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 addition 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 ammocoetes were studied. An area suitable for ammocoete habitat and the total number 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 suitable habitats for ammocoetes decreased markedly. In 1982 the total number of 1+ and older ammocoetes was estimated to be 1.4 million. In 1985 the estimate was 439000 individuals and in 1993 only 155000 individuals. As the result of the short‐term regulation, an ice cover was formed 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 production sites decreased. The embankment of the area above the reservoir caused erosion and nearly all suitable habitats for ammocoetes disappeared. As a result of regulation measures, the number of spawning habitats has decreased and their quality has worsened.

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