The coastal and pelagic foraging behaviour of grey seals and southern elephant seals : a stable isotope approach
Whisker samples were collected from grey seals from North Rona and from Isle of May in 1997-1999, from southern elephant seals from Pensinsular Valdes, Patagonia, from 1998 and 1999 and from South American sea lions also from Pensinsular Valdes in 1999. The whiskers were sub-sampled along their length and analysed for stable isotopes of δ(^13)C and δ(^15)N. There were significant differences between the two populations of grey seal and between years for each of the populations. Isotopic variations seen along the length of some of the whiskers were greater than could be attributed to background variation, indicating some seasonal variation. Possible prey items were herring, sandeels, some species of flat fish and gadoids. A strong temporal shift in apparent prey choice was seen only for the Isle of May, and the shift over a period of three years diminished the difference between the two populations. This could reflect changing prey choice or prey abundance in the North Sea over this time period. Male and female southern elephant seals showed significant differences in their isotopic values, with the male seals showing three different isotopic clusters that were distinct from one another and from the female seals. Two of these groups showed higher S'^C and §'^N values than those of the females and the third group. South American sea lions showed significantly higher values of δ(^13)C and δ(^15)N than female and most male southern elephant seals, suggesting that they were feeding at a higher trophic level. The South American sea lions and the group of male southern elephant seals with the highest values of δ(^13)C and δ(^15)N were possibly eating similar prey species which could include various fish and elasmobranch species. The female seals were feeding on prey with an isotopic signature similar to that of krill.