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

Abstract

This study quantified both the age- and sex-specific survival rates of juveniles and adults, and tested for interannual differences in age-specific survival rates of the southern elephant seal population at Marion Island. Pups were tagged on an annual basis from 1983 onwards at Marion Island, and a consistent recapture program yielded data that was analysed using the software package MARK to obtain maximum-likelihood estimates of survival and capture probability. On average, 1st-year survival was 0.58 and 0.62, and survival 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 are based on very small sample sizes. Averages of age-specific survival estimates from the earlier (mostly 1983-1987 cohorts) and later (mostly 1988-1992 cohorts) periods were compared and considerable reductions were observed in 4th- and 5th-year male survival, and 4th-year female survival. The comparatively low adult survival is suggested as the proximate cause, and food limitation as deduced from the decline in survival of elephant seals with comparatively high energetic demands as the ultimate cause behind the population decline at Marion Island. Although not tied in with the decline of the population, 1987, 1990 and 1993 were identified as high-mortality years.

Evidence for density dependent population regulation in southern elephant seals in the southern Indian Ocean

“…Food resources are probably a limiting resource in our study population as has been previously speculated (Pistorius et al 1999b;McMahon et al 2005). Adult female southern elephant seals haul out for between three to four weeks during both the moulting and breeding seasons.…”

“…Several research papers have stemmed from this long-term data series detailing temporal patterns in survival (e.g. Pistorius et al 1999b;McMahon et al 2003;Pistorius et al 2004) and fecundity (Pistorius et al 2001).…”

Cohort effects and spatial variation in age-specific survival of Steller sea lions from southeastern Alaska

“…We expected lower survival of males than females for this polygynous, sexually-dimorphic pinniped (Pistorius et al 1999, Pendleton et al 2006). Sex differences in survival have been observed for California and New Zealand sea lions but only among adults and not juveniles (Hernandez-Camacho et al 2008, Chilvers and.…”

Advances in New Zealand mammalogy 1990–2000: Pinnipeds

“…Preweaning mortality was around 5% (Hindell & Burton 1987;McMahon et al 1999b). By contrast, on Marion Island, where the population declined at 2.5% a year from 1986 to 1991, and 4.3% from 1991-1997, survivorship from 1-3 years was only 69% for males and 74% for females; from 4-9 years, male survivorship was 66% and female 75% (Pistorius et al 1999). Adult mortality appears to be an important component of the decline at Marion Island.…”

“…At Marion Island males do not generally live beyond 13 years, and 93% of females do not reach 14 years (Hindell 1991;Pistorius et al 1999). However, females branded as pups on Macquarie Island have been seen alive and still successfully breeding at 23 years of age (Hindell & Little 1988).…”