by Carol Heiser and Dean Fowlerillustrations by Spike Knuth

isheries biologists are a modest, hard-working bunch. They lumberabout in clumsy, rubber waders, slog around the shorelines of

streams and ponds, spend countless myopic hours at the computer analyzingdata, and drive around in muddy vehicles that smell like, well, fish. Passionateabout aquatic resources, they devote enormous energy to the task of assessingthe health of Virginia’s fish populations and the key habitats that fish dependon. Fisheries biologists use several techniques, such as electrofishing, gill-net-ting, and tagging, to discover clues about various fish species and the watersthey live in. It is important information on which management decisions arebased.

Pumpkinseed sunfish are commonin warmwater Virginia streams.

A Snapshot Underwater

Fish populations fluctuate over time for a va-riety of reasons. Weather and disease are twonatural factors that can influence fish abun-dance. For example, excessively low or hightemperatures or flooding can cause suddendie-offs. Also, because fish are cold-bloodedand their body temperatures fluctuate withchanges in the water, temperature affects justabout everything they do: breathing and feedingrates, spawning location and timing, and evenmigrations. Weather can also affect the abun-dance of organisms that fish feed on or that feedon them.

Frequently, human activities have a more sig-nificant influence on fish populations than nat-ural factors. People catch large numbers of fishfor food and sport, and land-use and develop-ment affects water quality and fish habitat. Fish-eries management, therefore, is much morethan simply stocking fish, it is the job of evaluat-ing a resource and making sure that fish popu-lations stay at healthy levels despite adverse fac-tors, both natural and human, like those listedabove.

The very first step a biologist takes is to findout how many fish are “out there” now and howthey are doing. However, fish are elusive andhard to see underwater; frankly, it’s impossiblefor us to swim around a pond and count themall. Since we can’t count or see them all, we’llhave to be satisfied with looking at a portion ofthe population, called a sample, and with col-lecting or sampling some individuals. Thesample gives biologists a “snapshot” of what’sgoing on in that body of water at that particularpoint in time. The technique chosen for sam-pling depends on the type of fish and the type ofhabitat they’re in, whether that be a stream,lake, shoreline, open water, or shallow water.

Stunning FishIn freshwater, the most commonly used sam-

pling technique is called electrofishing orelectroshocking. The term might conjure up ascene of Frankenstein’s lab, with a white-coatedscientist wielding a plugged-in fishing pole, butthat would be a flawed impression. Imagine in-stead a chilly, burbling mountain stream inwhich biologists and crew members walk cau-tiously over the streambed cobbles wearingrubber hip waders. One of the members car-ries a portable, gasoline-powered electric gen-erator (or battery) on a backpack, or it istowed along using a tote barge. In his hands aretwo grounded, metal poles, one with a nega-tive electrode on the end and the otherwith a positive. A device called a pul-sator is used to adjust an electric cur-rent that runs between the two poles,the ends of which are placed beneaththe water’s surface. The water, inessence, serves as the “wire” that con-nects the two electrodes (see illustration).

When the current is turned on in the water,the other crew members are strategicallypoised a few meters downstream at variouspoints in the streambed, with dip nets ready toscoop up any fish that are stunned by the biolo-gist doing the “shocking.” Within moments, anotherwise empty-appearing stream reveals aremarkable array of hidden treasure: red-striped candy darters, silver and saffron shin-ers, black-spotted and mountain redbelly dace,gold-speckled brook trout, and shimmeringrainbow trout.

Fish are generally unharmed by electrofish-ing but can display several different responsesto the electric field. Many of them escape by ex-hibiting a fright response when they first sensethe electricity. Some exhibit a muscle response

which causes them to swim toward the positivepole. Others are rendered temporarily uncon-scious. Those that are caught provide invalu-able data about the species composition of thestream, the size and age classes withina species, and general growthand productivity rates.Once examined andmeasured, the fishare released liveback into thestream.

Althoughcrew members areinsulated from the electricityin the water by wearingrubber waders, this

s a m p l i n gmethod involves a degreeof risk, because a crew member could fall intothe electrical field or develop a leak in thewaders. The power output of electrofishinggenerators is quite capable of stopping ahuman heart; so, strict safety measures are ob-served. Wading is generally only done in smallto medium-sized shallow streams that are inac-cessible by boat.

In larger streams, electrofishing by boat ispreferred. The electrical field from an elec-trofishing boat is effective on fish to a depth ofabout 8 feet. Consequently, the boat is generallyused for sampling fish along shorelines and inother shallow areas of lakes and streams. