Minnesota’s Heritage Lake Trout – A Natural Legacy
Northeastern Minnesota is famous for the Boundary Waters Canoe Area Wilderness (BWCA) containing more than a million acres of protected wilderness and over two thousand lakes.
The 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 of deep, cold, clean lakes that are home to native lake trout that have survived since the Ice Age.
Lake trout, and some other cold-water species like mysis and deepwater sculpin, spread to Minnesota from their original glacial refugia via the large meltwater lakes formed as the glaciers retreated.
Lake trout likely were originally present in many more Minnesota lakes as the glaciers receded, and those we see now are the most resilient survivors of 10,000 years of climate and landscape change.
The location of most of these trout lakes in the BWCA has left them free from the impacts of lakeshore development, overfishing, displacement by stocked fish, and declines from invasive species.
Minnesota is one of only a handful of states with populations of naturally reproducing lake trout, and along with Alaska the only state with significant populations of wilderness lake trout.
Some indications are that isolation for thousands of generations has resulted in trout with locally unique genetic strains, perhaps making them well adapted for survival in Minnesota’s inland lakes.
In the second half of the 20th century, fisheries managers began to appreciate the potential value of these legacy trout populations, classifying these lakes as “heritage trout lakes” if they had no known history of lake trout stocking.
About four dozen heritage trout lakes were identified. In addition, strong evidence supports an additional 20 to 40 additional lakes having native lake trout without stocking or failed stocking.
Ancient, Unique and Fragile Ecosystems
Many heritage trout lakes have just a handful of fish species (even including minnows) along with various aquatic invertebrates. These elegantly simple communities have been able to survive for thousands of years.
The success of lake trout in Minnesota is often understood to depend upon the success of cisco populations, but in these small interior lakes cisco are typically not present. Instead, lake trout in many lakes feed mostly upon invertebrates. Thus, even among Minnesota’s relatively small number of lake trout lakes, these small heritage trout lakes are unique.
Those invertebrates have their own unique cycles, whether it be from day to night, or season to season, or from deep water to shallow. A key invertebrate in many lake trout lakes is the relatively large opossum shrimp (“mysis”; Mysis relicta). However, even mysis are lacking in many lake trout lakes, and lake trout stomachs examined during the summer are often full of much smaller organisms, such as cladocerans and copepods.
Lake trout are the apex fish predator in many of these waters, but other predators feed on the trout, such as otters, loons, eagles, and ospreys. So the health of these heritage trout lakes also impacts the other denizens of this northern lake country.
Lake trout require the cleanest, coldest water with the highest oxygen levels. Thus, the presence of lake trout indicate a lake is particularly clean and healthy. In fact, even relatively light development on non-BWCA lakes in northern Minnesota has been associated with loss of those lakes’ trout populations.
Thus, Minnesota’s heritage trout lakes contain ecosystems that are ancient, unique, and fragile.
Status and Risks to Heritage Trout Lakes
The heritage trout lakes are often difficult to reach, requiring at least a day of wilderness travel. That remoteness, plus tight budgets, reduced fisheries staff numbers, and alternative priorities has meant that relatively little is known about most heritage trout lakes.
Unfortunately, there are a number of BWCA heritage trout lakes that were known to once contain lake trout but which have become trout-free. Even the lakes where trout are survive, typically little is known about the trout populations or the key parameters relating to their ability to sustain cold-water inhabitants, such as oxygen and temperature conditions throughout the open-water season.
Many of the fish populations and water quality parameters of these heritage trout Lakes have been surveyed only once or twice in the last fifty years, and frequency at which these lakes are surveyed has been severely reduced in the past 20 years.
Thus, despite being perhaps the cleanest, wildest, most unique and fragile lakes in Minnesota, surprisingly little is known about the heritage trout lakes and their current health and status.
Significant questions include the health of the fish populations, whether invasive species have reached their lakes, and whether the lakes are warming in a manner that will impact their long-term survival.
A need exists to assess the status of the heritage trout lakes, as well as to begin long term assessments of key indicators of lake health, including lake temperature and oxygen profiles; presence of invasive species, and health of the trout populations themselves. If we can understand these ecosystems and their current health, and monitor them for years to come (such as for temperature, oxygen, and invasive species), we can both value and protect them.
Unanswered questions include:
1. What is the oxythermal habitat status of Minnesota’s heritage trout lakes, such as TDO3 (water temperature at DO = 3 mg/L) or alternatively thermal habitat volume (THV), of Minnesota’s remaining trout lakes, and how is that THV likely to change in coming decades?
2. How many lakes are at risk of losing cold-water habitat needed by lake trout, and which lakes are likely to continue to support the species through a prolonged period of climate change?
3. How many, and which, lakes in Minnesota still support their original lake trout populations?
4. Are there genetic adaptations in the isolated populations that have allowed them to survive for thousands of generations, and if so which populations potentially possess such adaptations and how can those populations be preserved?
5. How many lakes have been affected by introductions of undesirable fish species (like walleye and smallmouth bass), and how many are still vulnerable to such introductions?
6. How many lakes have already been infested by other undesirable or invasive species (e.g. rainbow smelt, spiny waterflea, and zebra or quagga mussels)?
7. What effect will future infestations have on lake trout populations?
8. How many lake trout lakes are currently harvested at levels near or above long-term sustainability?
9. How will extreme weather, such as long-lasting heat waves that warm lakes or unprecedented rain storms that could flood lakes and spread non-native fish, impact heritage lakes?
10. What will be the effect of droughts that result in large, intense fires in forested landscapes, releasing large pulses of nutrients?