After hundreds of studies, it has been difficult to link fish predation by cormorants (Phalacrocorax auritus) to the reduction of fishing quality in Minnesota lakes. It appears that game fish such as walleye and northern pike make up from less than 1% to nearly 3% of the bird’s diet. They eat only small fish. Many of the fish they eat are perch, which prey on walleye, and it is even possible that by culling small walleye or northerns, they increase the growth rate for those fish in two way. One is by reducing competition between fish for food, and the other is by exerting selective pressure for faster growth.
When cormorants were heavily culled on Leech Lake a few years ago, the Walleye fishing got better. The fishermen and resort owners hailed the killing of the birds as a great thing and attributed the improved fishing to the culling policy. However, the lake simultaneously underwent a very aggressive restocking, and slot limits had been imposed at the same time. The fisherman and resort owners are, sorry to say, being stupid about this, rejecting the science, and possibly shooting themselves in the foot.
Two Minnesota Congressmen have been behind changing federal law to allow widespread killing off of cormorants in the state. This, I believe, is unbecoming of a member of Congress who have the responsibility of paying attention to the science, and of being stewards of our national resources at a level greater than a few local whiny mayors and resort owners.
This issue has been brewing for a few years and will continue. I am hoping that the recent focus on the importance of science in developing public policy will mean a more intelligent, less immature and misinformed cormorant policy in Minnesota over the next few years.
Sources and resources on Cormorants (Phalacrocorax auritus):