Improvement in Survivorship: The Key for Population Recovery?

GRANDI, M.F.; DANS, S.L.; CRESPO, E.A.

ZOOLOGICAL STUDIES

ACAD SINICA INST ZOOLOGY

Lugar: Taiwan; Año: 2016 vol. 55

1021-5506

In northern Patagonia,commercial harvesting of South American sea lions, Otaria flavescens,from 1920 to 1960, decimated its population abundance. Population recovery wasnot immediate after hunting ceased in 1962. The population was stable until1989, and since then has grown at an annual rate of increase of 5.7%. Alongwith this growth there was an increase of the juvenile fraction and changes inthe social composition of colonies, which could be related to changes in somepopulation vital rates. The aim of this study was to analyze changes in thesurvivorship pattern of Otariaflavescens through time. The ultimategoal was to contribute to a better understanding of changes that could haveoperated on the ecosystem after the decline and recovery of one of the mainmarine top-predators in the southern South Atlantic Ocean. The comparisons of survivorshipcurves of males and females, obtained from the life tables of two periods withdifferent population trends: 1981-1987 (stationary) and 2000-2008 (recovering),showed that there were differences in survivorship between sexes, where recentfemale age-specific survival was higher than that of males at any age. Thecomparison of survivorship between periods showed differences in both sexes.Both juveniles and adults, both male and female, from the recent period showedhigher survival than those of the 1980?s decade. This improvement insurvivorship could be one of the essential factors that drove populationrecovery in the last decades. Here we discuss the possible hypotheses of whichfactors could have changed in the ecosystem to favour juvenile and adult survivorship,such as an increase in the availability of food recourses, a decrease of exogenousmortality causes, or a combination of both factors.