DIVING PERFORMANCE OF ADELIE PENGUINS IN RELATION TO FOOD AVAILABILITY IN FAST SEA-ICE AREAS - COMPARISON BETWEEN YEARS

1. Between-year variation in adelie penguin Pygoscelis adeliae foraging behaviour was studied using time-depth recorders at a colony in Lutzow-Holm Bay, Antarctica in the summers of 1990 and 1991 in areas where fast sea-ice remained. Poor chick survival and growth, long foraging trip duration and low meal delivery rate indicate that food availability was poor in 1991 when compared to 1990. However, mass of food brought to chicks per shore visit and rate of decrease of parental mass did not differ between these years. 2. In 1991, the penguins on average dived deeper (12.3 +/- 4.2 m) and for longer durations (1.9 +/- 0.2 min) than they did in 1990 (7.1 +/- 1.6 m depth and 1.5 +/- 0.2 min duration). However, time under water, the number of dive bouts per day and dive bout duration did not differ significantly between 1990 and 1991. 3. Foraging trip duration of birds carrying the time-depth recorders in mid to late January was longer in 1991 (861 +/- 419 min) than in 1990 (502 +/- 199 min) because travelling/searching time within trips in 1991 was about four times as long as it had been in 1990. Total dive bout duration within foraging trips did not differ between 1990 and 1991. These indicate that probability of locating a prey patch was lower in 1991 than in 1990, but once a prey patch was found total diving effort was similar to that in 1990. 4. These data show that when food was less abundant adelie penguin parents increased the duration of their foraging trips, thereby decreasing meal delivery rate, but did not increase total diving effort to compensate for poor food availability. The reasons for this might be that (i) their foraging sites were highly restricted to small ice holes or tide cracks; therefore, prey abundance within sites might decrease rapidly by depletion and the penguins might not be able to get more food even if they forage for a longer period; or (ii) they have a priority to maintain their body condition for their own future survival at the expense of offspring production.