Counting Fish

By Greg Stoner | July 2, 2007
From Missouri Conservationist: Jul 2007

One night in April, two bass anglers on Lake of the Ozarks are pitching jigs and spinnerbaits around sunken brushpiles. Using an electric trolling motor, they quietly make their way around a rocky point and approach a craft with three people in it, all of whom are hovering around a tank in the middle of the boat.

As they get closer, they can hear one person speaking while another seems to be taking notes. They hear subtle splashes as the speaker slips objects over the side of the boat. The anglers then watch as the boat starts its engine and moves to the other side of the cove.

Curious, but still fishing, the anglers are surprised to hear a gasoline generator start and see the area in front of the boat suddenly bathed in bright light. They also notice two people standing in the bow of the boat dipping nets into the water.

They keep an eye on the boat, and when the generator shuts down and the lights go out, they slide up close to the boat to find out what its occupants are doing.

“I’m a fisheries biologist with the Missouri Department of Conservation,” the driver responds to their question. “We are conducting our spring electrofishing survey to collect data about the black bass population in the lake.”

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, Trap nets and Hoop nets

Electrofishing works well for counting some, but not all, species of fish. We also use trap nets and hoop nets 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 trap nets in October. We survey channel catfish in the summer by using hoop nets baited with aged, foul-smelling cheese. We set gill nets 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.

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 reproduction. Our fish sampling efforts reveal the year-class structure of various species in a body of water and helps us predict how good the fishing might be in future years.

We can also determine habitat conditions by noting the relative weight index of fish for their length. A fish with a relative weight of 1.0 is considered healthy, whereas .85 is on the skinny side and 1.2 is a fat fish that is finding plenty of food.

Sampling Anglers

During creel surveys, we’ll approach anglers and ask them for information about how long they fished and how many fish they caught. This harvest and fishing pressure data, combined with the information from our fish counting and sampling expeditions, gives us a good understanding of the dynamics of a fish population in a body of water.

We need this understanding in order to make adjustments to fish populations through length limits, daily limits, the setting of seasons, stocking and habitat enhancement.

Should you have a late-night encounter with a Conservation Department survey boat, pull over and say hello. We’re out there counting, instead of casting, so that we can improve your fishing.

Data Mining to Manage a Fishery

In 2001, we found that the bass population in mossy lake consisted of a good variety of sizes from small to large with 20 percent of the bass over 15 inches in length. Also in 2001, we collected bluegill at a rate of 250 per hour with few fish measuring longer than 8 inches. By 2005, few bass were over 12 inches in length, but we found several large bluegill between 8 and 10 inches.

What happened during these five years to cause such a change in the fishery?

Large year-classes of bass produced in 2002, 2003 and 2004 caused an increase in the number of bass in mossy lake. During this same period, the relative weight index dropped steadily from 1.2 in 2001 to .80 in 2005. They became skinnier. Bass growth rates also decreased from 13 inches at age 3 in 2001 to 9.5 inches at age 3 in 2005.

This bass population had become overabundant and steadily ate away at the numbers of bluegill. Meanwhile, the decline in bluegill numbers meant less competition for food for that species, so that the remaining bluegill grew fast and to a large size.

If mossy lake anglers are content to catch many small bass, but not many of them over the legal length of 15 inches for harvest, or if they don’t mind waiting through slow action for the chance to catch large bluegills, we might not do anything more than continue to monitor the lake. Angler surveys, however, tell us that there is general discontent with the fishery and we need to take action to improve it.

In this case, a 12- to 15-inch slot limit on bass and an increase in the daily limit of bass from two to four or six may be all that is needed. Anglers could take more bass, but they would not be able to harvest bass between 12 and 15 inches long.

These regulations would improve bluegill production by allowing a bigger bass harvest, reducing the number of bass and permitting more small bluegill to survive.

The new regulations would also provide a great catch-and-release fishery for 12- to 15-inch bass, many of which would find enough food to grow longer than 15 inches, making them eligible for harvest or catch-and-release trophy fishing.

The changes won’t happen overnight, but these regulations should steadily improve the fishing in mossy lake.

Catch Rate per Hour in Mossy Lake


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