Mike Duval of Brainerd and Tom Jones of Aitkin are Minnesota Department of Natural Resources (DNR) fisheries biologists. They work extensively on fish habitat issues. In the interview below, they discuss this spring’s early ice-out, and what it means for fish behavior and fishing patterns.
Minnesota lakes became ice free very early this year. Was this a fluke or part of a larger pattern?
Duval: Early ice-out is occurring around the globe. What Minnesota experienced this year is part of a larger global pattern. Earlier ice-out dates have been observed throughout the Midwest, continental U.S. and more broadly in Canada, Russia, Scandinavia and Japan. John Magnuson, a limnologist, and his colleagues at the University of Wisconsin Center for Limnology in Madison, have concluded that the length of time that lakes in the northern hemisphere are ice-covered has become shorter.
When did this pattern develop?
Duval: According to Magnuson’s research, the average ice out date for Minnesota lakes has been occurring earlier and earlier over the past three decades. Ice-out in Minnesota is occurring approximately three to five days earlier every 10 years in the central part of the state and nearly a week earlier per decade for southwestern Minnesota
So does that mean spring is coming earlier to Minnesota lakes?
Duval: That’s basically correct. The mid-April ice break-up has advanced northward nearly 100 miles over the past three decades. The east-to-west mid-April ice break-up line used to cut through the Montrose area. Today it’s closer to Brainerd. At this rate, by the end of this decade spring will come to Minnesota lakes nearly a month earlier than it did 40 years ago.
What are the implications of early ice out?
Duval: It will mean a variety of things: longer open water periods, warmer water temperatures, more evaporation, and more inter-mixing of lake water because the ice “lid” that capped the lake will have been removed for a longer period of time. Over time, Minnesota waters and their fish populations will tend to be more characteristic of states to the south of us.
Will warmer water improve or reduce fishing quality?
Jones: That depends. Temperature is a limiting factor for many fish species and, thus, a critical component of their habitat. Clearly, warmer water temperatures will be detrimental to tullibee, lake trout and other species that depend on cold water.
Tullibee, for example, could disappear in the next few decades from some southern and central Minnesota lakes due to a combination of higher water temperatures higher in the top portion of the water column, and insufficient oxygen in the lower portion of the water column where temperatures are cooler.
On the other hand, bass will do just fine because they can tolerate warmer water temperatures. In fact, bass abundance is already increasing across Minnesota. Growth rates should improve as well because of longer growing seasons. Overall, the early ice-out trend will create winners and losers, depending on the temperature habitat requirements of each particular species.
What’s the long-term forecast for walleye and northern pike?
Jones: In northern Minnesota lakes, higher water temperatures may benefit walleye and northern pike by increasing the length of the growing season. However, in southern lakes, temperatures may become too warm and lead to periods of mid-summer stress. If this stress becomes too severe, fish weights could decrease and walleye mortality could increase.
How will the early ice-out in 2012 affect walleyes this year?
Jones: For walleye, early ice-outs can lead to poor year classes. The early start for walleyes means that they often hatch before their food supply develops. After walleye absorb their yolk sacs, they depend on zooplankton for forage. If the water is not sufficiently warm, zooplankton will not be present in large numbers, and many of the young walleye can starve. The cool weather this April increases the chance that zooplankton abundance will be low when walleyes hatch this year.
What about impacts to bass and sunfish?
Jones: Bass and sunfish species depend on a good spring warm-up to spawn. Bass spawn when temperatures reach the low 60s, usually in late May or early June. In very cool years, they may not spawn at all if the water does not get warm enough in time. While early ice-outs don’t guarantee that aquatic processes occur very early, they do increase the likelihood that bass and sunfish will be able to spawn in time to have an adequately long growing season.
Did early ice-out affect DNR operations this spring?
Jones: The early spring meant fisheries crews set up walleye egg take sites a week earlier than ever before. However, because of the cool spring, the walleye egg takes peaked and ended at about average dates. The extra time of cool, open water also allowed fisheries crews to harvest “carryover” walleye from some of last year’s walleye rearing ponds. Removal of the carryover fish amounted to more than 12,000 pounds of walleye stocking spread over dozens of lakes, and also will increase survival of fry raised in these ponds this summer.
Do you think this will be a “normal” opener?
Jones: Opening success varies from year to year and that will be true this year, too. Walleye, like most gamefish, exhibit seasonal changes in behavior and distribution. Walleye spawn within a few weeks of ice-out, when water temperatures are in the low 40s. After spawning they tend to remain in shallow water for a few weeks, feeding on minnows and other small fish. As water temperatures warm, walleye tend to gradually migrate into deeper water. The ice went out early, but cool April weather has prevented water temperatures from rising quickly. As we speak, about one month after Mille Lacs Lake opened, and about two days after average ice-out, water temperatures in Mille Lacs are still in the low 40s, about 3 degrees warmer than average and only about six days ahead of average. On the other hand, in 2010, ice-out occurred April 5, and temperatures by the end of April were 50 degrees. My point is that the fish behavior around opening day depends not just on ice-out dates, but also on weather after ice-out. Walleye fishing for the 2012 opener probably won’t be as advanced as anglers might expect given the early ice-outs.
So where would you fish?
Jones: My sense is that many walleye and pike will probably still be in their traditional early season locations on the 2012 opener. I would start there, but be prepared to try a little deeper if fishing is slow.
Duval: I agree. While walleye may be a little further along than usual, the April weather has kept lakes cool, so most of the walleye will be close to where anglers usually find them on opener.
Do you think the fishing opener will move to an earlier date sometime in the future?
Duval: Climate change is a long-term process that cannot be determined based on short-term weather patterns. Biologically, our agency’s interest is delaying the opening date until walleye and northern pike have completed spawning and have begun to disperse from spawning areas. The timing of spawning is based on both day length and water temperature, so it is uncertain how much of a shift would occur based on water temperature alone. Any future changes in the opener date should only occur based on scientific monitoring and documented long-term trends. We want to ensure the long-term sustainability of our fisheries.