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Counting Fish

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Published on: Jul. 2, 2007

Last revision: Dec. 1, 2010

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.

Age and Growth

In addition to recording the lengths of fish they catch, biologists often remove a few scales. Fish scales contain rings, much like the rings in the trunk of a tree, allowing biologists to determine a fish’s age.

We determine the growth rate of fish by comparing their age to their size. Slow growth in a fish population is often a result of an overpopulation of that species, or a shortage of food. Knowing the growth rate of fish populations helps us make decisions about the fishery.

Lakes and ponds generally have uneven fish production from year to year. Some years, the number of young may be small because of an abundance of competition from older fish, too many predators or a lack of food.

Natural reproduction of fish in any given year is strongly influenced by the fish already living in a body of water. Strong year-classes occur when conditions are right for lots of young fish to survive. These strong year-classes eventually affect future natural

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