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illustrations of largemouth bass, walleye and lake trout.

Forecast: Warmer Waters

How will Minnesota fish, anglers, and fisheries managers adapt to warmer lakes and streams?

By Jason Abraham

A stiff north wind cut at our backs as my 5-year-old daughter, her cousin, and my brother-in-law huddled with me over four holes drilled into the thick ice of Silver Lake, a 114-acre panfish and walleye hot spot outside Moorhead.

Our New Year's Eve ice-fishing expedition was off to a bad start. The fishing holes were freezing over, the kids' feet were getting cold, and my arms ached from hand-auger duty. We'd been on the lake for all of 10 minutes when my daughter pointed to a nearby portable shelter and asked, "Why don't we have one of those?"

The truth was, we hadn't really needed one in recent years, when my daughter and I had happily fished in the fresh air, perched on a bucket. But this past winter was much different. To many Minnesotans, myself included, it seemed like a throwback to the good "old-fashioned" winters. During the past 10 years, mild temperatures and less snow have become the norm during winter. Since the winter of 1996-1997, only two winters have been cold and snowy enough to rate above "mild" according to the DNR winter severity index, which gauges the severity of winter weather based on snow depth and daily temperature.

"Climate scientists from across the globe are virtually certain that concentrations of greenhouse gases in the atmosphere will lead to warmer temperatures in the next 50 years," says Don Pereira, fisheries research manager with the Department of Natural Resources. "Given that, we need to begin thinking about adapting our fisheries management to a warmer climate."

Protect, reduce, adapt

Pereira has been compiling climate change data to get a better picture of how a warming climate might affect Minnesota's fisheries. In presentations to anglers and fisheries scientist colleagues, he paints an honest but hopeful picture of how protecting habitat, reducing carbon emissions, and adapting fisheries management can help maintain Minnesota's fishing traditions even as the climate warms. "Climate change will exacerbate the habitat challenges we already face," Pereira says. "We can start preparing simply by focusing on our existing habitat issues, such as emphasizing best management practices for lakeshore owners, maintaining buffer strips along lakes and streams, and reducing erosion by improving development, logging, and agricultural practices."

No going back

No matter how many drivers swap gas-gulping sport utility vehicles for high-mileage hybrids, or how many incandescent light bulbs are switched for fluorescents, there is compelling evidence that Minnesota's climate -- and lakes -- will warm in the next 50 to 100 years.

According to the Intergovernmental Panel on Climate Change and the American Association for the Advancement of Science, the average temperature across North America will increase roughly 5 degrees Fahrenheit by the year 2100. The reasons are twofold: Enough carbon dioxide and other greenhouse gases are already aloft to warm the atmosphere for at least the next 90 years, according to the IPCC. And global industrial development will continue to contribute greenhouse gases to the atmosphere.

By 2095 Minnesota's climate could be warmer than the North American average because of its location in the middle of the continent, where oceans do not mitigate temperature, Pereira says. According to IPCC projections, central North America could be approximately 4 to 11 degrees warmer across winter and summer seasons by 2099.

"The biggest effects and most noticeable already are in winter," Pereira says, "especially when we look at ice cover." John Magnuson, professor emeritus at the University of Wisconsin-Madison, has compiled a database that shows a steady decline in ice duration on 38 lakes across the globe. One example shows that ice on Lake Mendota in central Wisconsin stayed for 120 days in 1860. In the winter of 2000, it was ice free in about 80 days.

Minnesota's long-term data suggests ice out has occurred three days earlier on average over the past 20 years, according to Heinz Stefan, a University of Minnesota professor specializing in environmental hydrodynamics and water quality monitoring.

Because warm air holds more moisture, some scientists believe climate change could result in more frequent heavy rainstorms. Heavier rainfall increases stormwater runoff and reduces the amount of water that infiltrates into soil to benefit plants and recharge groundwater aquifers, Pereira says. Since runoff does not seep into the ground, it's warmer and adds sediments and other pollution when it enters lakes and streams.

Walleye and northern pike

Generally, as Minnesota's climate warms, lakes in the forested north, which currently have a relatively short period of optimal growth for walleye and northern pike, will become more productive for cool-water species. However, because these game fish depend on cold-water prey species such as tullibee, the temperature shift could cause cascading effects that are difficult to predict, Pereira says.

For example, if carbon dioxide in the atmosphere doubles in the next 90 years as predicted by the IPCC, the surface water temperature of Lake Vermilion would stay in an average range between 59 and 77 degrees for about six weeks longer than it does now, according to Stefan. That could lead to better walleye and northern pike growth as both species grow faster at water temperatures around 71 degrees, Pereira says. However, that temperature range may not fully benefit walleye and northern pike because Lake Vermilion tullibee, which thrive in 62-degree water, would start to experience summerkill.

"The thermal niche for walleye and northern pike would be better," Pereira says, "but the quality and quantity of food might be disrupted. We could see compromised growth of game fish because of that."

