In order to understand interspecific trophic relationships among top predators, we analyzed diet information on nine bird and two seal species collected in the austral summer from 1996 to 2000 at South Orkney Islands. Overall, the diet of most of the predators was mainly composed of krill, fish or penguins. The re-occurrence of prey among predators was intermediate and ranged from 25.3 to 36.7 and fish, krill and squid re-occurred most frequently. The re-occurrence of fish among predator pairs was low and ranged from 8.1 to 28.1. The species that re-occurred most frequently were the nototheniids Gobionotothen gibberifrons, Nototheniops nudifrons and Nototheniops nybelini, and the myctophid Electrona antarctica. Prey overlap was greatest between Chinstrap and Adélie penguins. Most predator pairs had high overlap of fish prey. Predators that could forage on demersal or water column prey had yearly variable diets. This variability may be explained by fluctuations in krill availability. In years when krill is scarce (e.g. 2000), these predators can diversify their diet by increasing the consumption of fish, which increases the re-occurrence of these preys in the diets. Our samples suggest the recovery of G. gibberifrons stocks around the South Orkney Islands and draw the attention to the potential increase of interspecific food competition among predators under scenarios of decreasing krill availability.
Dr María Mercedes Santos (Argentina)
Dr Enrique Marschoff (Argentina)