Collecting information about fish populations is fundamental to understanding and managing fisheries. Each year, fisheries management crews conduct routine surveys to maintain or improve fishing opportunities. Most of these surveys are conducted in a similar way to allow comparisons of results across waters and from one year to the next. In this way, results can be used to help make decisions about stocking, regulations, and other activities.
Surveys of fish populations are conducted to determine the survival, abundance, growth, body condition, and sizes of fish in specific waters. A variety of methods are used to collect information in the field and a great deal of time is spent by biologists reviewing and analyzing data. The primary methods of fish collection from inland waters are shoreline electrofishing, trapnetting, gillnetting, and acoustics (SONAR).
Shoreline electrofishing involves specialized equipment that applies electrical current to shallow waters where fish congregate at certain times of the year. Collecting fish with electrical current can only be done by trained professionals using the appropriate safety equipment. The current resides in a field immediately adjacent to the sampling equipment, typically a boat, and when properly applied stuns fish which can be collected with dipnets. Fish sampled with this method are measured, weighed, and usually released in good condition. The Division of Wildlife uses shoreline electrofishing to assess black bass and sunfish populations during April-June, stream fishes during July-August, and stocking success of walleye, saugeye, or hybrid striped bass during September-October. Sampling is typically conducted during one or two consecutive days, with night surveys common on lakes with clear water and day surveys on darker-colored waters.
Trapnets are sampling devices that fish voluntarily swim into during their daily movements, and then become captured in an enclosed net “car” or “pot”. Division of Wildlife crews set trapnets along shorelines of reservoirs each fall to assess crappie populations, with an emphasis on learning about their abundance, size distribution, and growth. These nets are constructed of dark brown nylon mesh, a lead that encourages fish to move into the collection pot, and a 3’x6’ frame connected to smaller hoops, where fish congregate and cannot escape. Nets are set overnight and fish are removed the following day. Sampling is typically conducted for three to four consecutive nights during October-November. Division of Wildlife trapnets are marked by our crews to avoid confusion among anglers fishing while we are conducting surveys.
Gillnets are monofilament nets of varying mesh sizes used to entangle fish for capture. Fish swimming into gillnets are trapped in the mesh as they swim through, or become tangled in the net with their teeth or spines. Gillnets are used on one or two consecutive evenings during October-November and are typically set for 2-4 hours each. Data collected with gillnets are used to assess adult populations of walleye, saugeye, white bass, and catfish.
Acoustic surveys, a technique sometimes referred to as hydroacoustics, is a specialized use of SONAR. The equipment that the Division of Wildlife uses is a scientific-grade version of fish finders that most anglers have aboard their boats. Acoustic surveys are conducted to assess prey fishes, primarily gizzard shad, in 10-14 reservoirs each year to better understand their capacity to support stocked fishes. These surveys take place during evenings in late July through early September, when transects of the survey boat are steered to a pre-determined, GPS-plotted course. Results provide estimates of the total weight of fish in a reservoir, their number per acre, and average size. Concurrent sampling with a small trawl indicates the percentage of fish in the sample that are likely gizzard shad, the primary food of most fish-eating sportfish.