Biological indices for monitoring population status of walrus evaluated with an individual-based model

Demographic parameters were investigated as potential biological indices of population status (i.e,, population size relative to ca. capacity) for wal rus using an individual, age-based population model. Density-dependent chan ges in juvenile survival rates, birth rates and maturation rates regulated growth in the simulated population. Because only qualitative knowledge of p opulation regulation is available for large, long-lived mammals, like walru s, different functional relationships between the regulatory parameters and population size were used to examine changes in potential biological indic es. Age-specific rates of survival and reproduction specified in the model yielded a maximum growth rate of 8%. Because survival rates are unknown for walrus, the model's growth rate should not be considered an estimate of th e maximum growth rate for walrus. Only birth rates, measurable as successfu l ovulation rates, appear to be relatively robust to the assumptions of pop ulation regulation incorporated in the model, and thus may be a potential i ndex of population status. However before biological indices can be incorpo rated into a management scheme for walrus, variability in parameter estimat es due to the availability bias of the harvest needs to be quantified and a measure or index of population size identified.