- Annual rate of increase ( r ) was estimated for the Hooker's sea lion (Phocarctos hookeri) using demographic models that incorporated age-specific estimates of reproductive rate (m,) and survival
In the absence of empirical estimates of natural mortality (n,), survival schedules were derived by combining a range of incidental mortality rates (h,) in commercial fisheries with estimates of n , for two species that exhibit similar life histories to Hooker's sea lions: northern fur seals (Callorhinus ursinus), model A, and Himalayan thar (Hemitragus jemlahicus), model B. Model A represented a population that would have a limited capacity for population increase; incidental mortality rates of mature females above about 1% would result in a population decline. Model B represented a population that, even in the absence of incidental mortalities, would just remain stable.
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If either of these models accurately represents the population demography of Hooker's sea lions, then these animals have limited capacity for population increase. They are constrained by relatively delayed maturity, moderate reproductive rates and short life spans.
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While absolute estimates of r from our models must be interpreted with caution, it is the responses of r to incidental mortalities and to increased natural mortality of pups that are most informative. Both models show r decreasing by 0.007-0.008 with a 0.01 increase in incidental mortalities of mature females. Both models also indicate that an increase in the natural mortality of pups by 0.027 will reduce r by 0.0055 or more.
5 . Given the paucity of available empirical data, our results suggest that incidental mortalities of Hooker's sea lions in the trawl fishery for arrow squid (Nototodarus sloanii) should be minimized if a population decline is to be avoided.