Plant Exclusion of a Herbivore; Crayfish Population Decline caused by an Invading Waterweed

The population of the crayfish Astacus astacus was recorded in Lake Steinsfjorden (southeast Norway) over the period 1979–1996 by studies of total catch effort (baited traps) and catch per unit effort over the season. Over the years 1979–1987, total population of exploitable crayfish (>9.5 cm) was 194,000 (±62,000 SD), and the catches were evenly distributed over the shallow areas. After 1987 there was a sudden decrease in population size, reaching a level of 92,000 (±22,000 SD) for the 10 years that followed. From 1977 onwards the Canadian pondweed Elodea canadensis invaded the lake and established dense covers over large parts of the shallow areas. The crayfish were gradually excluded from areas covered by Elodea, while the yield from remaining areas remained relatively constant. Direct observation by test-fishing and diving confirmed the virtual absence of crayfish within the Elodea stands. Dense stands may directly interfere with movement of adult crayfish, and also strong fluctuations in O₂ and pH were recorded within the stands, probably acting as a stress factor. Although anoxia was not observed in this survey, it may occur during years of massive dieback of Elodea. Thus, while crayfish are able to feed on Elodea, they are unable to control the rapid growth over large areas of this invader, and somewhat paradoxically the herbivore is spatially excluded by its potential resource.