Revealing environmental forcing in the different trophic guilds of fish communities off the Western Mediterranean Sea

The dietary preferences of 61 Western Mediterranean species of fish belonging to different trophic levels were studied. Specimens were collected during the annual bottom trawl survey MEDITS for the period 2011-2018, with a total of 16,588 stomach contents analyzed, providing a highly valuable raw dataset for advanced studies in trophic ecology. The software EstimateS 9.1 was used, for the first time in a study of Mediterranean fish species, to estimate cumulative prey curves in order to determine the adequacy of the sample size for each species in representing the species diet. The main findings revealed the existence of nine well-identified feeding strategies, or trophic guilds, based on food preferences. Indicators, namely the Shannon diversity index, biomass estimated from standard surveys, mean trophic level and fish community composition, were used to categorize the structure of the fish community in Western Mediterranean marine food webs. In addition, the effects of latitude, depth and habitat type on fish community structure were investigated. Results show all these environmental factors investigated drove fish composition structure. Regarding the three variables investigated, depth had the greatest impact on the fish community structure, particularly affecting diversity and fish community composition. Latitudinal gradient only seemed to affect fish community composition, showing consistency along a latitudinal north-south axis. Habitat type was found to be significant in fish community structure. Mean trophic level was the only indicator that was not affected significantly by environmental variables. The present study shows the relevance of environmental forcing in fish community structure. These findings highlight the need of ecosystem studies, since information about the trophic networks in the study area is still scarce, jeopardizing the development of ecosystem models. The present work aims to fill this gap for the effective implementation of an ecosystem approach to fisheries management in the Western Mediterranean Sea.