Meta-analysis review of fish trophic levels in marine protected areas based on stable isotope data

Abstract

Stable isotopes (δ15N) have been used to determine trophic levels in marine food webs. This study assessed if Marine Protected Areas (MPAs) affect the trophic levels of fishes based on stable isotopes in the western Mediterranean. A total of 22 studies including 600 observations were found and the final dataset consisted of 11 fish species and 146 observations comparing trophic levels inside and outside MPAs. The database was analysed by meta-analysis and the covariate selected was the level of protection (inside vs. outside MPAs). The results indicate significant differences between trophic levels inside and outside the MPAs; however, results differ from expectations since the trophic level was lower inside than outside MPAs. Three habitats were analysed (coastal lagoons, demersal, and littoral) and significant statistical differences were found among them: trophic level was higher in demersal habitats than in coastal lagoons and littoral areas. No significant differences were found in species classified by trophic functional groups. Several hypotheses are considered as possible explanations linked to protection level, time since protection, and MPA size. We debate the suitability of using the stable isotope δ15N as a direct indicator of trophic level in evaluating MPA effects on food webs.