July 2007 - Volume 68, Issue 7
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Counting Fish continued...
Fisheries biologists use electroshocking to survey fish populations in Missouri rivers and lakes. Biologists often work at night because many species of fish tend to be closer to shorelines after dark, and the fish are less likely to be spooked by a boat. The fish are only stunned by the electric current and recover quickly.
Electrofishing, Trapnets and Hoopnets
Electrofishing works well for counting some, but not all, species of fish. We also use trapnets and hoopnets for surveying populations. What’s important is that we collect each species at the same time of the year and using the same method so that we can compare our data with other years.
For example, we survey largemouth bass with electrofishing gear in April and May once the surface water temperature reaches 65 degrees. We collect crappie in trapnets in October. We survey channel catfish in the summer by using hoopnets baited with aged, foul-smelling cheese. We set gillnets below wing dikes in the Missouri River during late winter for sturgeon.
Our knowledge of when and how to survey fish populations was not accumulated overnight. Present-day biologists have learned from past efforts how and where to obtain “representative” samples of fish populations.
When possible, a biologist may collect 500 or more fish of a given species to get a representative sample. To be accurate, a sample should contain all sizes of the fish in the population from fingerlings to large adults.
As is the case with any survey, obtaining a good sample provides solid information about the whole population. In the case of fish sampling, the data we collect helps us gain an understanding of the population structure of a species.
For example, we might determine that 25 percent of the fish are less than 12 inches long, 50 percent are between 12 and 15 inches long and 25 percent are longer than 15 inches. By tracking this data through several years we can identify changes in population structure and take action to improve the fishery.
How fast we are able to capture fish—our catch rate—gives us a good indication of relative abundance of a species. Although it is often not possible to determine exactly how many bass, crappie or catfish live in a lake, we can compare catch rates one year to the next to determine whether fish numbers are increasing or decreasing. By comparing catch rates from one body of water to another we can forecast which waters might provide better fishing. Catch rate information also is valuable in helping us set harvest regulations.
About This Article
Author
GREG STONER has been the Department’s
fisheries management biologist at Lake of the
Ozarks since 1991. He also assists landowners
with pond-related issues throughout Camden,
Miller and Morgan counties. When not at work,
Greg spends as much time as possible fishing in
the Niangua Arm of Lake of the Ozarks with his
wife, Sherri, and his kids, Alex and Katie, who
luckily all like to fish.
Photographers
CLIFF WHITE is a life-long Missouri resident. After nearly 10 years as a staff photographer for the Missouri Department of Conservation, Cliff recently took on management duties as Art Director. Now he supervises on an unruly gang of super-creative folks. In his spare time Cliff likes to play old-time folk and bluegrass music, float fish and chase the occasional turkey.
Department of Conservation photographer NOPPADOL PAOTHONG discovered his love and passion for wildlife photography in college in 1995. Born in Thailand, he came to the United States in 1993 to study graphic art before switching to journalism. He has worked as a full-time photographer at the Joplin Globe and the Springfield News-Leader, and has achieved more than 60 regional and national awards.
