An international study based on 35 years of oceanographic data has identified a progressive deeping of warm-water masses in the Mediterranean Sea, a trend that could increase thermal stress, health challenges and production losses in marine fish farms.
Published in the Journal of Environmental Management, the research analysed thermal stratification patterns across twelve aquaculture zones in Spain, Croatia, Tunisia, Greece, Türkiye, Cyprus and Egypt between 1987 and 2021.
The authors found that isotherms between 20ºC and 28ºC are moving deeper into the water column, particularly in the eastern and southern Mediterranean.
The study focused on areas where gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) are farmed. Together, these species account for more than 95% of Mediterranean finfish production. For both species, the optimal temperature range lies between 18ºC and 24ºC, while temperatures approaching 29ºC are close to their upper thermal tolerance limits.
More warm water at depth
Researchers used Copernicus Marine reanalysis data to compare a historical baseline period (1987-2010) with a more recent period (2011-2021). One of the key indicators examined was the depth of the 25ºC isotherm, considered a critical threshold for feeding performance, growth and physiological balance in seabream and seabass.
The largest changes were recorded in Tunisia, with a cumulative anomaly of +127.7 metres, followed by Cyprus (+122.6 m) and Greece (+80.9 m).
In Spanish waters, increases were more moderate, reaching +52.5 metres along the eastern coast, +37.5 metres in the south and +19.9 metres in the south-eastern region.
According to the authors, this deepening means that waters warmer than 25ºC now occupy a larger proportion of the depth range typically used by offshore cages. As a result, fish have less thermally suitable space available during the warmest months of the year.
Implications for aquaculture management
A greater penetration of warm-water masses can increase metabolic costs, reduce feed efficiency, constrain growth and heighten susceptibility to opportunistic pathogens. Stronger stratification can also limit vertical mixing within the water column, reducing the natural exchange of oxygen and nutrients.
To help anticipate these risks, the research team developed the Thermal Anomaly Stratification Index (TASI), a new indicator designed to quantify how current thermal stratification conditions deviate from historical norms.
The authors suggest that such indices could support management decisions including adjustments to feeding protocols, reduction in stocking density, strengthened health programmes and, where appropriate, the relocation of farming operations to deeper or better-flushed sites.
Beyond its climate-related findings, the study argues that thermal stratification should be incorporated into marine spatial planning and the selection of future aquaculture sites.
The broader objective is to move from reactive management towards a more anticipatory approach as external warming events become increasingly common across the Mediterranean.
Reference
Ortiz-Navarro, M., Sanchez-Jerez, P., Atalah, J., Ballester-Berman, J.D., Forcada, A., Toledo-Guedes, K. (2026). Climate change and vertical thermal stratification of the water column in the Mediterranean Sea: implications for marine aquaculture. Journal of Environmental Management, 411, 130178.
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