Environmental effects of marine cage aquaculture in the Western Mediterranean: A meta-analysis of monitoring data.

Environmental monitoring plans (EMPs) are essential for assessing and limiting the impacts of marine cage aquaculture. In the Mediterranean, EMPs are legally mandatory in many countries. However, inconsistencies in variable selection, analytical methods, and experimental designs limit regional comparability and hinder environmental assessments. This study evaluated EMPs from five finfish cage farms in southeastern Spain (2017-2022). We qualitatively compared sampling methodologies, measured variables, and experimental designs across physicochemical and biological components, and quantitatively assessed, using meta-analysis (logarithmic response ratios, random-effects models), which variables are most sensitive for detecting aquaculture-driven changes. Significant methodological inconsistencies were identified among consulting firms, particularly in variable measurement and analytical protocols. The meta-analysis identified suspended solids, chlorophyll-a, nitrate, ammonium, sediment organic matter, sediment redox potential, and dissolved oxygen as the most informative physicochemical indicators. Among biological variables, macrobenthic abundance showed the largest effect (substantially higher in impact areas), followed by faecal coliforms. Shannon-Wiener diversity and species richness were recorded across all farms but showed negligible effects, likely reflecting methodological inconsistencies rather than true ecological insensitivity. These results demonstrate the environmental effects of fish farming on surrounding ecosystems and highlight an urgent need for standardised EMP frameworks at the regional scale. Such frameworks must integrate both physicochemical and biological indicators with appropriate experimental design to enable reliable long-term assessment of marine aquaculture impacts.