Counting Fish

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Published on: Jul. 2, 2007

Last revision: Dec. 1, 2010

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

Fish 2001 2003 2005
Bass 80 120 175
Bluegill 250 135 92

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