Relations among foraging, disturbances to foraging, and vital rates in southern elephant seals

Background/Question/Methods Southern elephant seals show a limited range of behaviors, simplifying the possible links between changes in individual behavior due to disturbance and impacts these have on the population’s vital rates. As capital breeders the seals go on extended foraging trips at sea to build lipid reserves before returning to land to molt or breed. Disturbances that limit the seal’s ability to forage will also reduce their lipid stores, which, in the case of females, will lower their pup’s weaning mass and thus its survival. Since southern elephant seal populations are most sensitive to changes in this demographic rate, this could lead to changes at the level of the population. We built a state-space model describing elephant seal behavior while at sea and fit the model to data collected from satellite tags. The output from the model was then used to simulate the possible effects of disturbance on individuals and vital rates.

Results/Conclusions Our model successfully estimated the lipid mass of female southern elephant seals while at sea, under certain assumptions pertaining to the females’ lean mass. Given a viable predictor of elephant seal condition, we simulated disturbance by excluding females from preferred feeding grounds, which would result in increased movement and decreased energy intake. Increasing the duration of the disturbance successively decreased female lipid mass. This led to a decrease in pup weaning mass, reducing pup survival. Large changes in maternal lipid mass were needed to significantly reduce pup weaning mass, so pup survival did not fluctuate widely. The duration of the disturbance, combined with the proportion of the population exposed to its effects, determined the magnitude of the impact on the vital rate for the population. Relatively small declines in overall pup survival resulting from realistic disturbance scenarios have the potential to negatively impact the population. This has conservation implications, since widespread, minor disturbances could affect species abundance.