Effects of an erroneous natural mortality rate on a simple age-structured stock assessment

The abundance of many stocks is estimated by fitting an age-structured mode l to catch-at-age and relative abundance data from the commercial fishery a nd scientific surveys. The natural mortality rate used in the model is usua lly estimated externally and its value is uncertain. An erroneous natural m ortality rate will bias the stock size estimates obtained by fitting the mo del and will also bias the yield calculations that are done to choose a har vest rate and recommend quotas. This paper describes the general features o f both effects by analyzing a simple age-structured model fitted to artific ial data. It is shown that an erroneous natural mortality rate mainly affec ts the estimates of fishing mortality and hence abundance but not the estim ates of age-specific selectivity. Errors in estimated abundance and target harvest rate are always in the same direction, with the result that, in the short term, extremely high exploitation rates can be recommended (unintent ionally) in cases where the natural mortality rate is overestimated and his torical exploitation rates in the catch-at-age data are low. A conservative (low) estimate of natural mortality can avoid that danger. Long-term yield under either an F-MSY or F-35% strategy is not very sensitive to error in natural mortality rate unless it is grossly underestimated.