Utilisation of the oceanic habitat by king penguins over the annual cycle

The distribution and behaviour of foraging seabirds depend on the physical features of the ocean at different time and space scales, but little is known for penguins. We investigated the for- aging behaviour of king penguins in relation to oceanographic features over the birds' complete annual cycle. A total of 44 birds was followed between 1994 and 1997 at the Crozet Islands to moni- tor foraging habitat, diving behaviour, and sea temperature of the water column, using satellite- tracking and time-temperature-depth recorders (TDR) carried by the birds. The study included breeding in summer, the winter period of chick raising, and the post-moult period in spring. King penguins foraged in 2 specific regions in response to the seasonal changes in local prey availability. In summer, satellite-tracked birds during the incubating and brooding stages (n = 14) preferentially exploited the polar front located 340 to 450 km to the south of their breeding site. TDR-equipped birds (n = 12) also foraged at the polar front in summer as indicated by the vertical temperature pro- files. In autumn and winter, satellite tracks (n = 8) and sea temperature measurements of TDR- equipped birds (n = 8) showed that birds with creching chicks instead foraged in antarctic waters, with 70% of individuals reaching the latitude of the pack-ice limit (1600 km from the colony). This suggests better prey availability than in the polar frontal zone at that time. When the birds were at the latitude of the polar front, the thickness of the surface mixed layer (SML) ranged from 80 m in summer to 140 m in winter, the SML temperature was ~4°C and the thermocline had a mean maxi- mum gradient of -0.5°C for each 10 m depth. When the birds were at their most southerly position, the depth of the SML ranged from 100 m in autumn to ~150 m in winter, while its temperature ranged from -0.8 to 2°C. The temperature gradient of the thermocline showed an inversion in autumn, and this gradient was positive in winter (mean maximum gradient of 0.3°C for each 10 m depth). Except for spring birds (n = 4) and for 1 winter bird, where the SML exceeded the diving range, all TDR- equipped penguins (n = 19) dived preferentially in and below the depth of the thermocline, thereby minimising diving in the SML. Therefore, their prey may have been predictably concentrated below the SML through oceanographic processes. In king penguins, the strategy of going south could then have evolved in relation to the thinning of the SML towards the south at any time of the year.