The Big Fish Picture

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


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Andy Whitcomb

Andy Whitcomb

Andy Whitcomb is a columnist, outdoor humorist, and stressed-out Dad. He says there are “people who fish”… and there are “fishermen”.  One of the few things he knows is that he is a “fisherman”...  To the point it could be classified as borderline illness.  Sharing this obsession is rewarding, therapeutic. He likes to encourage people to “stop and smell the crappie."  Enjoys catching fish, but gets a greater thrill out of helping someone else hook up.
Born in Florida, but raised on the banks of Oklahoma farm ponds. Now relocated to western Pennsylvania. He has fished, worked, lived all around the US.  He has a B.S. in Zoology from Oklahoma State as well...
And he met his wife while electrofishing. He has been contributing weekly to www.takemefishing.org since 2011.