1999 CRAYFISH DISTRIBUTION SURVEY OF THE ST. CROIX NATIONAL  SCENIC
RIVERWAY, ST. CROIX AND NAMEKAGON RIVERS, MINNESOTA AND WISCONSIN

Byron N. Karns, Robin Maercklein, and Randy Ferrin.  
St. Croix National Scenic Riverway, National Park Service, St. Croix Falls, WI 54024.

The purpose of this survey was to determine the extent of the distribution of the 
exotic rusty crayfish (Orconectes rusticus) and native crayfish on the St. Croix 
National Scenic Riverway.  Previous surveys conducted over the past twelve years 
(Langlois, 1995; Hobbs and Jass, 1988; Helgen, 1988), indicates three native 
crayfish species have been found to inhabit the Riverway.  These studies, coupled 
with Park records, suggest a marked increase in rusty crayfish densities, and a 
noticeable reduction or elimination of native crayfish species.  In order for Park 
Resource Managers to determine what, if any, action could be taken with respect of 
O. rusticus, density and distribution of St. Croix-Namekagon River crayfish should be 
obtained.

Nineteen sampling locations were selected on the St. Croix and Namekagon Rivers 
north of Stillwater, Minnesota (12 on the St. Croix  ten mile intervals and 7 of the 
Namekagon  fifteen mile intervals).  The trapping site at each location was 
determined by the best suitable habitat.  All traps were placed in the river via land, at 
a depth no greater than 70cm.  Traps are standard minnow traps with one entrance 
kept at 2.5 cm, the other widened to 4 cm (this is to reduce crayfish size bias).  Bait 
consisted of rank cow liver.  Traps were tagged with name, address and phone, and 
staked into the substrate for 24 to 48 hours.  Additionally, dip net and hand searches 
augmented collection efforts.  Information gathered included, site/location GPS & 
legal description; substrate; current speed; water depth; vegetation; air/water 
temperature; crayfish species; carapace length; sex; form; number(s) collected; non-
target species/incidental catch.  After site and crayfish information was recorded, one 
voucher (per sex) for each native and all the exotic species were preserved using 10% 
formalin.  The remaining native species were returned to their collection site. 
Vouchers were deposited at the Bell Museum, University of Minnesota, in St. Paul.

Eighty-seven crayfish were collected from fourteen locations on the St. Croix and 
Namekagon Rivers (eight on the St. Croix, six on the Namekagon), during the late 
summer of 1999.  Fifty specimens were retained as vouchers.  Of thirty-six crayfish 
collected from the St. Croix, all but two were P. rusticus.  Orconectes virilis was 
found at two locations along the upper stretch of the St. Croix River.  Fifty-one 
crayfish were collected on the Namekagon: forty-three Orconectes propinquus, and 
eight O. virilis.  The eight O. virilis were taken virtually throughout the Namekagon. 
O. rusticus were not found on the Namekagon River.  O. rusticus seem to be 
pervasive and appear locally abundant on the middle third of the St. Croix River.  No 
native crayfish species were discovered below the confluence with the Namekagon. 
No rusty crayfish were found above the confluence.  O. propinquus were widespread 
on the Namekagon, and at one location collected in large numbers.  Collecting more 
than one species per location happened only once, on the upper Namekagon.  A rain 
event during trapping on the upper St. Croix may have affected results along this 
stretch.

Orconectes rusticus and O. propinquus have been shown to hybridize and produce 
sexually viable offspring.  In fact, the crosses appear to be more successfully than 
either parent, and backcrossing does occur (William Perry, pers. comm.).  Therefore, 
continuing to survey for ranges of all Riverway crayfish species, and limiting the 
spread of O. rusticus (esp. by human means), seems paramount if native crayfish are 
to remain in the St. Croix and Namekagon River.

Keywords:  St. Croix River, crayfish, exotic species, distribution, invasion
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