CCAMLR

This paper describes in detail a method for obtaining food samples from Pygoscelid penguins and some suggested modifications to CEMP Standard Method A8. It proposes a new method for draining the water from samples, an extension of the sampling period to include the guard phase, and that birds found to be empty of food be included in the data set if they are bona fide breeders. It also suggests that the first regurgitate of each sample be kept separate from the remainder because it often consists of organisms that have been collected adventitiously by the bird on its return journey to the colony. Previous papers which have reported diet composition on the basis of a single regurgitate per bird are likely to be biased toward neritic species (amphipods, Pleuragramma antarcticum and Euphausia crystallorophias) and may underestimate the importance of E. superba in the diet of penguins from some region

The diet of lactating female Antarctic fur seals Arctocephalus gazella at South Georgia was investigated during the pup rearing period (January - March) of 1991-1994. Antarctic Krill Euphausia superba was the main prey item, occurring in 88% of all scats (n=497), whereas fish occurred in 47% and squid in 5%. There was considerable intra and inter-annual variation in the characteristics of krill taken by fur seals. The distribution of krill sizes taken suggest that fur seals are not actively selecting particular sizes of krill and, therefore, that the krill in the diet reflects the krill available around South Georgia. The absence of group 3 krill (44-48 mm in length) in the South Georgia area, as indicated by there absence in the diet of seals, is suggested as a possible reason for low availability of krill and the subsequent reproductive failure among krill predators. The frequency of occurrence of fish was much higher than in previous studies; the pattern of fish consumption showed a consistent seasonal pattern in three of the four years studied. Of the total number of the myctophid Protomyctophum choriodon, the most numerous - fish taxon, 98% were taken between early February and the middle of March. Champsocephalus gunnari and Lepidonotothen larseni agg., which both feed on krill, dominated the fish component of the diet outside this period and together constituted 94% of the total estimated biomass of fish consumed. The intra and inter-annual variability in the diet of Antarctic fur seals emphasises the need for diet studies to be conducted during the entire pup-rearing periods of several years.

1. This study examined the costs of reproduction in terms of future survival and reproduction in female Antarctic fur seals from Bird Island, South Georgia. It used mark-recapture data from 11 consecutive years, including 3 years when several indices showed that food availability was well below average.
2. Population age structures were used, in conjuction with the measured age-specific survival rates, to estimate the rate of increase of the population as 10•7% per annum.
3. The average annual survival rate was 0•83 (SD = 0•10) with a range from 0•65 to 0•93. Survival rate showed no trend through time but was weakly correlated with pup growth rate, suggesting that it may be influenced by availability of food. Survival rate was unrelated to any other environmental or demographic parameter including population size.
4. There was no evidence of senescence. Survival rate was not related to year of birth or age, after accounting for variation due to pregnancy and calendar year. Survival was reduced as a result of pregnancy which accounted for 40–50% of adult female mortality. This effect was greatest in the age classes with the highest reproductive output (ages 5-8 years).
5. Mean pregnancy rate was 0•70 (SD = 0•11) with an interannual range of 0•59–0•88. Although females normally produced their first pups at age 3–4 years, pregnancy rate peaked at age 8 years and declined thereafter. Otherwise pregnancy rate was unrelated to the environmental or demographic variables we tested. Food availability during the pup-rearing period had no effect on pregnancy rate. 40–50% of failures to become pregnant related to animals having been pregnant in the previous year.
6. Reproduction incurs costs to females, in terms of reduced survival and future fecundity, and consequently, on average, females which survive longest tend to do so because they have lower fecundity.

During sustained bouts of diving, predators like fur seals may adjust the time they spend diving to maximise the time they can spend foraging and minimise the time spent at the surface between dives. To examine this, swimming speeds and the time allocated to different parts of the dive cycle were measured in 10 adult female Antarctic fur seals (Arctocephalus gazella) while they were foraging at sea. Mean swimming speeds during diving ranged from 1.32 to 1.99 ms-1 and 90% of diving swimming speeds within individuals were between 1 and 2.5 ms-1. This represented a narrower range of speed than was exhibited by animals when swimming at the surface. Swimming speeds were fastest during the descent and ascent phases of dives with a reduction in speed near the bottom of the dive, when the seals were assumed to be feeding on krill. Surface interval increased as a curvilinear function of dive duration and was influenced by diving swimming speed. This relationship was, on average, close to that predicted by a model (Houston & Carbone, Behav. Ecol., 3, 255-265) which suggested that the metabolic rate was greatest during the foraging phase of dives (5.9 times predicted BMR) than during the ascent or descent (3.6 time predicted BMR). Deep diving (>30 m) and high swim speeds also incurred costs in terms of reduced foraging time and may explain why mean dive depths for individuals were within the range of 12-33 m. These data broadly support the predictions of models of diving behaviour based on balancing the supply of, and demand for, oxygen and the principles that fur seals are attempting to maximise the time they spend within the foraging area.

The pattern of prey distribution can profoundly affect the foraging behavior and success of a predator. In pelagic marine ecosystems, where prey is often patchily distributed, predators must be able to adapt quickly to changes in the spatial patterning of prey. Antarctic fur seals feed primarily on krill, which is patchily distributed. When combined with information about swimming speed on the surface, the time taken for a fur seal to locate a new patch after leaving an old one is an indication of the distance between patches. The frequency distribution of intervals between bouts of foraging showed that fur seals foraged at two spatial distributions: (1) a fine-scale (median distance 0.18 - 0.27 km) represented by short (5 rain) travel durations. In a study lasting 5 years, the distributions of travel durations between bouts of feeding changed between years. These changes suggested either that the structure and/or the spatial distribution of krill swarms varied between years. The behavior of fur seals suggested that there was overall clumping of prey at the fine-scale but there was a more even spacing of prey patches at the meso-scale level. Only in one year of the study (1990/91) were there indications that fur seals had difficulty in finding enough food. Fur seal behavior suggested that there was no reduction in the number of prey patches available in that year but that prey patches were of poorer quality. The study showed how predator behavior can provide valuable information about the functional relationship between prey dispersion and predator performance.