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17 February 2010 Growth and reproduction of female New Zealand sea lions
Simon J. Childerhouse, Stephen M. Dawson, David J. Fletcher, Elisabeth Slooten, B. Louise Chilvers
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A sample of 834 female New Zealand sea lions (Phocarctos hookeri), which were aged and measured, was obtained between 1998 and 2001. In addition, the reproductive histories of 505 marked females from the Auckland Islands were recorded from 1998 to 2005. These data sets were used to investigate growth and reproductive rates. Length and weight ranged from 134 to 197 cm and 49 to 156 kg, respectively. A Gompertz growth model best described growth and predicted that females attained 90% of asymptotic length (161.7 cm) and weight (112.0) at ages 4 and 11 years, respectively. No significant differences were found in growth rates among years, nor between the 2 major breeding colonies in the Auckland Islands. Females reproduced between the ages of 3 and 26 years, with evidence of reproductive senescence starting at age 23 years. Although females up to age 28 years were observed, no females over 26 years were recorded as reproductive. Age-specific reproductive rate p(x) increased rapidly between ages 3 and 7 years, reached a plateau between ages 7 and 23 years, and then declined rapidly after age 23 years. Mean observed reproductive rate was p(x)3–28  =  0.67 (SE  =  0.01). This is the 1st robust estimate of reproductive rate for this species, is consistent with rates reported for other sea lions, and is considerably lower than assumed rates used in recent population modeling for this species. This calls into question the current method for estimating levels of sustainable bycatch. Low growth and reproductive rates are consistent with a population that is occupying a marginal foraging environment. These factors, along with a recent significant decline in pup production, suggest that current management is insufficient to ensure population stasis, let alone meet the statutory goal of recovery.

Simon J. Childerhouse, Stephen M. Dawson, David J. Fletcher, Elisabeth Slooten, and B. Louise Chilvers "Growth and reproduction of female New Zealand sea lions," Journal of Mammalogy 91(1), 165-176, (17 February 2010).
Received: 26 March 2009; Accepted: 1 July 2009; Published: 17 February 2010

New Zealand sea lion
Phocarctos hookeri
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