Abstract
The natural mortality rate (M) of a fish stock is typically highly influential on the outcome of age-structured stock assessment models, but at the same time extremely difficult to estimate. In data-limited stock assessments, M usually relies on a range of empirically or theoretically derived M estimates, which can vary vastly. This article aims at evaluating the impact of this variability in M using seven Mediterranean stocks as case studies of statistical catch-at-age assessments for information-limited fisheries. The two main bodies carrying out stock assessments in the Mediterranean and Black Seas are European Union’s Scientific Technical Economic Committee for Fisheries (STECF) and Food and Agriculture Organization’s General Fisheries Commission for the Mediterranean (GFCM). Current advice in terms of fishing mortality levels is based on a single “best” M assumption which is agreed by stock assessment expert working groups, but uncertainty about M is not taken into consideration. Our results demonstrate that not accounting for the uncertainty surrounding M during the assessment process can lead to strong underestimation or overestimation of fishing mortality, potentially biasing the management process. We recommend carrying out relevant sensitivity analyses to improve stock assessment and fisheries management in data-limited areas such as the Mediterranean basin.
Highlights
The natural mortality rate (M) is a key parameter for modeling age-structured fish population dynamics
Our results confirm that the choice of the M vector has a major impact on the output of stock assessment models
When considering the median stock status across all stock assessment outputs based on all 18 M scenarios, none of the stocks would be sustainable, and four out of the seven stocks would be in severe overfishing, suggesting that choosing a single M scenario from the extremities of the spectrum is associated with high risk of misclassifying the stock status
Summary
The natural mortality rate (M) is a key parameter for modeling age-structured fish population dynamics. Fishing mortality (F), the main concern for fisheries managers, is commonly estimated by deducting M from an estimate of total mortality (Z) (Quinn and Deriso, 1999; Haddon, 2011). As a result, both the perceived stock status and the associated fisheries advice rely greatly on the chosen value of M. In contrast to several other stock assessment parameters that describe somatic growth, maturation and longevity, M is rarely directly estimable from the available data of exploited stocks, as M is essentially confounded with fishing mortality F and recruitment
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