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Report on small pelagics and environmental forcing off the west coast of Africa

Derhy, Ghoufrane; Navarro, Gabriel; Rincón Hidalgo, Margarita; Khalil, Karima; Elkalay, Khalid; Macias, Diego

The north west coast of Africa, particularly the Moroccan Atlantic coast is characterized by a high primary production and high derived biomass of small pelagic species. The fishery of this group plays a key role in the Moroccan fisheries sector. The recent registered increase of the fishing mortality levels during the last decades requires intervention and analyses of the fishing pressure level on the small pelagic stocks, as there are several biological signal indications of a decrease in the stock capacity under a highly dynamic hydro-climatic environment. This report aims to represent the contribution of different oceanographic variables explaining the stock abundance fluctuations. There is an overall lack of detailed biological data (length-age composition, weight, maturity, etc.) regarding fisheries in the west coast of Africa. After reviewing and collecting all available data, especially surveys data about the abundance of small pelagic populations, the stock of chub mackerel in the center and south of Moroccan Atlantic coast was selected based on the data availability. This stock dynamics was evaluated using an approach of data limited methods (Surplus Production model in Continuous Time, SPiCT model) and subsequently we explored the relationship between this model results (Relative Biomass trend) and different environmental forcing. We carried out a correlation analysis of the stock abundance trend and the environmental covariates of the study area. A strong correlation between environmental forcing and population dynamics was captured and allow us to identify the factors that can affect the stock abundance. Among the environmental drivers studied, salinity, chlorophyll concentration, net primary production, oxygen concentration and nitrate concentration are the main variables significantly correlated with the spatio-temporal variations of chub mackerel abundance, which are the main factors that may describe the upwelling intensity and the estimated fluctuations of the relative biomass of the stock. These variables can be included in this stock assessment model or other fisheries management approaches in order to estimate the different scenarios of the stock dynamics under different environmental conditions. Our results bring crucial information due to the typical fisheries management models need to be adapted to consider both the fishery impact and the environmental effects, to produce a series of outputs on the assessment and management of exploited fish stocks. The results of this approach can be used to achieve agreement, develop and support policy advice that should be taken in response to changing resources condition. Reaching these management measures will be a high challenge for fishery managers, but it seen as a necessary step in the long term.

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