Abstract

The crayfish plague (Aphanomyces astaci) and watercourse modifications devastated once productive noble crayfish (Astacus astacus) stocks in the large river systems in Western Finland during the first half of the 1900s. Since then, there have been several attempts to reintroduce noble crayfish to rivers as well as other efforts to improve the aquatic habitat to better suit the general requirements of crayfish. The River Pyhäjoki has been affected by crayfish plague twice (1960 and 1979), and in 1988 crayfish mortalities also occurred, but the reason for this event remains unknown. The most recent reintroduction program was initiated at the turn of the millennium (in the year 2000) when a carefully planned project was funded by fisheries administrators from within the EU and by other financers from national and private sources within Finland. The aim of the program was to reintroduce noble crayfish into the River Pyhäjoki. The project included massive restockings of noble crayfish from several other populations as well as education of the local fishermen on the management of crayfish stocks. After two years of reintroductions, a complicated lesson was learned, since among other coincidences, the crayfish plague was introduced into the river system. Recently the spread of crayfish plague has been sporadic in the river system and, quite amazingly, it did not affect all the crayfish stocks, even in the main channel of the river. The crayfish plague also wiped out one of the source stocks of crayfish 50 km from the River Pyhäjoki. Later it was discovered that the strain of the crayfish plague fungus indicated that it could have originated from a signal crayfish (Pacifastacus leniusculus) population either directly or indirectly, although there are no known occurrences of signal crayfish in the study area. Furthermore, the stockings were more successful in the small rivers upstream, than in the main channel of the River Pyhäjoki itself. Due to severe lack of rainfall and resulting drought, some populations of crayfish were also lost from the smaller upstream rivers. The dynamics of the crayfish plague and experiences from the River Pyhäjoki crayfish introduction project are discussed in detail.

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