Foraging habitat and diving activity of lactating Subantarctic fur seals in relation to sea-surface temperatures on Amsterdam Island
Résumé
This study investigates the foraging behaviour of lactating Subantarctic fur seals Arctocephalus tropicalis breeding on Amsterdam Island, Indian Ocean, in relation to sea-surface temperatures (SST) of the surrounding waters over 3 consecutive breeding years (1995 to 1997). Foraging habitat and diving activity were investigated using time depth temperature recorders (TDRs), deployed on 29 individuals during the first trip after parturition in December (n = 7), later in the summer (n = 13), and in winter (n = 9). Argos satellite transmitters (n = 4) and a direction recorder (n = 1) were also used in the 1995 and 1996 austral summers, respectively. Sea temperatures recorded by TDRs in conjunction with the IGOSS SST database were used as a locational cue to estimate the foraging range. The foraging habitat of lactating females was associated with the northern part of the Subtropical Front (STF) (axial SST = 14.2°C), where their main prey, myctophid fish, are known to be abundant. The organisation of the foraging trips, in terms of diving activity, showed seasonal changes but remained similar among years. During the first trip after parturition, females foraged within the STF, 60 to 130 km from the colony, and exhibited a diving activity that did not vary significantly throughout the trip. Later in summer, when STF was south of Amsterdam Island, most females travelled in a straight south-east direction without diving (suggesting that they travelled regularly to reach a known area). They then concentrated their diving activity in the middle of the STF during 50% of the foraging trip duration, and continued diving on the return trip to the colony. In winter, the 14°C surface isotherm was 250 km north of Amsterdam Island and SST gradient was very low. Females increased their foraging range up to 530 km, and there was no evidence for females concentrating their diving activity within a given area, suggesting that they did not encounter dense patches of prey. In winter, seals also increased their diving effort probably in response to a decrease in food availability. Annual changes in SST surrounding Amsterdam Island, and in the surface temperature gradient of the STF appear to affect the time spent at sea, and the relative diving activity throughout the trips in summer. These results suggest that Subantarctic fur seals adjust their foraging behaviour according to both seasonal and annual changes in oceanographic conditions, and thus probably, food availability.