Climate change and vertical thermal stratification of the water column in the Mediterranean Sea: implications for marine aquaculture

and other greenhouse gases, the oceans are warming under the global phenomenon of climate change. This effect is especially pronounced in semi-enclosed seas like the Mediterranean, where marine heatwaves have become more frequent and intense, destabilizing marine ecosystems. This study analyzes Mediterranean Sea thermal stratification trends using Copernicus Marine Service reanalysis data at 12 aquaculture facilities in Spain, Croatia, Tunisia, Greece, Türkiye, Cyprus and Egypt, covering 35 years (1987-2021). Data were averaged over approximately 14 × 14 km around each facility to evaluate water-column thermal structure across latitudinal and longitudinal gradients. Two periods were compared: a reference period (1987-2010) and a post-period (2011-2021). Depth anomalies of the 25°C isotherm (calculated as the deviation from the reference period mean) and the Thermal Anomaly Stratification Index (TASI) were calculated to quantify deviations from historical baseline conditions. A consistent deepening of isotherms between 20 and 28°C was detected across the basin, with the most pronounced changes in the eastern and southern Mediterranean, following a clear longitudinal and latitudinal gradient. Statistically significant isotherm deepening was confirmed at five stations by a Wilcoxon signed-rank test. These shifting thermal patterns represent a growing threat to Mediterranean aquaculture, increasing heat stress on farmed species and compromising growth and productivity. A provisional risk classification framework based on the TASI is proposed to support managers and regulators in translating climate indicators into specific adaptation strategies, including adjustments to feeding protocols, stocking densities and farming calendars.