Abstract

Canadian waterweed Elodea canadensis invaded Lake Steinsfjorden in 1977, and by 1982 the weed covered some 72% of the lake floor within the 0-6 m depth range. Length distribution of Astacus astacus in shallow areas with dense stands of Elode a outside, were skewed and the fraction of small crayfish were low compared to areas without Elodea. However, length distribution in early August trap catches in 1980-86 did not indicate reduced survival of young crayfish, and a possible explaination is that young crayfish exploit the areas with dense Elodea for food and shelter, whereas large crayfish avoid these areas. The local fishermen did rapidly adapt to the new situation with Elodea, and trap effort was not reduced in sections of Lake Steinsfjorden with dense Elodea. Mean catch per unit effort and yield have increased slightly in 1983-86 compared to 1979-82. Canadian waterweed may have enhanced the crayfish fishery by increasing survival of young crayfish, and by protecting the crayfish from overexploitation.

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