Variation in prey selection of a piscivorous fish after the impoundment of a neotropical reservoir: prey size and type

Abstract

The relative abundance and size of prey fish in the stomachs of the predator Acestrorhynchus pantaneiro were compared with those recorded in the field to estimate prey selection. Fish samples were taken monthly in the Manso Reservoir (State of Mato Grosso, Brazil) immediately after the impoundment, from March 2000 to February 2001 (period I) and from March 2003 to February 2004 (period II). In period I, the small relative dominance of the prey in the environment seemed to have lead to random foraging. In period II, however, when the forage fish Moenkhausia dichroura was dominant in the environment, the predator shifted its diet, foraging mainly on this prey. Species with short relative body depth were positively selected. The prey size classes between 30 and 49 mm, and 50 and 69 mm standard length (L(S)) were the most abundant in the environment. Small prey were predominantly selected by A. pantaneiro. Even when a given prey or prey size was predominant in the environment, A. pantaneiro was a selective predator and maintained its preferences associated to prey type and L(S), although it consumed the most abundant resource.