Survivorship of a declining population of southern elephant seals, Mirounga leonina, in relation to age, sex and cohort

This study quantified both the age- and sex-specific survival rates of juve niles and adults, and tested for interannual differences in age-specific su rvival rates of the southern elephant seal population at Marion Island. Pup s were tagged on an annual basis from 1983 onwards at Marion Island, and a consistent recapture program yielded data that was analysed using the softw are package MARK to obtain maximum-likelihood estimates of survival and cap ture probability. On average, Ist-year survival was 0.58 and 0.62, and surv ival rate averaged over the first 3 years of life, 0.69 and 0.74 for males and females, respectively. From years 4 to 9, the average survival rate was 0.66 and 0.75 for males and females, respectively. Survival estimates for elephant seals in their 10th-13th year are also presented, although these a re based on very small sample sizes. Averages of age-specific survival esti mates from the earlier (mostly 1983-1987 cohorts) and later (mostly 1988-19 92 cohorts) periods were compared and considerable reductions were observed in 4th- and 5th-year male survival, and 4th-year female survival. The comp aratively low adult survival is suggested as the proximate cause, and food limitation as deduced from the decline in survival of elephant seals with c omparatively high energetic demands as the ultimate cause behind the popula tion decline at Marion Island. Although not tied in with the decline of the population, 1987, 1990 and 1993 were identified as high-mortality years.