Who is Eating Who on Mille Lacs?

With generation after generation of young walleye disappearing before adulthood, there simply aren't enough walleye to go around a problem made all the more controversial because the lake's fish are shared by American Indian tribal members who, by court ruling, don't have to follow Minnesota laws and can net a portion of the lake's walleye.

The scarcity of young walleye and the fear that without them, sooner or later, the population could crash — has prompted the DNR to institute strict regulations this year. Only two flaky-fleshed walleye a day can be kept: either two between 18 and 20 inches or one in that slot and one longer than 28 inches. In addition, night fishing is banned for most of the season; no one on the water can be in possession of fishing tackle of any kind from 10 p.m. to 6 a.m. until Dec. 1.

The result is that while Mille Lacs offers some of the best fishing in the state, especially for large walleyes, muskies, northern pike and smallmouth bass, the reports from longtime business owners say that fishing boats, which in good years crowd like bumper cars over throngs of fish, are relatively scarce this year.
Meanwhile, the DNR is trying to figure out what's going on with the walleye in a lake that is changing: Its waters are getting clearer and, in most recent summers, hotter than in decades past.


Diet Study

One of the DNR's main programs of inquiry is a "bioenergetics" study. It's essentially an attempt to figure out how biological energy — think calories — flows in the 207-square-mile lake. A key part of that is a "diet composition" study — what fish are eating. The method: Capture fish, cut open their stomachs, and see what's inside.
One of the goals is to figure out what, if anything is eating young walleye — and whether it's enough to explain their disappearance. The study began last year and will continue at least through this fall.

"It's really too early to draw many conclusions," cautioned Brad Parsons, the project manager for Mille Lacs. The DNR is also in the process of a population study — a census of sorts — to estimate how many of several species live in the lake.

In discussing some of the early data, Parsons laid out several cautions:
The contents of a few fish's stomachs might or might not be representative of the entire population, and among some fish during certain times of year, sample sizes are small.
Until total populations for all the key fish are known, one can't know the impact of one pattern of predation. For example, if northern pike ate nothing but 4-inch walleye — which they don't — you'd still need to know the entire population of 4-inch walleye and the entire pike population to know how much of an impact the pike would be having.
What fish eat can be as much a result of what's available as what they want to eat. Mille Lacs began last spring with a weak forage base, but tullibees and shiners had a productive spring — and fish appeared to take advantage of that. Among the impacts: Walleye looked healthier. All the data is from 2013 thus far; this year, fish might eat different things, depending on what's available.

The percentage of diet doesn't necessarily translate to quantity. Examples: Most fish eat less in the fall than in the summer, when water is warmer. And five 1-pound fish eat more in total than one 5-pound fish.

Absolutes might never come from the study. "For a lake the size of Mille Lacs, getting the total consumption is going to be really hard, but we can find out the relative importance of each kind of fish," Parsons said.

The good news is that walleye hatched in 2013 — a good crop — appear to have grown fast and survived the winter.
As far as what walleye eat, the DNR broke them down into four categories:
Walleye under 13 inches:
In summer and fall, mostly yellow perch. In winter, too few fish were caught to have a good sense.

Walleye 13 to 18 inches:

Early in the year, a mix of fish, including shiners and darters, with some mayflies mixed in, made up the largest components of their stomachs. In midsummer, perch dominated the diet. In August and September, cannibalism picked up, with 10 percent to 15 percent of the diet consisting of walleyes. The winter sample size was too small.

Walleye 18 to 23 inches:
Throughout most of the year, these fish followed similar diet patterns as 13- to 18-inchers, except they ate fewer walleyes. In the winter, they ate mostly perch.

Walleye 23+ inches:
Through early summer, perch and shiners were the main food, with a mixture of other fish, such as an occasional bluegill or crappie, showing up. The rest of the year, they ate yellow perch. Cannibalism was noted in September and October, with 10 percent to 15 percent of their diet consisting of walleyes.

Northern Pike
Young northern pike are increasingly showing up in DNR research gillnets, and biologists think they might be benefiting from clearer water. Pike feed by sight. Clearer water has allowed more weeds, which pike prefer, to grow in deeper water. Meanwhile, young walleye, which don't like bright areas, have moved away from their traditional shoreline habitat.
Researchers are examining two categories of pike — and are learning they eat different things.

Pike less than 25 inches
In July and August, it's all about yellow perch — perhaps 90 percent of their diet. From fall into winter, walleye became a notable part of their diet, with young walleye making up 20 percent to 25 percent of their food intake.

Pike over 25 inches
Most of the year, 50 percent of their diet was tullibee, also called ciscoes. Yellow perch, crayfish (up to 20 percent) and other fish rounded out their eats. Walleye never made up more than 10 percent of their diet.

Smallmouth Bass

"Smallmouth was the easiest to figure out," Parsons said. "The diet that was observed was dominated by crayfish — significantly more than half in the summer. In the fall, they ate a few more fish. Smallmouths really don’t eat a lot of fish, but if they do eat them, they'll generally eat yellow perch, and occasionally bluegill, crappie, darters and shiners. The proportion of walleye they eat is very small."
Not enough smallmouths were collected in the fall to glean any useful data.