The Mutual Impact Between the Population Dynamics of Mytilus galloprovincialis and the Harvesting of Mussels in the Massa-Tifnit Area (Southwest Morocco)

This study provides a comparative analysis of the population dynamics and growth parameters of Mytilus galloprovincialis, based on monthly monitoring in three natural beds along the Moroccan Atlantic coast south of Agadir: Sidi R'bat, Douira, and Tifnit. Conducted over two years, from January 2018 to December 2019, the research aimed to assess the impact of mussel harvesting on their spatial distribution and ecological sustainability. Parameters were determined from size frequencies using FiSAT II software (FAO-ICLARM Stock Assessment Tools). Results showed significant variations in density, biomass, and condition index across all three sites, influenced by environmental and physicochemical factors, as well as the life cycle of the mussels. Douira exhibited the highest levels in the first year. Size class frequency distributions revealed slight differences among stations, with recruitment occurring throughout the study, peaking in winter-spring and summer. The growth rate, measured using the von Bertalanffy equation, was K = 0.5 yr⁻¹ at all sites, with maximum asymptotic length (L∞) and growth performance index (Φ') at Tifnit, recorded at 112mm and 3.7, respectively. Mortality rates (total mortality Z, natural mortality M, fishing mortality F) and exploitation rate (E) were at their highest at Douira (Z = 2.84 yr⁻¹, M = 0.87 yr⁻¹, F = 3.45 yr⁻¹, E = 0.81 yr⁻¹), resulting in a low survival rate (S = 0.014 yr⁻¹). The findings underscore the importance of sustainable management practices by local communities and cooperatives to ensure ecosystem sustainability and product quality.