Dieoffs are striking a cold-water species key to walleyes, muskies.
BRAINERD, MINN. - This spring's summerlike weather may be embraced by Minnesotans, but it spells trouble for ciscoes, a cold-water fish that serves as food for gamefish such as walleyes, northerns, muskies and lake trout.
Large ciscoe dieoffs -- likely caused by higher water temperatures and surface runoff that robs lakes of oxygen -- have become more common in recent years, and their populations have declined sharply in some lakes, including popular Gull Lake near Brainerd.
Researcher Andy Carlson calls ciscoes the "canary in the coal mine,'' an indicator that Minnesota lakes are changing. Ciscoes -- also known as tullibees -- will be among the first fish to feel the impact if Minnesota's summer climate becomes more like that of present-day Kansas over the next 85 years, as some studies have predicted.
"They're getting squeezed,'' said Carlson, a Department of Natural Resources fisheries biologist. The trends mean dieoffs could become more common and their populations could "blink out'' in some lakes. This year already is off to an ominous start: Many northern lakes set records for early-ice out, and air temperatures already have hit 90 degrees.
Carlson and two co-workers netted the silver fish last week on Pillager Lake near Brainerd and implanted some with tiny sonar transmitters to track their movement and measure water temperature, part of a study to help determine what the future holds.
"It could affect how we manage our walleye, northern and lake trout, over the next 10 to 20 years,'' he said.
Ping, ping, ping
The research is a fascinating blend of speed-surgery, 21st-century technology, biology -- and old-fashioned fishing.
On a warm morning, Carlson and fellow DNR fisheries biologist Rod Pierce and student intern Matt Gunderson placed a 300-foot fishing net in 220-acre Pillager Lake, waited just a few minutes, and pulled in more than a dozen ciscoes, most 11/2 to 2 pounds. The lake is loaded with them.
The goal was to implant four with small cylinder-shaped transmitters weighing less than a quarter ounce. But some of the fish failed to survive the delicate procedure.
After putting the fish in a tub and rushing to shore, the crew worked fast. Ciscoes, like trout, their relatives, are more likely to succumb to human handling than walleyes or northerns. Carlson rested a fish belly-up in a V-shaped platform, and used a small battery-powered pump to squirt water over its gills while he quickly made a 1-inch incision, popped the transmitter in and closed the wound with three stitches. Then he squirted some disinfectant on it and slipped the fish back into the lake.
Total time: 2 1/2 to 3 minutes.
Pierce, clad in waders, "walked'' the recovering patients in the lake by walking in the shallows while holding onto their dorsal fins, hoping to help reorient and stimulate them. Each transmitter costs about $700, and the batteries in them function for six months.
Some of the recovering ciscoes swam off, but others went belly-up.
"I don't see these making it,'' Pierce said has he guided two fish. When he let go, each flipped sideways.
So they recruited more "volunteers" and eventually sent four fish into the lake with transmitters, joining five others implanted earlier in the spring.
The transmitters emit a "ping'' like sonar in a submarine. They identify the fish and tell researchers the depth and water temperature where it is located. Other listening devices placed in the lake pick up the data, which Carlson regularly downloads into a computer.
Ciscoes are found in about 650 northern Minnesota lakes, but their population has been trending downward since the mid-1970s, hitting a record low in 2007. That was a particularly warm year, and major dieoffs occurred on Gull and Mille Lacs lakes, among others. Carlson expects the populations have rebounded somewhat, because 2008 and 2009 were cooler years.
But 2010 doesn't look promising.
Weather is just part of the likely cause of their decline. Carlson's research shows the fish are literally being squeezed. Here's what's happening: Ciscoes prefer water no warmer than 63 degrees. (The temperature was 68 on the surface of Pillager Lake this week.)
So as the surface water warms, they go deeper. But at the same time, nutrients and materials decomposing on the lake bottom consume oxygen, creating an oxygen-depleted "dead zone'' near the bottom. With more nutrients from runoff, that area grows, too.
The ciscoes are caught in the middle. They move to the small layer of water between those extremes. At Pillager Lake, they were limited to a 3-foot zone about halfway down in 42 feet of water. "There's nowhere for them to go,'' Carlson said.
While climate change solutions are long-term, there are some things that can be done short-term.
"If we can limit some of nutrients coming in, that could help,'' Carlson said. Protecting shorelines could help, too.
For now, ciscoes in central Minnesota are at the southern edge of their range -- a range that appears to be shifting north.
"It's possible they won't survive in some lakes,'' Carlson said.
Doug Smith • 612-673-7667
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