THE ENVIRONMENTAL BASIS OF DIET VARIATION IN PUMPKINSEED SUNFISH, LEPOMIS-GIBBOSUS, AND LARGEMOUTH BASS, MICROPTERUS-SALMOIDES, ALONG AN IBERIAN RIVER BASIN

The diet composition of largemouth bass, Micropterus salmoides, and pu mpkinseed sunfish, Lepomis gibbosus, and its relation to environmental factors was studied in intermittent watercourses of the lower Guadian a basin (southern Iberia) during the particularly dry summer of 1994. Overall, both species took food items similar to those found in their North American and acclimatised ranges, with pumpkinseed consuming inv ertebrates (chiefly Chironomidae) and bass preying on invertebrates (c hiefly Micronecta meridionalis) and fish (chiefly L. gibbosus and Gamb usia holbrooki). Both bass and pumpkinseed were opportunistic feeders, feeding on the most frequent and abundant prey. However, Diptera larv ae were apparently preferred by pumpkinseed and avoided by bass, while the contrary occurred with respect to Heteroptera. The two main fish prey of bass were eaten on the basis of random encounter. The relative abundance of macro-prey (i.e. fish and Atyephira desmarestii) were th e principal environmental variables constraining bass dietary variatio n along the basin. On the contrary, pumpkinseed diet variation was mai nly related to pumpkinseed size and the presence of piscivorous bass n earby, although habitat size and cyprinid abundance were also influent ial.