No abstract

Knowledge of relationships between prey availability and predator performance is the key to using predators as indicators of the state of marine systems and to assessing potential consequences of competition between natural predators and man for common resources. Fluctuations in the abundance of Antarctic knll are beiieved to have a substantial influence on the reproductive performance of krill-dependent top predator species in the Southern Ocean; few quantifications of such interactions exist. At South Georgia, for 2 years in which acoustic surveys revealed a major difference in knll abundance, we compared diet, prov~sioning of offspring and breeding success in 4 main predator species ( 2 penguins, 2 albatrosses, with supporting data from Antarctic fur seal) whose dependence on k r d typically ranges from 20 to 90%. The 4-fold difference in krill biomass between 1986 (ca 30 g m-2) and 1994 (ca 7 g m-2) was accompanied by (1) an 88 to 90% reduction in the mass of krill in predator dlets (and some increase in the fish component), (2) greater prey diversity for most species, (3) reduced diet overlap between species and (4) a switch from krill to amphipods in macaroni penguin but no major dietary change in other species. Rates of provisioning of offspring decreased by 90% in gentoo penguin and 40 to 50% in the other 3 species; this was due to reduced meal size in penguins (by 90% in gentoo and 50% in macaroni) and to doubling of foraging trip duration in albatrosses. Breeding success was reduced by 50% in grey-headed albatross (the species least dependent on krill), by 90% in black-browed albatross and gentoo penguin (only 3 to 4 % of eggs producing fledged chicks) but by only 10% in macaroni penguin, presumably reflecting its a b 5 t y to switch to small prey unprofitable for the other species. However, all species (except for black-browed albatross), particularly macaroni penguin, produced fledglings significantly lighter than usual, probably affecting their subsequent survival. Some effects on adult survival could also be inferred. Our results show a coherent, though complex, pattern of within and between species similarities and differences. These mainly reflect the degree of dependence on krill, the feasibility of taking alternative prey and constraints on trip duration andlor meal size imposed by foraging adaptations (especially relating to travel speeds and diving abilities, whereby flightless divers and pelagic foragers differ markedly). The generality of these principles are explored through comparison with other studies, particularly of Shetland seabirds.

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At the sub-Antarctic island of' South Georgia 25 of the 29 breeding species are seabirds. Fifteen ot these have recently been studied in some detail. By examining the timing 01 their breeding seasons i t l i d their diet and feeding ecology (especially feeding techniques and potential foraging ranges), the nature ottheir ecological isolating mechanisms, and in particular the way in which they partition the resources of the marine environment, are reviewed.Although breeding season adaptations occur (winter breeding in Wandering Albatross and King Pcnguin; out 01' phase breeding in two species-pairs of small petrels) these are less important than tlillrrences in food and feeding ecology. There is a fundamental distinction between the nirhe 01' purbuit-diving species (mainly penguins) and the remainder which are basically surface-feeders. The two abundant krill-eating penguins show clear differences in feeding zones. Three albatrosses and a petrel lked mainly on squid and there are difterences in both the species and size of'the prey ofeach. The remaining seabirds chiefly take krill (although the giant petrels are extensive scavengers and sonic smaller petrels specialize o n copepods) and utilize different feeding methods and areas to do Various adaptations related to inshore and oftshore feeding zones are discussed. Although most species possess a combination of ecological isolating mechanisms additional evidence fbr the particular importance of dietary difterences is presented. \o.