Estimation of population dynamics of the Kuril harbor seals off Cape Erimo using density dependent age-structured production models

Background/Question/Methods

The population of Kuril harbor seals off Cape Erimo in northern Japan had dramatically declined by the 1970s due to overhunting, and it had once faced with a risk of extinction. Since then, owing to protection measures, the population size has shown a steady recovery while the damage to set net fishery by the seals has also increased. Conservation of the population should of course be prioritized, but it is also necessary to develop a resource management strategy focused to achieve a balanced objective between the conservation of population and mitigation of the damage to fishery. For this purpose, assessment works are required to know the carrying capacity and the current level of depletion of the population. Fortunately, surveys of the harbor seal population have been continuously conducted both during the breeding and molting seasons, over a long period. Based on the observations for population indices as well as partial information on the extent of past bycatch, we constructed density-dependent age-structured production models and then estimated parameters associated with the models through maximizing a joint likelihood function from multiple series of observations. In addition, we conducted simulation studies to evaluate possible management procedures for the population. 

Results/Conclusions

Results of the maximum likelihood estimation showed that the models used in the analyses fitted well to the data. As the estimation result, we found that the population level has exceeded at least 60% of the carrying capacity though the extent of recovery slightly depends on the model assumption. Regarding the future projection in the simulation study under the assumption of a stochastic stock-recruitment relationship, the population size will still increase at the rate of current level of bycatch. The simulation results also showed that some possible future management procedures involving culling adults while avoiding unintentional bycatch of yearling animals would be effective to achieve the balance objective for the conservation and management of the Kuril harbor seals off Cape Erimo.