Catastrophic fish mass mortality events in Moroccan freshwater ecosystems: alarming trends and impacts on biodiversity

Fish mass mortality events (FMMEs) represent an escalating ecological crisis, significantly threatening aquatic biodiversity, particularly in North African freshwater ecosystems. Addressing a critical knowledge gap in this region, our study presents the first comprehensive assessment of FMMEs in Moroccan aquatic ecosystems, including freshwater systems and estuaries, based on meticulous monitoring from January 2020 to December 2022. During this three-year period, we documented 18 FMMEs across 16 distinct ecosystems, with a notable increase in frequency observed during the summer and autumn months. Estuaries emerged as critical hotspots for these events, exhibiting the highest frequency of FMMEs and highlighting their vulnerability to climatic and anthropogenic pressures. Our findings indicate a staggering loss of at least 7.8 million fish, with Atherina boyeri, accounted and identified as the most affected species by FMMEs. The families Cyprinidae and Mugilidae experienced the most substantial impacts, including significant biomass losses in Chelon saliens, Chelon labrosus, and Cyprinus carpio. Additionally, endemic species such as Luciobarbus maghrebensis and Luciobarbus rabatensis also faced considerable declines. These events underscore severe ecological disruptions and provide novel insights into species distribution and interactions, including the first recorded presence of Oreochromis niloticus in previously undocumented regions. This research underscores the urgent need for targeted conservation strategies and proactive interventions to mitigate the ecological and socioeconomic ramifications of FMMEs. By addressing these critical issues, we can better protect Moroccan freshwater ecosystems that are at risk of further biodiversity loss.