How equally sized piscivorous birds and fish sharing common food resources may reduce possible feeding interactions between them?

Abstract

The diets of sympatric predators may overlap, especially when their body sizes are similar and foraging area is relatively small. It may be also supposed that some differences in their foraging strategies may counteract competitive interactions among them, and therefore be of advantage to these species. To reveal such phenomena the composition of food of cormorant and adult pikeperch was studied in the Dobczyce Reservoir (S Poland) from June to November 2002. The main prey species were the same and the range of prey size was similar for both piscivores. Despite these similarities, the potential for dietary overlap was strongly reduced due to two differences in their foraging patterns: (1) different preferred prey species (cormorants foraged mainly roach, whereas pikeperch selected juvenile percids); (2) different size of simultaneously selected prey (in summer, cormorants selected larger prey, while in autumn larger prey was selected by pikeperch). These differences may be explained by some general features of birds and fishes, which determine the costs to the individual of capturing prey. The observed selection of different prey species and sizes may be also important for the co-occurrence of other piscivorous birds and fishes sharing common food resources.