Freshwater
Crayfish Survey on the Upper St. Croix and Tributaries 2004
by Diana Downes, WDNR
Ten days during the fall of 2004, I participated in a statewide survey of freshwater crayfish which added to our knowledge of the Rusty Crayfish (O. rusticus) invasion in Wisconsin. Click here to see a photo.
Rusty Crayfish did not live in Wisconsin prior to the early 1900’s. It is thought that they were brought to the state as bait by anglers. Since their arrival local populations of Northern Clearwater Crayfish (O. propinquus) and Northern Blue Crayfish (O. virilus) seem to be disappearing. In some lakes in northeastern Wisconsin rapidly increasing, dense populations of Rusty Crayfish have been clearly detrimental to aquatic systems. They reduce the quantity of rooted vegetation in the water and even along shore for up to a half meter, which reduces cover for spawning sites and fish fry. Rusty Crayfish are ‘opportunistic omnivores’, able to thrive on a low protein, vegetable diet as adults. They are efficient scavengers as well as competent hunters. When Rusty Crayfish are abundant they can change the diversity of fish in a lake.
Like many invasive species, Rusty Crayfish are more aggressive and more tolerant of disturbed habitats than the Northern Blue or Northern Clearwater Crayfish. They mature faster and reproduce more quickly allowing them to outcompete the local species. While freshwater crayfish prefer still, clear waters with a firm substrate, Rusty Crayfish tolerate a wide range of temperatures and can tolerate a broad range of water conditions. They will migrate overland from their birth site when conditions press them to.
The survey began In mid-Spetember, using minnow traps and fish entrails for bait we began at Fox Landing. Along with DNR staff I set 14 traps and spent time trying to catch crayfish with nets or by hand.
The following day a volunteer who has been monitoring Rusty Crawfish on the Snake River in Minnesota,joined our group. We learned that setting traps in mid-stream this time of year was not effective, and that netting and hand capturing them yeilded better results.
As water temperatures cool, and the nights grow longer, female crayfish and young of year tend to hide in tunnels along the shore or banks during the day and forage only at night. Because crayfish preserved in ethyl alcohol seemed to be brittle when handled we did not record the sex of the crayfish we caught, nor did we measure their length, which is used to determine their age. We did our best to identify species using several different keys.
In all, at 8 sites, we found 37 crayfish: 26 Rusty Crayfish, 6 Northern Clearwater Crayfish, 5 Northern Blue Crayfish. As was hoped, Rusty Crayfish do not appear to have gone above the confluence of the Namekagon. However, there are undocumented reports that they exist in the Minong Flowage and in Lake Hayward. The Yellow River, and Yellow Lake are also reported to have Rusty Crayfish.
2004
Summary of sampling results
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