Fish communities and food web interactions in some shallow Mediterranean lakes

Fish communities of five shallow Spanish Mediterranean lakes were studied and stomach content analysed to determine the foraging pattern of each species and the influence on the lake food web. Lakes ranged from 0.5 to 2300 ha with less than 3 m mean depth, while the trophic state varied from mesotrophy to hypertrophy. We fished during summer 2000 and 2001, and for one lake also during spring 2002. Almost no piscivores species were recorded and in general omnivorous species dominated independently of lake trophic state. Fish dietary differences were found among lakes and prey availabilities. Maximum fish biomass and diversity were found in the Albufera lake, where mugilids (Mugil cephalus and Liza aurata) and Cyprinus carpio dominated. The other lakes had young populations of Atherina boyeri and Liza ramada, endemic species (Barbus guiraonis, Aphanius iberus) or introduced species (Lepomis gibbosus, Micropterus salmoides, Gambusia holbrooki). Fish were mainly detritivorous and shifted to zooplanktivorous depending on abundance of large zooplankton. Fish demographic changes and facultative preys influenced food webs of the lakes by both direct predation and indirect nutrient recycling from the sediment. Omnivorous structure of fish communities in these Mediterranean warmer lakes resembles that described in some food webs of tropical lakes. Bottom-up mechanisms seem to be more relevant in warmer lakes and submerged plants were special key factors to maintain clear waters in the study lakes.