Minnesota's two-story lakes can bring different fishing experiences to one spot

From the public access on its north side, Bad Medicine Lake in Becker County stretches out in a west-oriented line. The lake includes about 800 acres of freakishly clear water surrounded by towering pines. And yet early in the fishing season, seasoned anglers unfamiliar with the lake might be so perplexed by the apparent haphazard trolling of others that they forget to notice the lake's beauty. A look at the floating or shallow-diving Rapalas that anglers have tied on, or the fact they're dragging them over 60 feet over water or more, would serve only to confuse things.

What sort of joker drags lures near the surface over open water on opening day of the walleye season? This is where I'll raise my hand and offer an easy explanation: We're targeting rainbow trout, not walleyes.

Bad Medicine is known as a two-story lake, which in biological terms means it can support warm-water species such as walleyes and bass and cold-water species such as trout. In fishing terms, that simultaneous warm-cold state can make for an unmatched fishing experience. A group from our fishing camp can forgo walleye fishing on the opener and go to Bad Medicine. We can leave the public access, toss out our lures, and within several hundred yards of trolling have hooked a trout, its silvery sides reflecting the rays of the sun. We can travel a few hundred more yards, pass an island where the water is shallow and the bottom is rocky, and, if we're lucky, hook a chunky smallmouth bass and experience once of the best fights available in freshwater.

Given the vastly different environments cold-water and warm-water species require, there are relatively few lakes in Minnesota where they coexist. Yet two-story lakes are scattered throughout the state. The Minnesota Department of Natural Resources stocks these lakes primarily with rainbow trout, and other stream trout such as brown or lake trout. The fish don't reproduce, so regular stocking is required to maintain fishable populations. With the exception of the fish communities that live within them, two-story lakes for much of the year aren't all that different from other bodies of water. But during the summer, the differences are stark.

"Trout, of course, need cold water and they also need pretty high levels of oxygen in the water in order to survive," said Brian Nerbonne, a DNR stream habitat consultant. "A lot of lakes don't have the right mix of temperatures and oxygen, especially during the summer."

In many lakes, the water is cold and deep enough to support trout, but in most cases there isn't adequate dissolved oxygen within that part of the water column to keep them alive. In these lakes, warm-water species spend their lives in the relative shallows, though larger individuals — northern pike and walleyes, in particular, which have lower oxygen requirements than trout — may seek refuge in cold water, if it's available, during the summer months. At Bad Medicine and other two-story fisheries such as Grindstone Lake near Sandstone, trout are near the surface and easily accessible early in the year when the water is cool, which explains why anglers, including my group, troll shallow-running lures around the lake when the season commences. But during the summer months — when anglers still catch fish such as bass and panfish in the shallows — rising water temperatures force cold-water fish deep. Anglers still can catch them, but doing so often requires specialized equipment.

As they age, lakes generally become warmer and the deep water holds less and less oxygen because of a process called eutrophication. It speeds up when nutrients such as phosphorus, an essential element for plant life, enter the water. For that reason, it's highly unlikely a lake would become a two-story fishery. Far more likely is a two-story lake over time becoming suitable only for warm-water species.

"If you don't have that and it doesn't support trout in the dead of summer, we just aren't going to invest in it," he said.

If those conditions are present, consideration can continue.

The second thing to think about is what else lives in the lake. If there are too many northern pike, for example, stocked trout may simply wind up in the bellies of larger predators. Predation can be a real issue, said Chris Kavanaugh, DNR regional fisheries manager in Grand Rapids. Trout did well in one of the lakes in his area until someone released pumpkinseed sunfish. In a competition between the two species, trout lost, and the agency quit stocking them. A third consideration is that trout need food, too, so a lake must have a zooplankton community that provides sufficient forage. And then it comes down to whether people are interested.

At Bad Medicine creel surveys have shown that about 80 percent of fishermen on the lake are there to target trout, said Nathan Olson, DNR area fisheries supervisor in Detroit Lakes.

"It's a great way to utilize a different type of resource," he said.

In the early 1990s, the Leech Lake Band of Ojibwe introduced ciscoes, another cold-water species, into the lake to provide alternative forage for the lake's northern pike. In time, ciscoes established a population and pike sizes improved, which correlated with a decrease in the total number of pike in the lake. Given the lack of success with walleyes, the fact ciscoes persisted, and the somewhat reduced pike population, DNR officials began to ponder stocking trout in Long Lake to create a two-story fishery.

After collecting biological data and public input from people in the area, the agency stocked rainbow trout just before the lake froze over last winter, and again this spring.

"Hopefully it pans out," Schultz said. "It doesn't work out very often that you can have a two-story fishery. They're really unique and pretty cool for that reason."