The method of refining the natural mortality coefficient is based on the notion that the free component of the regression equation between the mean-weighted coefficient of natural mortality as defined by the VPA and fishing effort must be equal to zero if the natural mortality coefficient has been correctly identified and something other than zero if it has not been correctly chosen.

As a result of this mathematical modelling study using C. gunnari in Area 48.3 as an example, it was shown that the proposed method facilitates such a refinement. When a mistake is made in the choice of natural mortality coefficient, the estimate of the free component in the equation will be other than zero. A bias in the estimate can be positive if the assumed natural mortality coefficient is less than the “true" value and close to zero or slightly greater than zero if the assumed value is greater than the "true" one

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WG-FSA-91/25

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