Weddell Seal — Antarctic Cod Interactions in McMurdo Sound, Antarctica

The impact of Weddell Seals, Leptonychotes weddellii, on Antarctic marine fish communities appears to be influenced by the behaviour of the Seals, which concentrates predation temporally and spatially. Movements of prey species, possibly in response to the distribution of the Seals, may determine which members of the fish community are most affected by predation. The distribution and movements of the Antarctic Cod, Dissostichus mawsoni, were monitored under shore-fast ice near Ross Island, Antarctica, by set-line fishing. The distribution and the age of Weddell Seals were studied in the same area, in conjunction with long-term research dealing with the species’ population dynamics. During the austral spring adult Weddell Seals concentrate in discrete breeding colonies near the shore, while sub-adults remain scattered in small groups at isolated breathing holes throughout the fast-ice areas. Experimental fishing success for Antarctic Cod was related to distance of fishing sites from Weddell Seal breeding colonies. Adult Seals from both sides of McMurdo Sound leave their colonies in mid-December and gradually concentrate along the edge of the permanent ice-shelf, when fishing success decreases rapidly. The diet of Weddell Seals in November, January and February consists mostly of small nototheniid fish (Pleuragramma and Trematomus spp.), decapod and amphipod crustaceans, and cephalopods. Although Weddell Seals apparently exclude Antarctic Cod from their preferred fast-ice areas, the Cod do not appear to be an important food resource for more than a limited time in December.