Adelie penguin population change in the pacific sector of Antarctica: relation to sea-ice extent and the Antarctic Circumpolar Current

One of the longest continuing data sets involving a marine organism in the Antarctic is that of annual estimates of breeding population size of Adelie penguins Pygoscelis adeliae at colonies on Ross Island, Ross Sea, 1959 to 1997. The sizes of these colonies have displayed significant interannual va riability during the 29-yr period. We hypothesized that changes are related to natural environmental factors; and used path analysis to analyze annual variation in population growth in relation to physical environmental facto rs during that part of the record with comparable sea-ice satellite imagery from 1973 to 1997. The Ross Sea sector of the Southern Ocean lying north o f Ross Island, from 150 degreesE to 1300 W, comprised our study area. Annua l population growth measured during summer was explained best, and inversel y, by the extent of sea-ice in the study area 5 winters earlier, and in som e way related to the Southern Oscillation. Analysis of a subset of the sea- ice data from 1979 to 1997 indicated strong correlations to ice conditions in the eastern portion of the study area (174 to 130 degreesW), and virtual ly no correlations to the western half (150 degreesE to 175 degreesW). This result supported other indirect evidence that the Ross Island penguins win ter in the eastern Ross Sea/western Amundsen Sea. A demographic model indic ated that variation in survival of juveniles and subadults might account fo r the observed population variation, and would also explain the 5-yr lag as 5 yr is the average age of recruitment to the summer breeding population. Extensive sea-ice during winter appears to reduce subadult survival, expres sed subsequently when these cohorts reach maturation. We hypothesize that e xtensive (more northerly) sea-ice Limits access of penguins to productive w aters known to occur south of the southern boundary of the Antarctic Circum polar Current, with starvation or increased predation disproportionately af fecting less-experienced birds. The observed patterns of penguin population change, including those preceding the satellite era, imply that sea-ice ex tent has changed significantly over recent decades.