Dock Talk: DNR uses nets to measure fish abundance
Anglers commonly reference gill net, trap net and stocking data found in map books and on the Minnesota Department of Natural Resources website dnr.state.mn.us.
Although that public information assists naturalists, anglers and curious folks in finding lakes with, for instance, a greater population of walleye, the DNR gathers the current information to monitor the physical and chemical characteristics of a lake.
Doug Kingsley, Park Rapids Area Fisheries Supervisor, says that the more popular lakes in our area are surveyed once every five years.
Lakes without a public access or those which receive limited fishing pressure are surveyed every seven to 10 years.
During that process, data regarding fish population is gathered primarily via gill net and trap net sets and water quality is sampled, including temperature, dissolved oxygen and phosphorous levels.
The two types of nets play an important role in gathering fish for the survey and each one serves a specific purpose.
The gill net is essentially a "wall" of mesh, spanning 250 feet and containing five "panels" with square holes ranging in size from 3/4" to 1 1/2." The gill net is set between two identifying buoys and once a fish swims into the net it becomes entangled and unable to free itself.
Unfortunately a high percentage of the fish caught in a gill net die. However, DNR workers set the nets late in the day and empty them early in the morning to minimize mortality. Live fish in the gill nets are released if in good health.
Although gill nets have a higher mortality rate compared to trap nets, they are the best way to effectively trap fish species that travel in open water, such as walleye, suckers, perch, tullibee and pike.
Trap nets, on the other hand, catch shallow water roaming fish. And the vast majority of fish caught in a trap net are successfully released.
The trap net consists of a square cage, four-feet high by six-feet wide, which sits at or just beneath the water's surface near shore. A lead, usually about 50 feet long, has funnel shaped "throats" that sit perpendicular to the shoreline and lead into the square "trap".
Fish species that roam shoreline contours, like panfish and bullheads, follow those mesh passageways and can't find their way out. The fish are then gathered and weighed, measured and released.
Yet bass, both largemouth and smallmouth, tend to avoid the trap nets. So when the DNR needs a good sample for bass, they conduct electrofishing, usually in the spring when it's close to their spawning period.
Special assessments to analyze the size of certain species, like crappie or northern pike, on lakes with experimental regulations is done during the spring as well, using the trap net as the capture tool.
If you see a DNR net in the lake, please keep your distance and never tamper with them; it's against the law and can cause inaccuracy in the DNR survey. And those surveys ensure the lake is healthy and functioning properly.