标题：Relative abundance and habitat association of three crayfish (Orconectes virilis, O. rusticus, and O. immunis) near an invasion front of O. rusticus, and long-term changes in their distribution in Lake of the Woods, Canada
其他标题：Relative abundance and habitat association of three crayfish (Orconectes virilis, O. rusticus, and O. immunis) near an invasion front of O. rusticus, and long-term changes in their distribution in Lake of the Woods, Canada
摘要：We documented the abundance and habitat association of the native virile crayfish [ (Hagen, 1870)] and two invasive species, the rusty crayfish [ O. rusticus (Girard, 1852)] and the papershell crayfish [ O. immunis (Hagen, 1870)] in a 38 km 2 area in the Lake of the Woods (LOW; Canada). From 20 June to 1 September in 2006, traplines were set at 376 sites for approximately 24 hours. A total of 9833 crayfish were captured, of which 68% were O. virilis , 23% were O. rusticus , and 9% were O. immunis . The mean catch per unit effort (CPUE) was 4.4 crayfish per trap with a maximum of 29. At least one invasion front of O. rusticus was detected, where its CPUE decreased sharply from a mean of 1.3 to 0.03. The mean CPUE of both O. virilis and O. immunis was significantly higher outside than within the front. For the entire catch, CPUE values of all three species were negatively correlated. Water depth (0.5-12 m) was significantly related to crayfish abundance, positively for O. virilis and negatively for O. rusticus and O. immunis . The distribution and abundance of the two invasive species on islands and adjacent mainland sites indicated that water depths near and deeper than the thermocline limit the colonisation of islands by O. rusticus and, particularly, O. immunis , but that islands surrounded by shallower water may serve as shortcuts in crayfish expansion circumventing convoluted shorelines. Mean CPUE of all three crayfish species differed between bottom substrates, being highest on mainly rocky substrates for O. virilis and O. rusticus , and on organic and inorganic fines for O. immunis . Mean CPUE of O. virilis near macrophytes was similar to that in areas free of aquatic plants; CPUE near macrophytes was significantly lower for O. rusticus and significantly higher for O. immunis . In conjunction with crayfish captures during lake-wide fish surveys between 1973 and 2006, we show that by 2006 O. rusticus has spread into most parts of LOW since its first record from Long Bay on the east side of the lake in 1963, expanding its distribution by an average of 2.1 km per year. Our results further indicate that O. immunis is more widespread and more abundant in LOW than previously assumed.
