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Swim Up And Be Counted!

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Published on: Oct. 2, 2002

Last revision: Nov. 12, 2010

When I was in high school, my classmates and I often discussed our future careers. Most had hopes of becoming engineers, lawyers, doctors and physical therapists. When I announced that I would be a fisheries biologist, they asked, "What do they do?"

I didn't know enough to answer them very well then, but soon after I had acquired a degree in fisheries biology, I learned that the most fun part of being a fish biologist is counting fish.

Fisheries managers count fish the same way the government counts people during the national census, except that fish can't fill out forms and drop them in the mailbox. Instead, we have to go out and find the fish. This doesn't mean tallying every fish in a body of water. We just take representative samples, and from these we can come up with good information about the whole population. The fish from our samples are released unharmed.

Censusing fish is not just a matter of counting them. Before conducting a fish census, we have to evaluate the targeted species of fish, decide on the size of fish we'd like to sample and learn the habitats they occupy. Timing is essential because different fish species occupy unique habitats throughout the year. Time of year, age, sex, water level, cycles of the sun and moon all influence where a fish may be.

The Shocking Truth

We sample fish populations using a variety of gear and techniques. Since the 1950s, the most widely used method of capturing fish to count them has been electrofishing. We introduce electric current into the water that stuns any nearby fish. They float to the surface and we capture them in dip nets.

Usually, the first question people ask about electrofishing is, "Aren't you afraid of getting shocked?"

Although water and electricity usually prove a dangerous combination, electrofishing is safe - both for us and for the fish - if we take proper precautions. We electrofish using backpack units powered by a battery or from a specially equipped boat that has a generator for power. Current flows between electrodes that are placed in the water. A safety shut-off switch eliminates any risk to the operator, and a control box regulates the electric field to make sure it only stuns nearby fish and doesn't kill them.

After only three weeks as a fisheries biologist, I was going on fish-collecting expeditions. We used a 14-foot johnboat rigged with electrical equipment. Within the next few months we handled flathead catfish, crappie, largemouth bass, bluegill, tons of buffalo and many other species of fish.

Electrofishing works well in a variety of habitats, from small streams to big lakes. The technique is used for many different species of fish, but it works especially well for for largemouth bass and bluegill in the spring.

Springing the Trap

We also rely on Fyke nets (also called trap nets) in reservoirs and lakes. A fyke net has a lead line (sometimes two

or three) connected to the shore. The lead works like a wall to direct fish into the net basket, which is usually anchored in deeper water, perpendicular to the shoreline. Fish entering the net swim through a series of funnels, which are easy to enter, but difficult to exit.

Fyke nets are best at capturing fish that move along the shorelines migrating, feeding or searching for cover. They are especially good for capturing crappie in the fall.

Hoop nets are framed with four to eight hoops that are 2 to 9 feet in diameter. The hoops are covered with netting to form a trap that has one open end (mouth) and a closed end. Hoop nets are used in rivers and deployed by boat. A rope and anchor are attached to the back of the net. The mouth of the net faces downstream, and the current holds the net open. Hoop nets are most often used for capturing catfish. Managers may use cheese, soybean cake or anything that stinks to attract fish into the net.

Gill nets and trammel nets are vertical walls of netting typically set in a straight line. They are used in water ranging from 8 to 20 feet deep. They can be anchored to the bottom or allowed to float or drift. Fish swimming into the net are caught by the gills in a gill net, or tangled in a bag or layers of mesh in the case of a trammel net.

These nets are used in many different types of habitats, from rivers to lakes. Gill nets and trammel nets were first developed in ancient Egypt. They are time-tested tools for capturing most species of fish, and work especially well on walleye, sturgeon and paddlefish.

A seines is a long, rectangular piece of net. When the wall of netting is pulled through shallow water it encloses fish. These nets can be pulled by hand, boat or tractor. They work well in ponds and lakes. Commercial fish growers often rely on them to capture fish. They also are effective in small, shallow lakes and streams.

A trawl is a funnel-shaped net towed by boat. Trawls are altered in many ways to catch different aquatic organisms. Trawls are often used in the deep water of large lakes and rivers that are difficult to sample by other methods.

More than counting

We do more than count the fish we capture. We try to get information that can tell us about the population size, structure (the percentage of fish in each size-group), mortality, growth and standing crop. Depending on what we're looking for, we might record total numbers of fish captured, sampling time, gender, length and weight of the fish.

Fish managers sometimes remove a few scales. If the census subject is a catfish, they remove a spine. Scales and spines have annual marks on them that indicate when fish growth slows during the winter. Spines are cut into thin sections to be examined under a microscope. These annual marks yield information about growth history, age and mortality. This is important because you can't determine the age of a fish from its size, but you learn a lot from its growth rate. For example, it's good if largemouth bass grow 12 inches in their first three years, but a slower growth rate of, say, 12 inches in five years indicates the need for some adjustment in managing the fishery.

We also look for tags on fish. Since the 1600s, people have been tagging and marking fish to learn about their movement patterns, growth and mortality, and to identify individuals. We have several types of tags and marks available, some are external and some are internal. The commonly used "spaghetti tag" is anchored into the fish's flesh beneath its dorsal fin.

Fish managers don't always have to capture the fish themselves to glean useful information from tags. Anglers also play an important role when they report tagged fish to the managers whose address is printed on the tag. Cash rewards are sometimes given for these reports.In general, the numbers and sizes of fish in a population determine the potential of a fishery to provide benefits to anglers. This information is used to create graphs and charts that help managers identify a balanced or unbalanced fish population. All of this information gives us knowledge needed to regulate, improve, preserve, and predict future fish communities.

In some situations it is critical to get information from anglers. Creel surveys provide valuable information concerning angler activity, numbers of fish harvested and fish species preference among anglers. Creel clerks go to recreational fishing sites and interview anglers in order to get this information. The data they collect may be used to determine or evaluate lake or river regulations. Surveys also provide an excellent opportunity for managers to meet with anglers and learn what they think about a fishery.

Fisheries managers generally get into this field because they love fishing, and because they are deeply interested in preserving and improving fish resources. Their dedication to researching, managing, regulating and preserving fish communities ensures that our grandchildren will have quality fishing.

Sometimes the work can seem tedious, but when we step back and look at how good the fishing is in Missouri, all those hours tending nets, measuring and weighing fish, and even repairing boats, is worth all the effort.

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