Managing ecosystem uncertainty: critical habitat and dietary overlap of top-predators in the Ross Sea

We summarize three types of data in order to increase appreciation among fishery managers of the close spatial and temporal ecological overlaps among top predators in the Ross Sea Shelf Ecosystem (RSShE). This includes data on diet, foraging behavior, and habitat use. Murphy (1995) demonstrated that space-time overlap is critical to predicting the degree to which a fishery might affect a food web. The fisheries that we contemplate are those for Antarctic toothfish and the Antarctic minke whale, though other species might also soon be exploited in the Ross Sea region. In addition to those two predators we also include other trophic competitors and (and in two cases predatory species): killer whale (type C), Weddell seal, Emperor penguin, Adélie penguin, and 4 species of flighted birds.
Using data from satellite tags attached to top predators that occur at colonies and haul outs along the coast of Victoria Land from 1990 through 2004, we summarize the foraging ranges from these sites and the habitats used for foraging. We also summarize data on diet and overlaps in foraging behavior among these predators from analyses of scats and stomach contents and time-depth-recorders collected from 1976 through 2002. Finally, we present results of ship-based surveys of birds and cetaceans made from 1976 through 1981. Though many of those species have not yet been studied using satellite telemetry, their diets have been investigated.
Most top predators in the Ross Sea feed at relatively great depths, perhaps because this affords them access to waters under sea ice, which persists in this region except for late summer. Three of them are able to exploit the entire water column of the shelf, with others foraging from near surface to mid-depths. The major geographic habitats used include waters that are or were part of the marginal ice zone that rings the Ross Sea Polynya during spring and summer when primary production is in full swing. Waters over shallow banks, especially in the western region, also appear to be important habitats. Even for colonies of these predators that are near the shelfbreak, their foraging efforts appear to be restricted to waters overlying the upper slope and shelf although deeper waters are well within range. In the RSShE, the main prey species eaten by most of the listed predators is the Antarctic silverfish, which is a major predator of ice krill. Based on frequency of occurrence in the diet, the prevalence of silverfish among diving predators averages 70% (range 45-95%) and among near-to-surface predators averages 31% (range 4-53%). The other main prey species of RSShE top predators is ice krill. Antarctic krill replaces ice krill in the predators’ diets over the Ross Sea continental slope and outer shelf waters.
The key, and perhaps critical, foraging habitats of the seals and penguins from the colonies and haul-outs studied so far along the Victoria Land coast occur almost entirely within CCAMLR statistical area SSRU 88.1J and the southern third of 88.1H, one of the main SSRUs for harvests of Antarctic toothfish. We make recommendations for research needs related to top predators, including further assessments of population size and diet (including studies of fatty acid composition) from autumn through early spring when sea ice is most extensive, and simultaneous tracking of toothfish and cetaceans, especially the toothfish-eating killer whale.