Assessing natural mortality of anchovy from surveys' population and biomass estimates
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In ordinary catch at age models, natural mortality conditions and determines the catchabilities at age obtained for the surveys which tune the assessments. For the same reason, inferring the Natural mortality of a fish stock from surveys’ estimates, require some assumption of the survey catchabilities at age. The anchovy fishery in the Bay of Biscay has been closed since 2005 up to 2010, due to low biomass levels. In the mean time, and since 1989, the population has been directly monitored by two independent surveys, acoustic and egg (DEPM) surveys, which supplied the basic information for the assessment of this stock carried out by ICES. The closure of the fishery supposes a major contrast on total mortality levels affecting the population in comparison with the former period of exploitation, suitable to get estimates of Natural and Fishing mortalities, under the assumption of no major changes in M occurring between both periods. Log linear models and a seasonal integrate catch at age analysis were tuned to the fishery and two series of surveys under the assumption of constant catchabilities across ages for the two surveys’ population estimates. An analysis of the period 1987-2009, searching for a single and constant natural mortality at age, results in minimum residual SSQ for an M around 0.8. But a better result is obtained when a pattern of increasing natural mortality at age is allowed, a possibility suggested since a long time for this type of short living species. Keywords: Anchovy; Natural mortality, M at age, Integrate assessment.