Imagine a vast rolling plain dotted with wetlands teeming with fathead minnows, and you're picturing a walleye's dream smorgasbord.
Far from being just a fish fantasy, such wetlands have been used for years to coddle young walleyes until they're large enough for stocking.
Now imagine that the wetlands are in North America's famed duck factory, the Prairie Pothole Region, which extends from western Minnesota through the Dakotas into Montana. Wetlands here provide critical nesting habitat for much of North America's migratory waterfowl.
The seemingly harmless fathead minnows make good bait and walleye forage, but they eat benthic macroinvertebrates that are important duck foods. Fatheads also consume zooplankton, especially larger breeds such as Daphnia, which themselves consume phytoplankton (suspended algae).
Without predators, fatheads take over. Duck food disappears, algae blooms and, because of the ensuing low light penetration, aquatic vegetation suffers.
Striking A Balance
To find a solution for this problem, Minnesota Department of Natural Resources fisheries researchers wondered whether stocking walleye fry could reduce fathead populations, produce larger walleye fingerlings and enhance duck habitat.
They stocked fry in six potholes with booming fathead populations. Yearling 'eyes were stocked in another six ponds, and six potholes went unstocked to serve as reference sites. Responses varied, but after three years of study, it was clear that stocking walleye fry worked.
As fathead populations decreased, benthic macroinvertebrates and large zooplankton thrived. In turn, phytoplankton decreased and the water cleared. The study period was too short to evaluate the long-term effects on aquatic vegetation, but improvements were noted.
Stocking yearling walleyes was not as effective. Fathead populations were not consistently reduced, probably because the larger walleyes did not eat larval fathead minnows, and stocking density was too low to greatly reduce adult fatheads.
Big Lake Success
Predators have performed similar rescues in several large-lake experiments where excessive nutrients had caused noxious algal blooms. Stocking salmon, pike, walleyes, largemouths and striped bass has reduced zooplankton-eating shad, alewives and perch.
The resulting increase in large zooplankton then crops the phytoplankton. With less algae, the water clears, making conditions conducive to the growth of aquatic vegetation. Once established, the weeds use up nutrients that otherwise might fuel future algal blooms. Habitat is improved, eventually leading to better fishing.
The process is called biomanipulation-altering the abundance of organisms at different trophic levels to change the aquatic community. To succeed, it requires stopping the influx of nutrients and establishing high densities of large predators to reduce populations of zooplankton-eating forage fish.
It also requires the cooperation of anglers through the release of these large predators. So the next time you land a big walleye, striper, pike or other big piscivore, do the ecosystem a favor: Take a photo and let it go.