Abstract

Aggressive interactions between two species of crayfish, Orconectes virilis, native to eastern Canada and Pacifastacus leniusculus, native to western slopes of the Rocky Mountains were measured using the technique described by Bovbjerg (1953). While Orconectes virilis males are known to be dominant over females of the same size, it was the female Pacifastacus leniusculus that proved dominant over males of the same size. The dominance order for juveniles established that females predominated over males. Male Pacifastacus leniusculus proved dominant over male Orconectes virilis for crayfish exceeding 30 mm length. In mixed groups of juveniles of both species, male Pacifastacus predominated over Orconectes. In mixed groups of larger dominant Pacifastacus females and dominant Orconectes males no dominance pattern could be determined. At the sizes investigated (30-40 mm C.L.) mature Orconectes males withstand aggression from Pacifastacus females of approximately the same size. The most likely possibility for such a reversal of previous tests is that female Pacifastacus at the size tested were immature, while male Orconectes were mature. Overall, the laboratory results would support the hypothesis that in nature Pacifastacus might predominate over Orconectes if they were introduced into boreal lakes of eastern Canada.

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