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Tullibees (also known as cisco or lake herring) are a coldwater fish that live in many of the nicest lakes in Minnesota. They provide excellent forage for trophy walleye, northern pike, muskellunge, and lake trout. A requirement for cold, well-oxygenated water allows them to primarily live in deep lakes that have good water quality. This requirement makes them especially vulnerable to two of the greatest threats for Minnesota lakes - eutrophication and climate warming.

In the summer, tullibees live in the cold water below the thermocline in most Minnesota lakes. Unfortunately, oxygen concentrations below the thermocline decline throughout the summer in many lakes, especially in more eutrophic systems. Loss of water quality increases the rate at which dissolved oxygen is lost below the thermocline. As more phosphorus (the key nutrient limiting algal production in most Minnesota lakes) runs off from poor land use practices in a watershed, more algae is produced in the upper layers of a lake. As these algal cells die and settle into the deeper portion of the lake, decomposition consumes oxygen, and concentrations decline in the water below the thermocline.

As the upper layers of the lake warm, tullibees can experience a "squeeze" as they move up in the water column to avoid low oxygen concentrations and encounter the warmer water. In some summers, the squeeze is so great that some tullibees will die as they get forced into lethally warm temperatures. Climate warming affects tullibees from the other direction - increasing water temperatures above the thermocline further add to the possibility of mortality if tullibees are forced into the upper layers.

Fortunately, many deep lakes with good water quality maintain adequate oxygen conditions below the thermocline all summer long, even in warm summers. The Minnesota DNR Fisheries Research Unit, in conjunction with the University of Minnesota, have identified 176 "refuge" lakes that are deep and clear enough to sustain tullibees even after climate warming occurs in Minnesota.

Sixty one of these lakes are especially deep and clear and are termed Tier 1 refuge lakes. These are some of the nicest lakes in Minnesota and include lakes like Sea Gull, Snowbank, Gunflint in northeastern Minnesota and Ten Mile, Big Sand, and Big Trout in north central Minnesota.

Other lakes, called Tier 2 refuge lakes, are not quite as deep and clear will still offer adequate oxygenated water below the thermocline and include important lakes such as Whitefish, Kabekona, and Roosevelt lakes. Maintaining the water quality in these lakes is critical for them to function as refuges to climate change.

The Department of Natural Resources is starting to look at ways to protect the water quality in these important lakes. Partnering with other state agencies such as MPCA and BWSR, local units of government, and lake associations will be a key strategy for protecting water quality. These strategies will primarily focus on keeping the forested lands forested in the watersheds of these lakes (most are located in the forested portion of Minnesota). Finally, there will be many benefits for protecting the water quality in these lakes far beyond providing refuge for coldwater fish such as tullibees. So, you will likely see programs "Protecting tullibee refuge lakes" in the future!

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