CCAMLR

Data on breeding population size and breeding success in gentoo penguins at Bird Island, South Georgia from 1977-1992 are used, in conjunction with empirical (and some hypothetical) data on survival and recruitment rates, to model the fluctuations in breeding populations, taking account of variations introduced by good and bad years (as classified on the basis of breeding success). There is generally good agreement between observed and predicted breeding populations, except in four years, when major population changes (large decreases followed by substantial increases) occurred. Three of these years were associated with reduced availability of krill, one with very cold winter and spring conditions. Comparing model and reality indicates that deferred breeding could account for the discrepancy in one year, and for part of the differences in two other years, when mortality rates must also have been higher. In the remaining year, when the increase in population greatly exceeded the preceding decrease, it is likely that some immigration occurred. Detailed field studies from 1987-1991 established that the population decline in 1988 was indeed attributable to substantial deferred breeding coupled with higher rates of adult mortality. Emigration was most unlikely to be involved; no data are available on immigration. Gentoo penguin population dynamics are disproportionately effected by the consequences of infrequent bad years; any increase, natural or artificial, in the frequency of such events might have serious consequences for population trends.

An activity recorder weighing 24 g with on-board data storage, designed to record data relating to seabird activity and behaviour at sea, is described. The principles, the design specification and the circuit description of the device are presented, together with data from field tests on Wandering Albatrosses Diomedea exulans to illustrate performance.

Published in Ornis Scandinavica 00-0 (1992) Ornis 23795/1433

Population dynamics of Black-browed and Grey-headed Albatrosses were studied at Bird Island, South Georgia for 17 consecutive years (1975-1991). Over this period almost all the Grey-headed Albatross colonies decreased, at an average rate of 1.8% per annum. Although the total Black-browed Albatross population increased (at 0.8% p.a.), 14 of the 23 colonies (including both study colonies) decreased. Black-browed Albatrosses follow an annual breeding cycle, with over 80% of birds successful in rearing a chick and 75% of those failing to do so returning to breed the next year, 5- 10% of both categories delaying one further year (even when still paired). Grey-headed Albatrosses are essentially biennial,

Estimates are provided for crabeater seal life-history parameters, to be incorporated into a simple model of the functional relationships between krill escapement and crabeater seal demographic performance. The crabeater seal parameters were estimated from seals collected near the Antarctic Peninsula between 1964 and 1990. Average annual survival rate of adults was estimated to be 0.93. Age at sexual maturity was estimated to be 3.8 years. Of 44 annual estimates of historical cohort strength, 16 were judged to represent “ good" years for demographic performance, 18 as "poor" years, and 10 as "bad" years.