Benthic and pelagic dives: a new foraging behaviour in rockhopper penguins

The pattern and characteristics of diving of 55 daily foraging trips performed by 16 female southern rockhopper penguins Eudyptes chrysocome filholi were studied in coastal waters of Kerguelen Archipelago during the guard stage. Diving patterns and dive profiles indicated that birds used 2 foraging behaviours. First, they performed typical pelagic dives, as previously decribed for other penguin species. Second, they also performed series of consecutive square-wave dives reaching similar maximum depths, with no deeper dives within the series, all criteria which characterized benthic dives. Two groups of benthic and pelagic dives were subsequently selected to compare their parameters. In agreement with optimization concepts in foraging theory, rockhopper penguins maximize bottom time (= feeding time) of benthic dives through an increase in both descent and ascent rates, thus minimizing transit time between the sea surface and the bottom. Regardless of dive depth, bottom time was longer and diving efficiency higher in benthic dives than in pelagic ones. Penguins were also more active during benthic dives, as indicated by higher numbers of depth and light wiggles at the bottom of these dives. Bathymetry and dive depth indicate that penguins were able to reach about 80 % of the sea floor surrounding the colony. Abrupt changes in dive depth within series of benthic dives were identical in height to the thickness of lava flows, the main geological features of the landscape, strongly suggesting that birds followed the bottom topography at a fine scale. Dietary analysis showed that rockhopper penguins fed upon benthic prey (a few fish and the mysid Mysidetes morbihanensis) and pelagic organisms, including the major item Euphausia vallentini. There was a positive linear relationship between the mass of food brought ashore and an index of the proportion of benthic dives during the daily trips, thus emphasizing the importance for rockhopper penguins living in a coastal marine environment of feeding on pelagic migrators trapped at or near the sea floor during the day.