ABSTRACT. The relationsh~ps between the foraging strategy of seabirds, hydrographic features and food availability are poorly understood. We investigated the movements at sea, time spent per oceanic sector, food intake, and d~e t of king pengulns Aptenodytespatagonicus in the Crozet Islands (Southern Indian Ocean) during summer, as a function of the position of major frontal zones. Fifteen trips at sea were monitored using satelhte transmitters over 3 austral summers (1992 to 1994). During each season, satellite transmitters were used in conjunction with stomach temperature recorders in order to investigate feeding activity. The at-sea distribution of klng penguins was closely related to the localisation of major hydrographic frontal systems. Intense prospecting areas were observed mainly in zones corresponding to the northern h i t of the Polar Front (50" to 51°S), southern limit of the Sub-Antarctic Front (44.50' to 45O S], and a zone between 47' and 48" S. During trips directed south, 2 distinct phases based on travelling speed were detected. The myctophids Electrons carlsbergi, Krefftichtys anderssoni and Protomyctophurn tenisoni dominated the diet. The est~mated average amount of food ingested per day at sea was 2.4 kg. Between 17 and 64 kg of food was captured during 7 to 25 d at sea. Approximately 80% of the food intake occurred during the first phase of the trip. Food intake was related to trip duration and relative amount of time spent in particular oceanic sectors. The sections 47' to 48" S and 48.5" to 50.50" S appeared particularly favorable for food intake, the latter coinciding with the northern limit of the Polar Front. King penguins fed ~ntensively on several distinct patches when traveling towards the Polar Front. The foraging range seems to be related to the foraging success during the first phase of the trip. The foraging strategy of king penguins during the summer favors displacements toward frontal zones where food availability is optimal.

SUMMARY It is generally assumed that air-breathing aquatic animals always choose the shortest route to minimize duration for transit between the surface and foraging depth in order to maximize the proportion of time spent foraging. However, empirical data indicate that the body angles of some diving animals are rarely vertical during descent and ascent. Why do they choose shallower body angles that result in longer descent and ascent durations? To investigate this question, we attached acceleration data loggers to eight female macaroni penguins, breeding on the Kerguelen Islands(48°45′–50°00′S,68°45′–70°58′E; South Indian Ocean), to record depth, two-dimensional acceleration (stroke cycle frequency and body angle)and temperature. We investigated how they controlled body angle and allocated their submerged time. The instrumented females performed multiple dives(N=6952) with a mean dive depth for each bird ranging from 24.5±28.5 m to 56.4±75.1 m. Mean body angles during descent and ascent were not vertical. There was large variation in mean descent and ascent angles for a given dive depth, which, in turn, caused large variation in descent and ascent duration. Body angles were significantly correlated with time spent at the bottom-phase of the dive. Birds that spent long periods at the bottom exhibited steep body angles during ascent and subsequent descent. By contrast, they adopted shallow body angles after they had short or no bottom phases. Our results suggest that macaroni penguins stay at the bottom longer after encountering a good prey patch and then travel to the surface at steep body angles. If they do not encounter prey, they discontinue the dive,without staying at the bottom, ascend at shallow body angles and descend at shallow body angles in a subsequent dive. A shallow body angle can increase the horizontal distance covered during a dive, contributing to the move into a more profitable area in the following dive. During the ascent, in particular,macaroni penguins stopped beating their flippers. The buoyantly gliding penguins can move horizontally with minimum stroking effort before reaching the surface.

We investigated the use of Antarctic waters by king penguins in a 2 yr study based on the satellite tracking of 10 penguins from the Crozet Islands (SW Indian Ocean). All the penguins travelled towards the pack ice, with 3 of them ending their journey at the edge between the marginal ice and the dense pack ice. The mean maximum foraging range and minimal distance travelled were 1620 and 4095 km, respectively. The effect of the satellite transmitter (PTT) attachment on foraging trip duration and colony attendance was much more important in winter in comparison to the summer. The penguins spent around 24% of their trip at sea in the marginal ice zone. They explored the ice-covered habitat non-randomly as revealed by compositional analysis. The marginal ice was more used than free ice and floes areas. The strategy of travelling towards the marginal ice zone during winter ensures that the penguins have access to predictable feeding areas at a time when food availability is very low in the polar frontal zone. The diet of king penguins when foraging in Antarctic waters is unknown but may be different to their summer food at the Polar Front. KEY WORDS: Feeding ecology · Satellite tracking · King penguins · Marginal ice zoneResale or republication not permitted without written consent of the publisher

SUMMARYUsing a newly developed data logger to measure acceleration, we demonstrate that free-ranging king and Adélie penguins only beat their flippers substantially during the first part of descent or when they were presumed to be chasing prey at the bottom of dives. Flipper beating stopped during the latter part of ascent: at 29±9 % (mean ± S.D.) of dive depth(mean dive depth=136.8±145.1 m, N=425 dives) in king penguins,and at 52±20 % of dive depth (mean dive depth=72.9±70.5 m, N=664 dives) in Adélie penguins. Propulsive swim speeds of both species were approximately 2 m s-1 during dives; however, a marked increase in speed, up to approximately 2.9 m s-1, sometimes occurred in king penguins during the passive ascending periods. During the prolonged ascending, oblique ascent angle and slowdown near the surface may represent one way to avoid the potential risk of decompression sickness. Biomechanical calculations for data from free-ranging king and Adélie penguins indicate that the air volume of the birds (respiratory system and plumage) can provide enough buoyancy for the passive ascent. When comparing the passive ascents for shallow and deep dives, there is a positive correlation between air volume and the depth of the dive. This suggests that penguins regulate their air volume to optimize the costs and benefits of buoyancy.