Foraging habitat and diving activity of lactating Subantarctic fur seals in relation to sea-surface temperatures at Amsterdam Island

This study investigates the foraging behaviour of lactating Subantarctic fu r seals Arctocephalus tropicalis breeding on Amsterdam Island, Indian Ocean , in relation to sea-surface temperatures (SST) of the surrounding waters o ver 3 consecutive breeding years (1995 to 1997). Foraging habitat and divin g activity were investigated using time depth temperature recorders (TDRs), deployed on 29 individuals during the first trip after parturition in Dece mber (n = 7), later in the summer (n = 13), and in winter (n = 9). Argos sa tellite transmitters (n = 4) and a direction recorder (n = 1) were also use d in the 1995 and 1996 austral summers, respectively. Sea temperatures reco rded by TDRs in conjunction with the IGOSS SST database were used as a loca tional cue to estimate the foraging range. The foraging habitat of lactatin g females was associated with the northern part of the Subtropical Front (S TF) (axial SST = 14.2 degrees C), where their main prey, myctophid fish, ar e 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 va ry significantly throughout the trip. Later in summer, when STF was south o f Amsterdam Island, most females travelled in a straight south-east directi on without diving (suggesting that they travelled regularly to reach a know n 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 r eturn trip to the colony. In winter, the 14 degrees C surface isotherm was 250 km north of Amsterdam Island and SST gradient was very low. Females inc reased their foraging range up to 530 km, and there was no evidence for fem ales concentrating their diving activity within a given area, suggesting th at they did not encounter dense patches of prey. In winter, seals also incr eased their diving effort probably in response to a decrease in food availa bility. Annual changes in SST surrounding Amsterdam Island, and in the surf ace temperature gradient of the STF appear to affect the time spent at sea, and the relative diving activity throughout the trips in summer. These res ults suggest that Subantarctic fur seals adjust their foraging behaviour ac cording to both seasonal and annual changes in oceanographic conditions, an d thus probably, food availability.