The influence of cooling water discharge upon fish

Research on this influence was mainly carried out near the Flevo power station (846 MW) at the SW-border of Lake IJssel (120.000 ha). Temperature increase during the summer amounts to 6–7°C, during the winter up to 14°C. Fish were extremely abundant in the discharge area, the total fish population was at least 200 times as dense as elsewhere in the lake. The extent to which the various species congregated was quite different. Compared with the abundance in the lake the concentration factor amounted to approximately 2000 for pikeperch, some hundreds for bream, roach and perch, whereas ruffe and smelt, which form the bulk of the fish population in Lake IJssel, were nearly absent in the discharge are. It could be expected that fish, when exposed to temperatures above the preferendum, would leave this area. This, however, did not happen: whereas the preferendurn for these species lies some degrees below 30°C, pikeperch, perch, roach, bream and white bream were not driven away by temperatures of up to 31°C. Although the discharge of cooling water attracted fish, it certainly did not act as a fish-trap. Two identical groups of fish (pikeperch and perch) were tagged and released in the outlet and the inlet area, respectively. These two groups were recaptured in Lake iJssel in equal percentages, demonstrating that these species stayed only temporarily within the outlet area. Considering the large quantities of fish in the outlet area and the observed, rather limited number of dead fish, mortality among these fish was not above normal. Nevertheless conditions were in some respects unfavourable: It can be concluded that the discharge of cooling water does not considerably harm fish in a direct way. In some respects, mainly in inproving the growth rate, an increase in temperature may even be advantageous. The discharge has some unfavourable effects because it attracts fish and thus causes indirectly food shortage, deterloration of the condition and probably results in an increase of infective diseases. A potential danger for pikeperch arises when temperatures near the bottom exceed 20°C in May. Most of the negative effects of cooling water discharge can probably be eliminated by technical provisions in the outlet area.