Marine heatwaves in the western Mediterranean: Considerations for coastal aquaculture adaptation

Abstract

Climate change threatens marine aquaculture, impacting fish health and farming practices through extreme events such as ocean warming and marine heatwaves. These events can compound the effects of other stressors, necessitating adaptive measures for sustainable aquaculture, such as submergible cages. We harnessed ocean reanalysis products to enhance our understanding of ocean warming and marine heatwaves in key fish farming areas in the Western Mediterranean, focusing on fish welfare thermal thresholds that inform mitigation measures. Our analyses revealed a consistent temperature increase across depths and farms over four decades, notably peaking at 0.75 °C per decade in some areas. Marine heatwaves have become three times more frequent, with nearly 50% longer durations on average compared to the 1980s. This included the most severe event experienced in 2022, with anomalies up to 4.2 °C lasting the entire summer. Fish welfare thermal thresholds exceeded the average depth of pen net systems and increased by 4.3 m per decade. Moreover, the seasonal onset of thermal thresholds shifted 5 to 6 days earlier per decade. To secure optimal conditions for seabream and seabass, net pens should be submerged to depths of around 20 m and 15 m, respectively, ideally in the second week of July. However, in shallow areas, this may not be feasible. Our findings raise concerns about the well-being of Mediterranean farmed fish, which, although adapted to current conditions, may struggle to thrive under recent and projected environmental changes. Addressing these challenges, a multi-faceted adaptative approach encompassing research, technological innovation, regulatory measures, and industry collaboration.