The sensitivity of stock assessment’s outcomes to the variation of Natural Mortality (M) in the Spanish Mediterranean fisheries

Abstract

Stock assessments involve statistical and mathematical methods to make quantitative statements about the status of harvested populations and predictions about how they are likely to respond to alternative management choices. There are often considerable uncertainties in the population parameter estimates used in the assessment models, particularly for parameters such as growth rates, Natural Mortality, Fishing Mortality patterns and spawning stock biomass and its relationships with recruitment. Assumptions recurrently have to be made in order to facilitate the tasks for the decision-makers but also for built scientific bases for fisheries management advice. A strong focus on natural selection factors such as natural mortality may be necessary to better understand the stock’s behaviour as well as the relationship that exists with fishing mortality, so defining exploitation rates. As a result, to be able to develop a vision of a good management plan in the best estimate of the reference points which are considered as a scientific basis and necessary step for the decision-making process. This parameter is treated almost like an externally defined parameter which corresponds for the most of time to 0.2/ year. Regarding this, the present case of study, which focus on the Spanish Mediterranean trawl fisheries, has been undertaken to describe the importance of the natural mortality rate (M) in the stock assessment process and how this could be sensitive to change of its results. Following an argued method by the GFCM and used by the majority of the Mediterranean countries, the XSA was run with different values of M calculated with methods depending or not to age, using the FLR library, to test the sensitivity to this parameter and then describe possible changes in management advice.