Have you ever been out on the lake and noticed red or yellow buoys scattered across the water’s surface?

Or have you heard the gentle roar of a generator coming from a large, barge-like boat meandering along the shoreline?

If so, there is a good chance the added commotion was caused by members of a South Dakota Game, Fish and Parks fisheries crew sampling fish populations in the lake. These sampling efforts are often referred to as lake surveys. Each summer, GFP crews complete surveys on approximately 30 lakes across northeastern South Dakota. Some lakes are annually surveyed, while others are sampled on a two-to-five-year rotation. Regardless of the rotation, individual lakes are sampled about the same time of year so that fish population trends can be monitored over time.

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Because individual fish species differ in body shape, behavior, habitat preferences and so on, no single method effectively samples all fish species in a lake. Therefore, a combination of different fish surveying methods like experimental gill nets, trap nets or electrofishing might be used during the lake survey process. Selection of sampling gears depends on the fish species present and management objectives for the lake.

Experimental gill nets form a curtain-like wall of various-sized mesh that is submerged and rests along the lake bottom. Fish that encounter this wall of mesh typically become wedged and are unable to escape. Gill nets most effectively sample fish species that tend to be more torpedo-shaped, like walleye and yellow perch, and occupy open-water habitats during the summer months.

Trap nets have a long lead that attaches to shore and gently funnels fish toward the trap portion of the net where they eventually enter the cod end and become “trapped.” Trap nets are generally used to sample shoreline-orientated fish species, like black crappies and bluegill, that tend to seek cover.

Electrofishing, which is typically completed in the spring, effectively samples species that tend to avoid nets like largemouth and smallmouth bass. As the name implies, electrofishing is the use of electricity to capture fish. Right now, some of you are probably asking, how can it be safe to use electricity in or around water? Certainly, there are risks involved, but by using specialized equipment and following established safety procedures, electrofishing can be an effective method of sampling fish.

As the boat travels along the shoreline, fish that are encountered by the boat are stunned by the electricity. Target species are then netted and placed into a live well where they quickly recover.

During surveys, collected fish are identified, individuals of each species are counted and a portion (usually 100 or more, if available) are measured and weighed.

For popular species, like walleyes, otoliths (small bones inside the head) are collected from a subsample of fish for age estimation. Opaque bands, or rings, form on otoliths when water temperatures cool during the winter months and fish growth slows. Much like rings on a tree, counting the rings allows fisheries biologists to determine how many winters a fish has lived.

Once collected and processed, lake survey data provides a current snapshot of the status of the fishery that is useful to both fisheries biologists and anglers. After all, who doesn’t want to know where the highest numbers and largest fish were sampled in a given year?

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The real value of lake survey data comes into play on lakes surveyed each year as the data allows fisheries biologists to more closely examine how fish populations behave through time. The ability to track variables like recruitment, which is the addition of new fish to the population, growth and mortality can be tremendously valuable when evaluating past management actions, including the success or failure of recent stockings and length regulations.

Lake surveys are completed throughout the summer, and sampling results are periodically updated on the “Fishery Reports” page of the GFP website.

Have a safe and enjoyable rest of your summer, and please take a kid fishing if you get the chance!

Todd Kaufman is a fisheries resource biologist with the South Dakota Department of Game, Fish and Parks.