The effect of natural mortality on the estimation of stock state parameters and derived references for sustainable fisheries management

The present study focuses on sensitivity analyses regarding the effects of various assumptions about the magnitude of natural mortality (M) on resulting stock assessment parameters and derived references for sustainable fisheries management. The results revealed that the estimated exploitation rate is decreasing and the stock size is increasing with increasing M. The recommended and internationally agreed fisheries management references of sustainable exploitation F0.1 and Fmsy are also found to sensitively react to changes in M. Both F0.1 and Fmsy increase with increasing M. All simulations are based on data from the Baltic sprat (Sub-divisions 22-32), which has historically undergone quite large changes in M. Nevertheless, the maximum sustainable yield (MSY) is demonstrated to be a rather robust estimator over a wide range of M, including species at a rather low trophic level. The trend to underestimate fishing mortality and to overestimate the stock size with high M might deliver, in comparison with actual catches, a positively biased perception of the state of the stock and its productivity. The elevated risk for sustainable fisheries even increases when underestimated fishing mortalities are compared with overestimated management references of exploitation, like F0.1 and Fmsy. It is recommended to base M assumptions in the assessment of exploited resources and the advisory process to fisheries management to the longevity of the species concerned, if no quantitative information about M is available. Furthermore, M should account for the different ontogenetic stages and for changes in fish condition if observed.JRC.G.4-Maritime affair