Background. Two North American fish species have been particularly successful in Iberian reservoirs, the largemouth black bass, Micropterus salmoides (Lacepède, 1802) and the pumpkinseed, Lepomis gibbosus (Linnaeus, 1758), but information on their ecology is still scarce, limiting their effective management and control. To increase the knowledge about the ecological mechanisms underlying bass and pumpkinseed invasion of freshwater systems, their feeding ecology and population descriptors are contrasted in two Iberian reservoirs differing in available resources. Materials and methods. The studied reservoirs, situated in tributary streams of the Guadiana River, were sampled during the same months for a total of eight times, spanning two years. Fish samples were made by using electrofishing from a boat in the littoral area and trammel nets. Zooplankton and littoral benthic invertebrates were sampled concurrently with fish sampling. Fish diet was studied by examining individual stomach contents. The dominance of the different items in the diet of each species size-class was determined using the modified Costello diagram, diet overlap was estimated with Pianka’s index and prey selection was assessed with the Manly-Chesson’s alpha. Back-calculated length-at-age and body condition were also assessed for each fish. Results. Invertebrates were dominant in the diet of pumpkinseed whereas invertebrates and pumpkinseed dominated the diet of bass. Both species demonstrated an overall dietary opportunism and a considerable food spectrum, with pumpkinseed eating fish and macrophytes. Feeding selectivity was observed that varied with species, size-class, reservoir and sampling period. Fish became the main prey for larger bass and the shift to piscivory was related to pumpkinseed availability. The dietary changes associated with pumpkinseed growth in native environments were not observed in the reservoirs. Dietary overlaps varied with sampling period, being frequently high. Differences in abundance, growth and body condition of bass and pumpkinseed were evident between reservoirs. Conclusion. The ability to use several food items and to feed opportunistically likely facilitated the widespread success of the centrarchids in variable Iberian reservoirs. Moreover, our results seem to support the occurrence of resource competition between pumpkinseed size classes and between smaller bass and pumpkinseed.

invasive, Micropterus salmoides, Lepomis gibbosus, ecology, diet, reservoirs, Iberia, biotic interactions