Shelter competition between native signal crayfish and non-native red swamp crayfish in Pine Lake, Sammamish, Washington: the role of size and sex

Freshwater crayfish (Decapoda) communities worldwide are becoming increasingly similar from location to location by the intentional or accidental introduction of North American crayfishes. The red swamp crayfish (Procambarus clarkii, Cambaridae), which is native to the south-central United States and northeastern Mexico, is the most widely introduced crayfish in the world. It was first discovered in Pine Lake, Sammamish, Washington in 2000. The results of a 2005 baseline survey of the crayfish in Pine Lake suggested that the red swamp crayfish was displacing the native signal crayfish (Pacifastacus leniusculus, Astacidae). One mechanism through which non-native crayfishes displace native species is competitive interaction over shelter that influences susceptibility to predation. Field experiments were designed to explore how crayfish size and sex influence shelter occupancy in mixed-species pairs of signal crayfish and red swamp crayfish competing for limited shelter inside enclosures placed on the bottom of Pine Lake. In addition, the relative survivorship of signal crayfish and red swamp crayfish was quantified in experiments where mixed-species pairs were tethered outside of single shelters. Irrespective of species and sex, when paired with smaller heterospecifics, large crayfish readily monopolized the shelters inside the enclosures. When contestants were size-matched, the dominant crayfish or \u27winner\u27 was typically the one with longer chelae; frequently, this was the signal crayfish. Female crayfishes also were adept at monopolizing the shelter. The tether experiments revealed no significant differences in survivorship between species. These results suggest that additional mechanisms besides shelter competition are contributing to the possible displacement of signal crayfish at Pine Lake