Resource Partitioning and Niche Shifts of Bream (Abramis brama) and Eel (Anguilla anguilla) Mediated by Predation of Smelt (Osmerus eperlanus) on Daphnia hyalina

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Abstract
The resource partitioning of the bream (A. brama) and eel (A. anguilla) populations in Lake Tjeukemeer, The Netherlands, was related to the variation in abundance of their most important food organisms, D. hyalina and larval chironomids. Niche shifts of both bream and eel populations were related to the abundance of young planktivorous fish, particularly smelt (O. eperlanus). When these fish were abundant the D. hyalina population was dominated by small individuals and bream switched from a planktivorous to a benthivorous diet, the condition of mature bream deteriorated, and its gonads developed poorly. Under these circumstances the eel population switched from a diet of chironomid pupae and mollusks to one of predominantly fish. The condition of eels smaller than 35 cm decreased and the chironomid population decreased in numbers and biomass. When the recruitment of planktivorous fish was poor, the size of D. hyalina was large and diets and conditions of bream and eel populations changed again. Because of the hydrological regime the stock of the young planktivorous fish is determined to a great extent by the immigration of allochthonous larval smelt and varies markedly, but the biomass of the bream and eel populations is comparatively stable.