The DNR manages roughly 180 lakes for stream trout. Because stream trout don't reproduce in lakes, regular stocking is required to sustain these fisheries. Each year, the DNR stocks these waters with a total of roughly 400,000 rainbow trout, 150,000 splake, 90,000 brook trout, and 20,000 brown trout.
Small lakes managed only for stream trout are often designated stream trout lakes, where fishing seasons and regulations are more restrictive in order to better manage the fishery. Some larger stream trout lakes are also managed for both warmwater species such as walleyes and; fishing regulations on these lakes are not as restrictive to allow for pursuit of warmwater species. Because the trout in these lakes live in the deep, cold water and the warmwater fish in the upper layer, these are called two-story fisheries.
Stream trout lakes are managed to create conditions that are suitable for sustaining trout. The lakes often historically contained mainly northern pike and white suckers. The DNR rehabilitates these lakes in preparation for stream trout stocking by killing the existing fish species with a fish toxicant to eliminate competition and predation. Lakes selected for stream trout stocking must fit certain criteria: they have to have cold, clean, well-oxygenated water that trout need to survive, and the existing fishery would otherwise have been relatively poor.
Stream trout lakes are extremely popular with many anglers and can receive intense fishing pressure. For example, Courthouse Lake in Carver County gets roughly 635 angler hours per acre per year. That's more than 60 times the pressure on some remote BWCAW stream trout lakes.
Stream trout lakes are relatively expensive to manage compared with other Minnesota fishing lakes because trout populations must be maintained entirely by stocking. Because many of these are remote, it takes aircraft and additional staff to rehabilitate and stock them. In addition, they have to be rehabilitated again if unwanted species get into the lake and start crowding out the trout. This usually happens when other fish species are introduced into the lake by anglers, commonly due to the use of live fish for bait. That's why it is unlawful to use live minnows on designated trout lakes.
Minnesota is one of only a few states in the U.S. with naturally reproducing lake trout populations. And Minnesota, with 116, is second only to Alaska in the number of lake trout lakes.
The DNR manages lake trout populations in the 116 lakes by stocking in some lakes, adjusting harvest regulations and seasons, and protecting habitat. Some of these lakes are managed with lake trout as the only game species. Others are managed for lake trout along with other game species such as rainbow trout or walleyes.
Stocking: Lake trout eggs are hatched and reared in DNR hatcheries. Each year, roughly 100,000 fingerling (6-inch) and 380,000 yearling (8-inch) lake trout are stocked in inland lakes. Improvements in hatchery technology over the past few decades has increased the survival of reared and stocked fish, allowed the DNR to raise larger fish for stocking, and more than doubled egg production. Research scientists have conducted studies to determine the best native strains of lake trout to use for stocking to ensure the genetic health of populations.
On the 35 known native lake trout lakes, called Heritage Lakes, the DNR does not stock lake trout so that the genetic purity of the unique existing strains remains untainted.
Regulations: Because lake trout grow slowly, begin reproducing at a late age, and live in relatively sterile water, they don't repopulate quickly when their population decreases. These factors make it easy for anglers to overfish a lake trout lake and deplete the lake trout population. The possession limit of 2 fish per angler is intended to limit harvest of lake trout to sustainable levels.
Ongoing evaluations: Over the past 15 years, the DNR has surveyed the populations of about half the state's lake trout lakes. This involves collecting information on lake trout sampled in survey nets. The DNR also conducts creel surveys on some lake trout lakes to understand how many are being caught by anglers both in summer and winter. Biologists also monitor other fish populations to see if other fish such as the walleye or introduced species such as the rainbow smelt are excessively competing with or preying upon lake trout.
Habitat protection: Lake trout need clean, cold water to survive. Poor watershed land use practices and ineffective septic systems can add too many nutrients to lake trout waters and upset the ecological balance that sustains lake trout habitat. Climate change is also a significant threat to lake trout habitat, with potential to warm water temperatures beyond lake trout tolerances. The DNR has done research to determine lakes that have to greatest likelihood of supporting coldwater species like lake trout based on predicted future climate, and will focus efforts to maintain suitable land use in these watersheds through partnerships with non-profits and local governments.