Mass Marking Study Plan for Steelhead Stocked in
Lakes Michigan and Huron
BY MATTHEW KORNIS, GREEN BAY FWCO
A combined 2.3 million steelhead (a.k.a. rainbow trout) are stocked each year by the Michigan, Wisconsin, Indiana and Illinois Departments of Natural Resources (DNR) to enhance sport fishing opportunities in Lakes Michigan and Huron. These four states have proposed a coordinated study with the U.S. Fish and Wildlife Service (USFWS) to mark and coded-wire tag (CWT) all stocked steelhead to improve the efficacy and impact of their steelhead stocking programs. The proposed study would be managed through the USFWS Great Lakes Fish Tag and Recovery Laboratory, which is headquartered at the Green Bay Fish and Wildlife Conservation Office and has used automated trailers and specialized staff over the past five years to mass mark other species (lake trout and Chinook salmon) within the Great Lakes basin.
What does mass marking fish entail? The adipose fin (a small fin located on the back of the fish behind the larger dorsal fin) is clipped off of hatchery-raised fingerlings to distinguish them from wild fish. In addition, a CWT is injected into the cartilaginous snout of each fish. CWTs are small pieces of stainless steel wire marked with codes to identify groups of interest, such as a specific stocking location, strain, or hatchery. The Service owns four AutoFishTM SCT6 mobile trailers, manufactured by Northwest Marine Technology, that specialize in the fin clipping and CWT injection process by automatically measuring each fish and sending them through tagging equipment calibrated for a specific size range.
Dr. Matthew Kornis, a fish biologist and data analyst with the USFWS, recently completed an analysis demonstrating that several key objectives could be addressed by the proposed mass marking steelhead study. By marking all stocked steelhead with a fin clip, the proposed study would produce precise estimates (within ± 1 or 2 %) of the percent of steelhead populations in Lakes Michigan and Huron comprised of wild, naturally reproducing fish. These estimates would in turn provide a more accurate estimate of steelhead population size. In addition, Dr. Kornis’ analysis suggested that the number of CWTs recaptured from hatchery-reared fish would be sufficient for researchers to determine the stocking sites, strains, and/or hatcheries that produce the greatest number of steelhead returning to the sport fishery. This information would help state fisheries managers improve their steelhead stocking programs by increasing return on investment.
In late July, Dr. Kornis presented his findings to the Lake Michigan Technical Committee, which is comprised of representatives from the Michigan, Wisconsin, Indiana and Illinois DNRs, USFWS and the U.S. Geological Survey. The Committee discussed the potential benefits of the study and decided to move forward with developing a detailed study plan, with a goal to start mass marking steelhead sometime in the next few years.