Woman Lake Included in Walleye Stocking Study
The Minnesota Department of Natural Resources (DNR) fisheries researchers, with help from local fisheries personnel, have launched a study to determine the optimum number of walleye fry that should be stocked back into four egg-source lakes including Woman and Winnibigoshish in Cass County, Otter Tail in Otter Tail County and Vermilion in St. Louis County.
These lakes were chosen due to their ecological characteristics and the availability of historical fisheries data.
Researchers also hope to gain a better understanding of the natural reproductive processes that occur in these important walleye lakes.
Each year, the DNR collects walleye eggs from the spawning run in the Boy River tributary to Woman Lake as part of a statewide walleye production and stocking program. To compensate for possible impacts of these egg removals, large numbers of walleye fry (newly-hatched walleye) are stocked back into Woman Lake annually. However, the effects of these stockings were not thoroughly evaluated in the past because of the inability to distinguish between natural and stocked fry.
“Our primary interest is maintaining the health of walleye fisheries in our egg-source lakes,” said Dale Logsdon of DNR Fisheries in Waterville, who is leading the study. The DNR has typically returned 10 percent of the walleye hatch to those lakes where eggs were removed, Logsdon said. This 10 percent figure was based on limited observations from Lake Winnibigoshish back in the 1950s.
“However, new technology is now available,” said Logsdon. “It allows us to get a better idea of how many walleye in a population originate from stocking and how many were naturally produced.”
Walleye fry will be chemically marked for the study with oxytetracycline (OTC). Newly hatched walleye fry are immersed in a solution of OTC for several hours just before they are stocked. The fry absorb a small amount of this chemical, which results in a small, harmless mark being laid down in the otolith, or ear-bone, of the fry. Months, or even years later, this chemical mark can be detected by examining the otolith using a microscope and ultraviolet (UV) light.
“The OTC-marked bone glows under ultraviolet light,” Logsdon explained. “If a fluorescent mark is present, we know that we are looking at a stocked fish. Wild fish do not carry this mark.”
Though 2008 is the first spring season in which all four of these study lakes received
marked fry, Woman Lake got a jump-start on the project because all fry stocked in 2006 and 2007 were marked. Results thus far from Woman Lake, Red Lake, and Leech Lake indicate that walleye hatch rates are considerably lower than previously thought. This information has provided fisheries biologists with new insight on the number of fry needed to produce a strong walleye year-class.
Several recent strong year classes have been produced in Red and Leech Lakes at total fry densities of less than 650 walleye fry per littoral acre (stocked and wild fish combined). The littoral area of a lake is the portion under 15 feet deep and is the most productive area of the lake.
Put-back stocking in Woman Lake has ranged from 2,500 to 4,500 fry per littoral acre, well above the number needed to produce strong year classes. Logsdon commented that even in the fish world there is sometimes a danger in having too much of a good thing.
“We are concerned that putting in too many fry may contribute to poor growth and high mortality of both wild and stocked walleye,” said Logsdon, “which in turn, may result in fewer catchable walleye in the population.”
During the eight-year study, fisheries personnel will sample walleye populations in the study lakes using fall gill net surveys and other methods. They will attempt to manipulate total fry densities during the first five years of the study at pre-determined levels between 250 and 2,000 fry per littoral acre, then monitor the stocked year-classes for the remainder of the study in an effort to learn what densities maximize survival, growth, and abundance to provide the most walleyes for anglers. Achieving these target fry densities will likely require adjustments of existing stocking rates. In some cases this might result in rates that are lower or higher than historic rates.
“Our goal is to maximize the number of catchable sized walleye available for anglers in these egg-source lakes,” Logsdon said.
For more information about the study, people should contact the local area fisheries office nearest the study lake.”