Method of Determining Natural Mortality in the Northern Fur Seal ( Callorhinus ursinus ) from Known Pups and Kill by Age and Sex

• Published in: Journal of the Fisheries Research Board of Canada
• Main Author: Lander, Robert H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1975
• Subjects: General Medicine; Alaska; Callorhinus ursinus; Northern fur seal
• Online Access: http://dx.doi.org/10.1139/f75-282; http://www.nrcresearchpress.com/doi/pdf/10.1139/f75-282

From known values of the number of northern fur seals, Callorhinus ursinus, born in a given year and the commercial kill of older animals of this species, by age and sex, a method is given for approximating natural mortality or survival 1) between times of birth and the first kill and 2) during the successive ages of kill. An example with hypothetical data indicates small relative error in these estimators, although bias may be considerable in certain of the associated age-specific estimators of abundance and exploitation rates. The problem of bias warrants further investigation. Estimates of natural survival from birth to the start of the kill at age 2 yr agree closely with independent tagging estimates for the 1964–66 year-classes of males from the Pnbilof Islands, Alaska. For this stock, further applications of the present method to the 1961–66 year-classes of males indicate natural survival of 31–42% during ages 0–2 yr and 84–89% annually during ages 2–5 yr.