracks from the Año Nuevo, CA, USA colony (red point) and 14 tracks from the Islas San Benito, B.C., Mexico colony (yellow point).
Figure 2
Figure 2. Trip duration for female northern elephant seals observed with (n = 98) and without (n = 17) a pup after the post-molting migration from 2004-2010.
Most females that skipped breeding returned outside of the typical breeding season (January – February).
Figure 3
Figure 3. Approximate location of dominant oceanographic features in the northeast Pacific Ocean.
The stippled region indicates the annual range of the Transition Zone Chlorophyll Front (TZCF). The location of the gyre-gyre boundary remains stable in contrast to the annual migration of the TZCF.
Figure 4
Figure 4. Monthly kernel density distribution of female northern elephant seals from the Año Nuevo, CA colony from 2004-2010.
Tracking data were regularized to hourly positions prior to analysis and only complete trips were included (n = 195). The black line shows the monthly position of the gyre-gyre boundary, estimated from the 170 cm absolute dynamic topography climatology contour. White points indicate the position of the Transition Zone Chlorophyll Front, estimated from the 0.2 mg/m3 contour. Oceanographic climatologies include data from 2004 through 2008.
Figure 5
Figure 5. Mean daytime dive depth for northern elephant seals from Año Nuevo, CA seals with a matched and complete diving and tracking record from 2004-2010 (n = 95).
Dives are shallower in the northern half of the sub-arctic gyre and coastal regions compared to the transition zone waters.
Figure 6
Figure 6. Hotspot analysis (Getis-Ord Gi* statistic) across all years of the study (2004-2010) for female northern elephant seals using two foraging metrics: number of drift dives per day and daily transit rate.
Areas in red indicate statistically significant clustering of foraging activity, independent of the number of seals present. Grid cells informed by only one seal were removed to avoid high leverage.
Figure 7
Figure 7. Temperature profile and female northern elephant seal density along a transect of the ∼163W meridian from 40N to 50N.
The temperature profile was created from TDR data between 28-July-2005 and 24-August-2005 (seal ID: 2005037; post-molting season). The 8°C isotherm, indicated with a black line, highlights the temperature inversion. The seal density was extracted from the inter-annual August kernel density (see fig. 5). The grey bar shows the position of the gyre-gyre boundary.

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