Double-crested cormorants are a well-established punching bag for fishermen.

Walleye fishing in Lake Mille Lacs, in the central part of the state, has slumped. Fishermen and the businesses that serve them are unhappy. Cormorants make the list of reasons for poor fishing.

"Cormorants eat some small walleye, and they compete with walleye for food," said Paul Venturelli, assistant professor in the department of fisheries, wildlife, and conservation biology at the University of Minnesota.

"But these impacts pale in comparison to what we suspect are the big three: warming, water clarity and invasive species," he said.

"Birds are pretty low on the list," he said, adding, "Recreational fishing is worse."

A study commissioned in 2014 by the Minnesota Department of Natural Resources examines various possibilities for walleye decline at Mille Lacs. The report says, in essence, if this happens, then that happens. Exact interplay of "this" and "that" is very complicated. (The report is 7,400 words long, with many charts.)

Cormorant with fish.

In the report, Venturelli told me in an interview, there is no recommendation for control of cormorants. Ability to evaluate cormorant predation was limited by a lack of diet data for these birds on this lake.

Many fishermen on lakes from here to New York and into Canada have blamed cormorants for diminished fishing success. Many studies from here to there show no conclusive evidence that the birds are part of any walleye problem.

Cormorants have been shot regardless of evidence. About 1,000 per year are shot at Leech Lake, continuation of a culling program begun in 2006. Fishermen and merchants in Walker complained about declining walleye catch. Fishing is said to be better now.

If the people in pursuit of a solution to the Mille Lacs walleye issue ever get serious about the birds, lack of a study is unlikely to make a difference.

The impacts on fish population in a given lake are many, said Venturelli. Sifting them for a solid conclusion is difficult at best.

Mille Lacs is changing. It is warming. It's been invaded by zebra mussels (aka zeebs) and spiny water fleas. It is infected with Eurasian watermilfoil, a plant that clogs the water in thick mats.

The single Mille Lacs cormorant colony is on Spirit Island off the south shore of the lake. It is half an acre of rock. It and a companion island, also tiny, form the smallest National Wildlife Refuge in the nation.

An estimated 400 to 500 pairs of cormorants nest there. Migrant cormorants also are present on the lake in season.

The cormorants' impact, determined by a complicated formula in the report, puts the birds in the "maybe" category. They are midway between being no problem and a possible problem. This section of the report, as well as others, contains several sentences beginning with, "It depends. ... "

Problems facing walleye fishermen include water temperature, zeebs, water fleas, cannibalism, milfoil and competition between walleye and other predator fish.

The problem facing the cormorants is that none of the other problems can be shot.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com. Join his conversation about birds at startribune.com/wingnut.