By Darby Nelson
DNR fisheries biologist Pete Jacobson throttled up the motor as we left the no-wake zone and headed out onto Twin Lake near Annandale, 55 miles northwest of the Twin Cities. Jacobson came to gather oxygen data and temperature data to gauge the lake's condition. His research is part of an ongoing study of the plight of Minnesota's tullibee populations.
Tullibee (Coregonus artedi) -- also known as cisco or lake herring -- are silvery-sided, iridescent fishes of the open water beyond the shore. With dainty mouths that feed on small crustaceans and aquatic insect larvae, tullibee don't often end up on the ends of anglers' hooks because they don't typically eat bait-sized prey. These members of the salmon family can weigh up to several pounds in inland lakes. Rare 5 pounders have been caught, but they typically weigh less than a pound.
"Tullibee are a great 'canary in the mineshaft' species," says Jacobson. "If you have tullibee in a lake, you know things are pretty good. If they are declining, you know something is wrong."
Restricted to roughly the northern half of Minnesota, tullibee must have cold, oxygen-rich water to thrive. Even slight changes in temperature and available oxygen can squeeze these sensitive fishes out of their comfort zone, particularly in central Minnesota at the southern extent of their range.
But tullibee aren't the only living things affected by changes in oxygen and temperature, two major drivers of lake ecosystem dynamics. According to Swedish limnologist Christer Bronmark, "Low oxygen concentrations in the [deeper water] is one factor resulting in a severe deterioration of living conditions for most freshwater organisms." Because tullibee are more sensitive to these changes than most other species, their state of well-being can signal the well-being of their lake -- as well as the northern pike and walleye that feed on tullibee.
Protecting lakes from excess nutrients, such as phosphorus, is crucial to maintaining sufficient oxygen levels, particularly with global warming likely to increase water temperature. Warmer water increases oxygen requirements for creatures like fish.
To gauge the health of tullibee habitat, and by extension the health of Minnesota's cold-water lakes, Jacobson's study is a comprehensive and statewide analysis of temperature and oxygen data.
"Tullibee are declining across Minnesota," Jacobson says, "And we need to know why."
Because I was a tullibee researcher more than 30 years ago in my graduate school days, Jacobson invited me to accompany him gathering habitat data from lakes in the Annandale-Paynesville area in early August. Like their relatives the lake trout and whitefish, tullibee are mostly confined to the glaciated bedrock basin lakes of the north and require cold, well-oxygenated water to survive. Populations around the Annandale area live on the southernmost edge of the species range in Minnesota and are in significant decline here, according to gillnet surveys.
Anchored over the lake's deepest spot, some 88 feet down, Jacobson lowered the temperature-oxygen probe just below the water surface. In summer many Minnesota lakes are structured like a three-layer sandwich. Surface winds mix lake water to create a uniformly warm, well-oxygenated upper layer called the epilimnion (epi means above.) I recorded the numbers in the epilimnion as he called them out. The surface temperature was 77 F and dissolved oxygen was plentiful. Though the epilimnion had plenty of oxygen, the warm water made it uninhabitable for tullibee.
For the next 23 feet, the two values did not change much. Temperature began dropping sharply at 26 feet, and by 39 feet had dropped more than 20 degrees. Oxygen levels, though lower, were still adequate for fish. With its cooler but oxygenated water, this middle water layer, called the thermocline, was more to tullibee's liking. Jacobson continued lowering his sensor into the bottom layer, the hypolimnion (hypo means below), where the temperature was 30 degrees colder than at the lake surface. While the upper part of this deepest layer contained enough oxygen to support tullibee, the deepest 30 feet had nearly none. These results were similar to those I found in my thesis research on tullibee in Lake Itasca.
Jacobson examined the data. "That's great news for tullibee," he said. "Many lakes in summer cannot provide tullibee with oxygen in the deeper, cooler water. We need to protect lakes like this that can. The trick for Twin Lake will be to keep it this way."
After taking phosphorus and chlorophyll samples to compare the lake's nutrient content with data from other lakes, Jacobson lowered a Secchi disc to measure water clarity. The disc disappeared, due to absorption of light by water and algae, at 15 feet.
We pulled anchors and headed for the landing, but Jacobson suddenly cut the motor. "Look at the echo sounder," he said. It showed a solid band 62 feet down. "That's zooplankton -- tullibee food. They hover just below where oxygen is too sparse for tullibee to tolerate. They're hiding from the fish," he chuckled.
