Variability of Natural Mortality Estimates–the Case of Selected Demersal/Deep Water Species from Azores (ICES Xa2)

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Priscila Silva
Helena Krug
Ana Novoa-Pabon
Mário Rui Pinho


Natural mortality (M) is a powerful parameter in fisheries stock assessment and management; however, its precise estimation is extremely difficult. There is still a lack of estimates for several stocks. That is the case of demersal/deep-water species from the Azores archipelago, where demersal species are the second most important fishery based on landings and the most important in value. Species with higher commercial value and lacking M data were selected: Pagellus bogaraveo, Phycis phycis, Beryx splendens, Pontinus kuhlii, Helicolenus dactylopterus, Pagellus acarne, Beryx decadactylus and Pagrus pagrus. Life history parameters were gathered from literature. A literature review was performed to summarize all published indirect methods that are commonly used, as a simple and low-cost way to estimate M. This study aims to demonstrate the variability of natural mortality estimates based on the input parameters. It also constitutes the first attempt of M estimation for the most commercially important Azorean species. Twenty-six indirect methods were applied and about 70 % of the selected species had an average mortality estimate between 0.22 and 0.34 per year. The most well-studied species (Pagellus bogaraveo and Helicolenus dactylopterus) provided more accurate mean estimations (M = 0.30 and M = 0.27, respectively). Independent methods based on Tmax alone, or combined with L¥ or W¥, lowered the values. Methods based on reproduction parameters alone, or combined with growth, inflated the estimations. From the selection of life history parameters to the final estimation of M, the results of this study can fill the existing gap and represent a starting point for further studies.


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