In southern lakes, such as Lake Pepin on the Mississippi River, water could reach average temperatures above 84 degrees during the summer, according to Stefan's projections. If the water temperature warms in Lake Pepin, which currently has an average high temperature of about 77 F, large walleye would likely stop feeding and lose body mass during the warmest period of summer, Pereira says.

While a warmer climate might extend the growing season for cool-water fish species such as walleye and northern pike in southern Minnesota lakes, Pereira says conditions in midsummer will be too warm for those species to feed and grow. Moreover, fish that are stressed are more susceptible to disease and hooking mortality. "In general, prime walleye waters like Lake Pepin or Mille Lacs may become less than prime," Pereira says, "but they will remain good walleye systems."

Tullibee

The effect of climate change may be more dramatic on cold-water species such as tullibee. Currently, this species is found in about 650 lakes in the northeastern and north-central part of the state. An average temperature increase of 3 degrees in July would imperil those species living in shallower lakes at the southern and western edge of their range in Minnesota, Pereira says.

"Tullibee will persist in lakes that offer oxygenated deep, cool water during the warm months of the year," Pereira says. "There are probably 50 to 100 lakes that may offer safe refuge for tullibee in a warmer climate -- if we protect water quality."

Lake and stream trout

Lake trout, a species found primarily in northeastern Minnesota's coolest and deepest lakes, would also be imperiled by warmer waters. "It's likely that lake trout would be found only in the lakes that were deep, clean, and cold enough to offer safe refuge during the warmer months," Pereira says.

Stream trout of southeastern Minnesota could be conserved by restoring forested strips that provide shade to cool spring-fed streams, Pereira says. Restoring native prairies and other land cover would promote groundwater infiltration and thus help recharge aquifers that feed cool, clean water to trout streams.

Panfish and bass

While the state's best walleye fishing shifts to the north and east, bluegill, sunfish, and smallmouth and largemouth bass -- which thrive in water around 80 degrees -- would begin to expand in northern lakes. In some cases this change would be to the detriment of species like walleye and pike due to increased competition. "The longer growing season will greatly benefit fish species like bass and bluegill," Pereira says. "It won't get warm enough to harm these species."

Sentinel lakes

"There is tremendous interest in this issue," says DNR Fish and Wildlife director Dave Schad, who has been encouraging DNR efforts to engage hunters and anglers in the discussion by bringing the issue forward at the 2007 roundtable discussions and through Pereira's presentations. "But there's still uncertainty about the effects of climate change on specific landscapes. We need to engage the public to learn more and effectively manage natural resources in the face of this challenge."

To that end, the DNR and Minnesota Pollution Control Agency are planning a joint effort to monitor two-dozen lakes across Minnesota to learn how the state's water bodies are holding up to the stresses of climate change, watershed development, invasive species, and runoff. The sentinel lakes project will look at water temperature, oxygen, phosphorus, invertebrates, aquatic vegetation, fish growth, and a host of other indicators.

None of the state's largest or most famous lakes are among the sentinel lakes. "The lakes we chose for this project represent average lakes in each of the state's ecoregions," says Ray Valley, DNR fisheries research biologist.

For the next four years the DNR and the PCA will take frequent water samples from these lakes to determine how climate change and other factors are affecting all the indicators. Project leaders will determine which indicators are best for more expansive lake monitoring across the state, though the agencies plan to continue intensively studying the 24 sentinel lakes well into the future.

In the past three decades, census data show Minnesota's most popular lake regions have sustained significant population growth, while other rural areas lost or maintained a steady population. More cabins, homes, docks, and lakeshore development have put additional pressure on lakes.

With climate change and development both stressing lake ecosystems, lakes are becoming more vulnerable. "It will hit critical mass soon with climate change," Valley says. "We have to pay close attention if we are to mitigate climate change's worst effects."

The DNR and the PCA hope to engage counties, townships, lake associations, and soil and water conservation districts in the monitoring effort and in efforts to enact changes that will protect lakes with good watershed management practices.

"No one wants to hear the fish they've been fishing for are going away and that there's nothing we can do about it," Valley says. "We need to bring citizens along, show them the effects of habitat loss and climate change, and then work together to mitigate the impacts."

Focus on fish

Back on Silver Lake, my brother-in-law and I plied our young fishing partners with hot chocolate, candy, and extra hand warmers in hopes of extending our New Year's Eve fishing expedition. Our bobbers remained still in their icy holes and the kids' enthusiasm waned with each chilly blast of wind.

After 45 minutes on the ice and a raucous game of red-light-green-light, we decided to call it quits. It would have been easy on this frigid day to think that a little climate change would be a good thing. I knew better. Silver Lake is a tiny natural oasis surrounded by huge agricultural fields. For years, the lake's bluegill, bass, and walleye have survived silt and fertilizer-laden runoff. Hotter summers and warmer winters will stress the fish population more.

All across Minnesota, the future of fish populations depends on what happens above the surface of our water.

Shoreland property owners can learn more about keeping lakes and rivers healthy.

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