Characterization of oceanographic habitat of cetaceans in the Southern Indian Ocean between 82° - 115° E: Cruise report from World Ocean Circulation Experiment (WOCE) I8S and I9S

Abstract

Cetacean surveys were conducted aboard the R.V. Knorr during World Ocean Circulation Experiment (WOCE) hydrolegs I8S, I9S in the Southern Indian Ocean from December 1, 1994 to January 19, 1995. A total of 186 sightings of cetaceans were obtained. Sperm whales (Physeter macrocephalus) were the most numerically abundant large cetacean south of the Polar Front. Sperm whales occurred directly over or in close proximity to complex bathymetry, such as the southeast edge of the Kerguelen Plateau. Minke whales (Balaenoptera acutorostrata) were the second most numerically dominant species. Eighty-five percent of minke whale sightings were recorded south of 60°S near the retreating ice edge. The highest density of minke whales occurred along the eastern flank of the Kerguelen Plateau, along a tongue of ice advected northward with the western boundary current. Migrating humpback whales (Megaptera novaeangliae) were observed as far south as the retreating ice (64°S). Over 50% of the humpback whale sightings were associated with the Kerguelen Plateau.

The highest density of cetaceans was observed along the southern flank of the Kerguelen Plateau. This distribution aligned with the mean position of the Southern Front of the Antarctic Circumpolar Current (ACC) and the southern water mass boundary (Southern Boundary) of the ACC. In this region the mean positions of the Southern Front and Southern Boundary of the ACC are similar to the pattern of ice retreat, due to the topographic steerage of currents around the Kerguelen Plateau and the presence of a western boundary current. The combination of complex bathymetry, high latitude penetration of the Southern Boundary of the ACC, shoaling of nutrient-rich, warm Upper Circumpolar Deep Water (UCDW) at the Southern Boundary, the presence of the marginal ice zone, and the pattern of ice retreat, generates uniquely favorable conditions for cetaceans at the southeast corner of the Kerguelen Plateau. Further, in this region the Southern Boundary of the ACC and the Antarctic Divergence occur in close proximity, enabling subsurface nutrient-rich UCDW to be upwelled or entrained into the surface mixed layer. Cetaceans may benefit from the cascade of trophic dynamics reliant on the nutrient enrichment from shoaled UCDW at high latitude and the productivity of the marginal ice edge.


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