CCAMLR

The swept area method was used to estimate biomass density changes in the stocks of five fish species Champsocephalus gunnari, Chaenocephalus aceratus, Pseudochaenichthys georgianus, Notothenia gibberifrons, and Notothenia rossii marmorata in the South Georgia area in the seasons of 1916/77-1986/87. In most of the seasons analysed, the estimates covered a near-bottom layer in about 1/6 of the area of the island shelf.
Assessment results point to considerable variations in fish biomass density, in the studied period. Estimates of fish stocks biomass ranged from 43 to 158 thous.tons. The changes in the biomass level are first of all a result of periodical fluctuations in the biomass density of C.gunnari stock. High biomass density appeared with 2-4 years intervals after recruiting an abundant year-class to the exploited stock. Biomass of other bottom species is more stable, especially in case of N.gibberifrons. Recently, an increase of the C.aceratus stock is observed and, at the same time, a gradual decline of the P.georgianus.

Regression functions of total length on carapace length and total length on removed carapace length were calculated for Antarctic krill Euphausia superba Dana for those classes of sex and maturity stage which can be discriminated in predator food samples. The use of removed carapace length as a predictor of total length is highly recommended for animals where total length measurement would be inaccurate, providing that the animal may be staged.

The arrival of Antarctic fur seals at a breeding beach on Bird Island. South Georgia. was studied over five consecutive breeding seasons, 1983 to 1987. Experienced bulls arrived first and established breeding territories on the beaches in anticipation of the arrival of the cows. Male arrival, which is less synchronous within years than female arrival, was significantly later in 1987 than in any other year. Female arrival, estimated by pup birth date where necessary, was highly synchronous: it usually started when 80% or more of potential territory sites were occupied by males. Cows arrived significantly later in 1984 and 1987 than in 1983, 1985 or 1986. The late arrival of both males and females in 1987 is attributed to unusually severe climatic conditions during the preceding winter, The late arrival and reduced fecundity of females in 1984 is attributed to markedly reduced food availability during the austral winter and summer of 1983. Males Were not affected in 1984 because they could move away from the area of reduced food availability earlier than females and because they have a more varied diet. Factors inftuencing the winter distribution, the timing and pattern of arrival and the breeding of male and female Antarctic fur seals are discussed.

The breeding biology of the gentoo penguin, Pygoscelis papua, was studied over a three-year period (1986-1988) at Bird Island, South Georgia, with particular reference to birds of known age or breeding experience. Laying date varied significantly between all three years, being three weeks later in 1987, when the breeding population decreased markedly. Factors involved in the timing of breeding are discussed. Within years egg-laying was highly synchronous: 95% of clutches were initiated in 14.5 days or less. The incubation period was 35 days and the laying interval, between the two eggs, 3.3-3.4 days. Chicks creched when 25-30 days old. and this varied between years. possibly related to food supply and chick growth. Chicks left the colony for the first time between 75 and 85 days of age. The breeding population at Bird Island decreased by 20% and increased by 84% in successive years during the study period. Breeding success (chicks fledged per egg laid) varied between 0.33 and 0.65 within colonies, but for the whole island was very consistent over the three years: 0.45, 0.51 and 0.47. Overall. colony differences were not correlated between years. Disturbance from Antarctic fur seals, Arctocephalus gazella, is suggested as the cause of consistently lower breeding success at one colony. Mean egg weight varied annually, and with age of the breeding bird, nest location and, in one year, with laying date. Young, first-time breeders laid smaller eggs and had lower breeding success compared to older, experienced birds, similar to other seabirds. However, they differed from other species in laying on average earlier than older birds. The relationship between age, egg weight, laying date and breeding success is discussed in relation to predation and seasonal food supply.