Tullibee have never been as popular with fishermen as other species, perhaps because of their deep-water preference and disinterest in bait. But tullibee are a rich and important food for larger game fish. Jacobson cites studies showing that walleyes grow significantly faster, and more large northern pike and trophy muskies are produced, when these predator fish are able to feed on tullibee. The tullibee's decline on Mille Lacs likely negatively affects the lake's walleyes.
Healthy tullibee populations can also provide enjoyment for winter anglers. According to Eddy Lyback, businessman and avid fisherman from the town of Wahkon, tullibee have had their fans on Mille Lacs. "My dad was one of the first to plow roads on Mille Lacs ice for tullibee fishermen," Lyback said. "Winter angling was popular in the mid-1960s into the early '70s. There was a place we called the 'tullibee hole,' a square mile area of real deep water south of Hennepin Island.
"We fished over mud bottoms where tullibee fed. We'd use tear-drop jigs with a wax worm, corn kernel, or mousie [syrphid fly with a tail resembling a mouse]. Some guys used flashers -- like large, shiny, hookless spoons -- to attract them. We'd start on the bottom, pull our bait up three or four feet, and jig. If no fish hit, we'd lift the line higher and jig again. Usually they'd be concentrated three to six feet off the bottom."
During the tullibee heyday, Lyback said, the lake would have 500 to 800 cars parked on the ice almost every weekend. Jim Remer of Isle also recalled the good old days. "When a school came through, you could catch them as fast as you could pull them up," he said.
Remer has smoked thousands of tullibee over the years, but he doesn't fish for them anymore. "You can't catch many these days," he said. "Sometimes you wonder if there are any tullibee left." Sport netting in the fall -- when tullibee come into the shallows to spawn -- was also popular decades ago. But as tullibee numbers declined, so did netting and winter angling. Sport netting became illegal in Mille Lacs in 2002.
The next morning Jacobson and I sampled Lake Koronis, up the road northwest of Annandale. From the dock, the waters of Koronis appeared less clear than the waters of Twin Lake.
"Tullibee in Koronis are in serious decline," Jacobson said. "We're worried about them." Three pelicans floated on the lake near the maximum-depth hole, our destination. Koronis tullibee can experience significant summer die-offs. Jacobson said pelicans are excellent "cleaner-uppers" of dead tullibee. "They probably eat them as soon as they die," he said.
We anchored in 129 feet of water and began recording temperature and oxygen numbers. As on Twin Lake, the warm surface temperature changed little for the first 33 feet, but oxygen levels dropped rapidly after that. Incredibly, the last 96 feet of water depth was completely devoid of oxygen.
Such conditions are calamitous for tullibee. Only the upper water layer contained enough oxygen to sustain the fish. But water there exceeded 75 degrees, a temperature commonly lethal for tullibee. Squeezed between high temperature above and asphyxiating waters below, tullibee habitat had been obliterated. Lake Itasca tullibee died in droves under such conditions when I studied them.
Tullibee die-offs are common in Minnesota lakes during hot summers. Eddy Lyback remembers cleaning up windrows of fish along a 300-foot beach when he worked for a Mille Lacs resort owner as a kid in the 1960s. "I'd fill a wheelbarrow every five feet," he said.
Jacobson's studies show that larger, mature fish are the most sensitive to lethal conditions. Previous die-offs may have further reduced an already depressed tullibee population in Koronis, leaving few large fish to succumb to today's conditions.
The shorelines of Koronis and Twin lakes appear similar in development, giving no visible reason to expect Twin would provide better tullibee habitat than Koronis. Measurements of chlorophyll and phosphorus concentrations, however, revealed a critical difference between the two lakes. Jacobson filtered a sample of Koronis' water through a white filter pad to determine chlorophyll content, which is strongly linked to algae density. The green algae coating on the filter contrasted starkly with the sparsely scattered green spots on Twin Lake's filter pad. Secchi depth visibility on Koronis, only a third that on Twin, confirmed the evidence from the filters: Koronis is richer in phosphorus and other nutrients; and, therefore, produces more algae than Twin Lake does.
On Koronis dead algae and other organic material rain toward the bottom and feed oxygen-consuming bacteria. Bacteria have removed oxygen from 75 percent of Koronis' water column, from top to bottom.
