Distribution of common carp in a Spanish reservoir in relation to thermal loading from a nuclear power plant

Arrocampo Reservoir is used as a cooling basin for a nuclear power plant located in southern Europe. Its annual mean temperature is about 30 °C with maxims that achieved 41.5 °C near the hot water effluent. Common carp (Cyprinus carpio, Linnaeus 1758) is the dominant pelagic fish species in this reservoir. Hydroacoustic surveys were conducted bimonthly over 9 years to characterize common carp abundance and distribution in the reservoir on an annual and seasonal temporal scale. Mean fish density during the period was 0.029 fish m−3, varying from a maximum of 0.038 in 2003 to a minimum of 0.012 in 1998. There were no significant differences in the mean fish density among years. Each year, fish distribution showed significant seasonal variation. Carp were homogeneously distributed during autumn and spring; in winter carp occupied warmer outfall areas, whereas in summer, fish were distributed in the coldest area of the reservoir (dam area) but avoided the warm outfall area. These seasonal tendencies of carp distribution was repeated every years. Behavioural reaction appears to be particularly important in explain carp distribution in this reservoir.