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Since 1893, crayfish plague, Aphanomyces astaci Schikora, has devastated Finland’s most productive populations of the native noble crayfish, Astacus astacus, causing great losses to once very valuable fisheries and exports. Efforts to halt the spread of the plague having failed, it was decided in the late 1960s to attempt to revive crayfish production and fisheries by introducing the plague-resistant North American signal crayfish, Pacifastacus leniusculus, into infected waters. So far P. leniusculus has been stocked in more than 300 waterbodies and is currently known to reproduce in roughly 100 of them. In some lakes both species live in sympatry. We have studied the interactions of co-existing A. astacus and P. leniusculus in the small lake of Slickolampi for 30 years. According to annual trap catches and population size estimates, A. astacus was clearly dominant up to the end of the 1980s, but in the 1990s P. leniusculus became dominant. In the last sampling, in August 1999, it accounted for > 98 % of total catches. In the course of three decades, then, P. leniusculus has almost completely replaced A. astacus. P. leniusculus has many of the characteristics commonly found in the successful crayfish invader. Our comparative studies in this lake show that P. leniusculus individuals grow considerably more rapidly, and that females reproduce for the first time at a younger age, attain reproductive activity at a larger size, and have greater fecundity in all size groups than A. astacus. The greater abundance of chelae injury in A. astacus than in P. leniusculus and the increase in injuries with expansion of the P. leniusculus population point to agonistic encounters in which P. leniusculus is dominant. Long-term data on co-existing A. astacus and P. leniusculus demonstrate that they cannot live in sympatry without competition in which P. leniusculus with its larger body and chela size, faster growth, more aggressive behaviour and greater fecundity would be the stronger. However, a similar weakening of the A. astacus population at sites with only a minor population of P. leniusculus suggests that the elimination of A. astacus is due, not to competitive exclusion alone, but to a combination of several interacting mechanisms and that the ultimate reason for the collapse of A. astacus is the almost complete cessation of successful reproduction as a result of reproductive interference between the two species. It is therefore futile to stock productive A. astacus waters with plague-free P. leniusculus in the expectation of better total catches. P. leniusculus should be introduced only into chronically plague-infected waters. The preservation of the native A. astacus, along with efforts to secure its valuable catches, requires above all the formulation of an action plan and a greater input into research, especially on crayfish plague control, crayfish management, and interactions between P. leniusculus and A. astacus. |
