The Big Fish Picture

By Andy Whitcomb

Nov 22, 2014

The number and size of fish caught by anglers are the most popular information.

The number and size of fish caught by anglers are the most popular information. This is called “Catch Per Unit Effort” by fisheries biologists. However in an attempt to grasp the full underwater picture, they will use a range of sampling techniques. Each method has a degree of sampling bias or selectivity for a species or a size, so biologists must consider such factors as: time of day, season, temperature, habitat and turbidity of the water.

Here are some sampling methods used by fisheries biologists:

Electro fishing: Portable generators are used to stun fish with an electric current. This can be done with a backpack unit and rubber waders in small streams, or from a specialized boat for larger bodies of water. Sensitivities of the electric current vary between species and effective depth is limited by factors such as minerals in the water.

Seine: This long net requires a person at each end to sample along a shoreline. It is commonly used to determine presence of young of year after spawning, such as bluegill when managing as forage for largemouth bass.

Seine Missouri

Trammel net: This long net is a combination of two net mesh sizes, such that fish get trapped in a pouch of smaller mesh netting when they push through the larger holes of the second layer. It can be set in deeper water with buoys.

Trawl: A weighted bag net pulled by boat. The Ohio DNR uses trawl capture data of Lake Erie in August to help forecast safe harvest rates of walleye and yellow perch.

Trap nets: Fish are directed into a series of cone shaped net chambers. In Oklahoma’s Lake McMurty, biologists use them in the spring to obtain the status of crappie and saugeye. In Michigan, they are used to catch muskies for spawning at fish hatcheries.

Gathering this data for many years helps biologists predict trends and manage fisheries. For example, this information may help identify and protect spawning areas, detect presence of invasive species, or determine if there is adequate forage for big fish. Aquatic systems are always changing, but by continually using multiple sampling techniques fish biologists do their best to keep anglers and fish “happy.”

If you would like to learn more about fish conservation, visit the Take Me Fishing Conservation Section

Andy Whitcomb
Andy Whitcomb
Andy is an outdoor writer ( and stressed-out Dad has contributed over 380 blogs to since 2011. Born in Florida, but raised on banks of Oklahoma farm ponds, he now chases pike, smallmouth bass, and steelhead in Pennsylvania. After earning a B.S. in Zoology from OSU, he worked in fish hatcheries and as a fisheries research technician at OSU, Iowa State, and Michigan State.