Jacobson cautioned, "You've got to consider the entire watershed to understand a lake's dynamics." Twin has a small watershed. Koronis' watershed is much larger, containing much greater acreage of lawns, farm fields, parking lots, and erosion sites from which nutrients like phosphorus can leach or run off into the North Fork of the Crow River, ultimately reaching the lake.
Jacobson's study is among the first to comprehensively analyze temperature and oxygen data on Minnesota lakes, and he believes that he is documenting tullibee as they become caught in a double whammy -- in between rising lake water temperatures from global warming and depleted oxygen from nutrient enrichment through runoff from watersheds. According to Jacobson, the lakes at greatest peril are along the forest-prairie transition zone from Annandale through New London-Spicer then up through Alexandria and Fergus Falls. Phosphorus from watersheds, algae, and lack of oxygen in the lower depths are intricately related, and tullibee let us know when that relationship becomes unhealthy.
"Protect tullibee lakes from nutrient enrichment," Jacobson said, "and, despite global warming, tullibee will likely survive in many lakes. We can make that happen locally."
Protecting lakes from nutrient enrichment starts at the shore and ends at the farthest edge of a lake's watershed. Reducing lawn grass, leaving shoreline and aquatic plants in place to prevent shore erosion, leaving lakeshore wetlands intact to filter pollutants, and not fertilizing lawns with phosphorus will help reduce oxygen loss in lake hypolimnions. (See "New Visions for Lakeshores,"July-Aug. 2005.)
Around the Great Lakes tullibee are known as lake herring -- a staple at North Shore restaurants.
Lake herring make up the bulk of the commercial harvest from the cold, clear water of Lake Superior. Netting begins in the spring and ends in late fall when they are harvested for the roe and flesh.
Today the North Shore's commercial fishermen harvest approximately 350,000 pounds of lake herring annually. The lake herring population in Lake Superior is greatly improved from the 1970s when the total commercial harvest was less than 150,000 pounds.
A decline prompted DNR and other fish management agencies to restrict commercial harvest of lake herring beginning in 1973. A spawning season closure was also implemented to protect lake herring when they are most vulnerable.
Lake Superior area fisheries supervisor Don Schreiner said the regulations likely assisted the lakewide recovery of lake herring.
Mille Lacs tullibee populations are also stressed, according to DNR area fisheries biologist Tom Jones. "It's huge!" he says. "In the mid-1980s, catches were often 25 per net. After that, catch rates declined until we were catching less than one per net in 2002. After a slight recovery in 2005, numbers fell again to 0.3 per net in 2006 and 2007."
Jones says the decline in tullibee began around the time muskies were introduced into Mille Lacs in 1984. "We considered that muskies might be to blame for the tullibee decline and investigated it," he says. "But the data don't support that explanation. Lately, tullibee numbers have decreased when water temperature went up and increased when temperature declined. The same pattern occurred on Big Winnie and Leech, yet muskie presence was constant." Statewide, tullibee numbers are declining in both muskie and non-muskie lakes, according to DNR Fisheries lake surveys.
Tullibee in large, shallow lakes like Mille Lacs face a different situation than they do in Twin and Koronis lakes, according to Jones. On Mille Lacs, wind can mix warm, oxygen-rich surface water with water from the bottom, preventing lake layering.
Jones sees increasing water temperature as the main threat to Mille Lacs tullibee. Jim Remer, a maintenance manager at Father Hennepin State Park who is responsible for keeping beaches clear of dead fish, has noticed tullibee die-offs on Mille Lacs often come when the lake has been calm for days.
Jacobson says, "Relatively shallow lakes like Mille Lacs do not have a thermocline. Then, without a cold-water refuge below a thermocline, tullibee are at the mercy of the warming and mixing of surface waters."
Few Minnesota lakes can provide tullibee with oxygen in the deep summer waters. "We need to view hypolimnetic oxygen as a natural resource in and of itself," Jacobson says. "Those lakes with good oxygen levels in deep water should be top priority for protection against nutrient enrichment. That protection should come in the form of good land-use practices within lake's watersheds."
Tullibee, those finny mineshaft canaries, are telling us our lakes need help. "The ability of our lakes to produce thriving populations of those species is special and unique," he says. "The loss of that ability would symbolize a loss of Minnesota's special 'sense of northness.' It would be a sad day."