Stream trout lakes
The DNR manages roughly 180 lakes for stream trout. Because stream trout don't reproduce in lakes, this is mainly a stocking program. 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 called designated stream trout lakes. Some larger lakes are managed for both warmwater species such as walleyes and bass and for stream trout. 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 artificial fisheries in the sense that the trout aren't native to the waters. The lakes 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. Otherwise the northerns just eat up the stocked trout like dogs in a sausage factory and species such as suckers would compete with the trout for food. 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 has to be 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 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 bullheads or other fish mixed in with minnows get dumped into the lake by anglers. That's why its unlawful to use live minnows on designated trout lakes.
Lake trout lakes
Minnesota is one of only a few states in the U.S. with naturally reproducing lake trout populations. And Minnesota, with 99, is second only to Maine in the number of lake trout lakes.
The DNR manages lake trout populations in the 99 lakes by stocking, adjusting harvest regulations and seasons, and protecting habitat. Some of these lakes are managed solely for lake trout. Others are managed for lake trout along with other present species, such as rainbow trout and 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 conducted studies to determine the best native strains of lakers to use for stocking to ensure the genetic health of lake trout 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. To protect the older, spawning lake trout, the DNR has regulations restricting harvest to no more than three over 16 inches. Also, a break in the fishing season during April and early May protects some spawning lakers when they are spawning in shallow water.
Because many lake trout lakes are in relatively inaccessible areas of the Boundary Waters Canoe Area Wilderness, these regulations have been sufficient in protecting existing lake trout populations from overharvest. However, as angler pressure continues to expand on these waters--for example, from increasing numbers of ice anglers using dog sleds to reach them--the DNR and fishing conservation groups may propose further restrictions to ensure that the lake trout populations stay healthy.
Ongoing evaluations: Over the past 15 years, the DNR has surveyed the populations of about half the state's lake trout lakes. This involves counting the number of lake trout counted 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 species such as the walleye or exotics such as the rainbow smelt are competing with lake trout for food.
Habitat protection: Lake trout need clean, cold water to survive. One threat to lake trout populations are leaky septic systems on lakes cabins and homes. These can add too many nutrients to lake trout waters and upset the ecological balance that provides lake trout habitat. Because the DNR has no authority to regulate septic system, its lake trout habitat management consists mainly of informing landowners and anglers of the risks to lake trout fisheries from polluted water.