Linking foraging behaviour of the northern elephant seal with oceanography and bathymetry at mesoscales

Abstract

We examined the foraging behaviour of 12 adult female northern elephant seals in relation to mesoscale oceanographic features between 1995 and 1997. Females were divided into 3 groups based on the geographic region to which they migrated ('coastal', 'coastal-oceanic' and 'oceanic'). We defined focal foraging areas (FFAs) using satellite telemetry and average daily transit rates of ≤0.4 m s-1, and examined characteristics of FFAs with respect to sea surface temperature (SST), sea surface temperature gradient (SSTg), sea level anomaly (SLA) and bathymetry using logistic regression models. Shallow bathymetry was significant for the coastal grouping of females and we examined the prevalence of benthic dives using a dive index ratio (DI), calculated by dividing mean maximum dive depth (m) by ocean depth (m) at each location for all 12 females. An additional 6 adult males were included in this analysis to quantify and elucidate differences in benthic foraging mode between the sexes. DI comparisons revealed significantly greater prevalence of benthic diving in FFAs in males than females (χ2 = 10.588, p < 0.01). However, one coastal female did show a higher occurrence of benthic dives in FFAs than other females, and had significantly greater rates of mass gain. The importance of each oceanographic variable differed between the 3 groups of females. Overall, model results showed SST to be the most influential factor related to FFAs. Additionally, 6 females had FFAs that were characterised by greater SSTg and higher SLA. These characteristics are consistent with anti-cyclonic eddies that occur throughout the range of elephant seals. Semi-permanent eddies, such as the Haida and Sitka eddies of the Alaska gyre, may be particularly important in determining FFAs for some female northern elephant seals. © Inter-Research 2007.