Minnesota’s northeast border region is famous for the Boundary Waters Canoe Area Wilderness (BWCA), totaling 1.1 million acres of protected wilderness containing over 2,000 lakes within the Superior National Forest. The greater BWCA region was sculpted out of bedrock by glaciers that receded roughly 10,000 years ago. In the remote interior of the BWCA lies a little known, but remarkable, ice age legacy: a collection approximately 100 particularly deep, cold, and clean lakes that are perched near the top of the continental divide and have been protected from development. These lakes are among the most pristine anywhere in North America, and are home to native lake trout that have survived since the last ice age.
These cold, clean trout lakes are often quite small and are generally isolated from one another. In many of these lakes the native lake trout have never been replaced with, or displaced by, stocked fish. Often these lakes are also free of non-native species that have destroyed other lake trout lakes in Minnesota and elsewhere. Their isolation for thousands of generations seems to have resulted in some locally unique genetic strains, well adapted for survival in Minnesota’s inland lakes.
In the second half of the 1900s fisheries managers began to appreciate the potential value of these legacy fish populations, classifying lakes as “heritage” trout lakes if they had no (or minimal) known history of lake trout stocking. These heritage trout lakes are often difficult to access, requiring at least a day of wilderness travel to reach. That remoteness, plus tight budgets, reduced fisheries staff numbers, and alternative priorities has meant that relatively little is known about most individual heritage trout lakes: Many of their fish populations and water quality parameters have been surveyed only once or twice in the last hundred years, and frequency at which these lakes are surveyed has actually been severely reduced in the past 20 years.
Lake trout require cold water (55oF or less) and high levels of dissolved oxygen (over 5.0 ppm). During the summer months, when surface waters are warm, they must have access to well-oxygenated cold water in a lake’s deeper waters, an area known as thermal habitat volume (THV), essential for trout survival.[1] In small lakes, lake trout also seem to do best when they face little or no competition from other predators. Lake trout spawn in the fall, and successful spawning requires clean rocky or coarse gravel bottom types. Often, spawning is confined to just a couple small sites with the correct conditions. Although lake trout lakes support some popular fisheries in Minnesota, most cannot support much lake trout harvest. The remote nature of many of the Minnesota’s smaller lake trout lakes has helped keep harvest on most at sustainable levels.
Today, our lake trout lakes face profound challenges, including a rapidly warming northern climate, land use changes and increased nutrients, introductions of undesirable fish species and invasive species, and high angling harvests. The warming climate will inevitably increase lake water temperatures and increase the length of time over which lake trout must rely on deep cold-water summer refuges, thereby decreasing THV. Land use changes (mainly reduced forest cover) and increasing nutrients can cause greater loss of oxygen in cold water refuges, and cause an influx of sediment that can degrade spawning areas. Increasing forest fires from climate-related warming may contribute to significant nutrient loading in some lakes, and there is ongoing work being done to assess possible airborne delivery of nutrients into Minnesota lakes. The list of undesirable fish species includes two that might seem unlikely: walleye and smallmouth bass. Neither is native to most lake trout lakes, and ranges for both have been expanding, including into heritage lake trout lakes. Other invasives, such as the spiny water flea that are known to disrupt aquatic ecosystems, are found in some nearby BWCA lakes. Their impact on heritage trout lakes could be particular significant because many heritage lake trout feed primary on invertebrates. Finally, on Minnesota’s more accessible lake trout lakes, and even on several of its small remote lakes, lake trout harvest by anglers can, in some years, reach levels that are near or above those considered sustainable by the species in these relatively unproductive lakes. Excessive harvest reduces the number of larger fish in the population, and can eventually reduce or eliminate reproduction by removing too many adult fish.
Heritage lake trout and their supporting ecosystems are a unique and priceless natural legacy, but one for which we lack adequate insights. These lakes are among Minnesota’s most pristine, least-impacted ecosystems. Unfortunately, this incredible natural legacy is at risk of being lost even before it is fully understood.
This proposal outlines a plan for a collaboration of private and public entities to work together to assess the current range and status of heritage trout lake ecosystems, and to propose a long-term approach to research, monitoring and management actions to protect them during a period of profound climate change that will continue for centuries to come.