Gill nets have been used to estimate fish populations in Minnesota since the 1930s and, along with creel surveys, have been a standard tool since the 1950s.
But how accurate is the information supplied by gill nets? A recent study has raised interesting questions.
For the past two years Brainerd DNR fisheries biologists Gerry Grant and Paul Radomski have studied how walleyes react to gill nets. They have learned 'ol marble eyes can avoid a gill net with the same ease as a hook.
"The nets are far less effective than we thought they would be," said Grant, who in August will present his findings at a meeting of the American Fisheries Society. "We knew gill net numbers were crude before we started, but now we have a better idea exactly how inefficient they are."
Paul Radomski (left) and Gerry Grant, fisheries research biologists with the DNR, transport underwater cameras to a location where they are used to monitor gill nets. A video recorder is started when the nets are set. Grant must return in the middle of the night to change tapes and refuel the generator that powers the lights. (Dispatch Photo by Vince Meyer)
The study was conducted in a 43-acre DNR aquatic management lake near Brainerd. A 10-foot gill net was lowered into the lake. Two underwater cameras and four infrared lights were mounted on an aluminum frame and placed about 3 feet from the net. In 36 nights last summer the camera filmed 2,303 walleyes approaching the net, but just 18 were caught.
"That would indicate we're not catching most of the fish that swim up to a gill net," Grant said. "Either they avoid the net or they escape it once they come in contact. We're almost certain the fish see the net. We've seen them swim right up to it and then stop inches away."
The cameras have shown that fish sometimes escape the net by twisting and turning until they get free. A 1.25-inch square mesh was most effective for catching the small walleye in the lake, yet about half of the fish that contacted mesh of that size managed to escape.
The study also learned big walleyes are less prone to being captured in a gill net for the same reasons they're tougher to catch on hook and line -- there's fewer of them and they're more wary. Moody Lake didn't have big walleyes when the study began so 240 walleyes up to 26 inches in length were stocked. So far three have been caught.
This illustration shows the basic setup of the DNR gill net study being conducted in Crow Wing County by DNR fisheries biologists Gerry Grant and Paul Radomski. (Source: DNR)
The study has raised other questions. How does water clarity affect the probability that fish will contact a gill net? How far do fish range? If year after year gill nets are placed in the same locations within a lake are they collecting fish from just one stock in the lake?
The study will help fine-tune methods used to estimate Mille Lacs' walleye population. Grant and Radomski had planned to conduct the study in Mille Lacs, but low water clarity and the difficulty of working in a large lake with strong underwater currents forced the move to the smaller lake near Brainerd.
Mille Lacs' walleyes are counted each fall by placing 250-foot gill nets with five different mesh sizes in 32 locations throughout the lake. The present Mille Lacs walleye population is estimated at 996,502.
"There may be more be more big walleyes out there than our (lake population estimates) say there are," Grant said.
The underwater cameras and infrared lights used in the DNR gill net study are mounted on this frame. The power needed to run the setup is provided by the cable, which is run several hundred feet to shore. (DNR Photo)
Despite their limitations, gill nets won't be abandoned any time soon. Sixty years of gill net data has value from a historical perspective. If new data isn't collected the perspective is lost. And researchers will always need some fish in hand to get age and growth information.
But if Grant's next project succeeds -- he would like to develop a method for counting fish by camera -- the use of gill nets could be reduced. For now, Grant will settle for a step in that direction.
"We're making progress in determining what our gill net catches mean and that's the ultimate goal," he said.
Brainerd Dispatch ©2013. All Rights Reserved.