Minnesota Department of Natural Resources (DNR) fisheries biologists began an eight-year study in 2008 to determine the optimum number of walleye fry that should be stocked back into lakes where eggs are removed for stocking purposes.

The first phase of the study focused on marking walleye fry using oxytetracycline (OTC) to differentiate stocked fry from those naturally produced. The last phase will now focus on determining the growth and survival of these marked fish to catchable size.

“This is a cutting edge fisheries research project,” said Dale Logsdon, DNR fisheries research biologist in Waterville. “It uses technology to better understand biology and to help guide management practices that result in the maximum number of walleyes for the public to catch and enjoy.”

Annually, the DNR collects walleye eggs from 13 different spawning runs as part of a statewide walleye production and stocking program. Lakes supporting these spawning runs represent some of the most prolific walleye fisheries in the state. The importance of assuring that hatchery operations do not have negative impacts on these fisheries has long been recognized.

To compensate for possible impacts of these egg removals, the DNR has historically stocked at least 10 percent of the walleye fry hatch back into those lakes where eggs were removed. However, the effects of these compensatory stockings had never been thoroughly evaluated due to the inability to distinguish between natural and stocked fry. DNR fisheries biologists want to ensure that enough fry are returned to the lakes, but are also concerned that stocking too many fry could result in poor growth and survival of both wild and stocked fish, thus resulting in fewer catchable walleye in the population.

Too many young walleye in the system at one time can result in increased competition for food, reduced growth rates, increased foraging times, and greater vulnerability to predation, according to Logsdon. “We want to optimize fry abundance to help ensure that we are maintaining the health of the walleye fisheries in our egg-source lakes.”

With the advent of the OTC technology, fisheries researchers are able to mark newly-hatched walleye fry by immersing them in a solution of OTC for several hours, just before they are stocked. The fry absorb a small amount of this chemical, resulting in a harmless mark left on the fish otolith, or ear bone, that can be detected years later using a microscope and ultraviolet light.

“This new technology enables us to determine how many walleye in a population originate from stocking versus natural reproduction,” Logsdon said. “If a florescent mark is present, we know we are looking at a stocked fish.”

Four egg-source lakes – Woman, Winnibigoshish, Otter Tail and Vermilion – are included in the study. These lakes were chosen because of their ecological characteristics and the availability of historical fisheries data.

The study’s has two main objectives are to better understand the natural reproductive processes in these lakes, and to use this information to identify the optimal number or replacement fry to stock in relation to natural changes in spawner abundance.

Fisheries personnel stocked marked fry at predetermined levels during the first five years. Gill net surveys will continue the next three years to learn what fry densities maximize survival, growth and abundance. Achieving these target fry densities will likely require adjustments of historical stocking rates.

This study continues the DNR’s history of implementing research projects that aim to improve the understanding and management of the state’s fishery resource. Minnesota, one of the nation’s top five angling destinations, continues to provide some of the nation’s best fishing.

For more information on the walleye fry marking research project, contact the local DNR fisheries office nearest the study lake.