DISTRIBUTION AND ABUNDANCE OF RIVER LAMPREY (LAMPETRA-FLUVIATILIS) AMMOCOETES IN THE REGULATED RIVER PERHONJOKI

The Kaitfors hydroelectric power plant was built, and subsequent short -term regulation was started, in the River Perhonjoki at the beginning of the 1980s. Nine kilometres of the river were impounded and, in add ition to this, regulation measures made below and above the reservoir. The effects of regulation and transplantations of mature lampreys in the areas above the dam on the distribution and abundance of ammocoete s were studied. An area suitable for ammocoete habitat and the total n umber of 1+ and older ammocoetes were also estimated. An inventory was performed in 1982 before the power plant was completed and then again in 1985 and 1993. Over the 11 year period ammocoete densities and sui table habitats for ammocoetes decreased markedly. In 1982 the total nu mber of 1+ and older ammocoetes was estimated to be 1.4 million. In 19 85 the estimate was 439 000 individuals and in 1993 only 155 000 indiv iduals. As the result of the short-term regulation, an ice cover was f ormed on the most important production sites of the river banks below the dam, making them even more uninhabitable for ammocoetes during the winter. The thick, mobile ice cover and sudden fluctuations in water flow increased erosion and therefore the quality and quantity of the p roduction sites decreased. The embankment of the area above the reserv oir caused erosion and nearly all suitable habitats for ammocoetes dis appeared. As a result of regulation measures, the number of spawning h abitats has decreased and their quality has worsened.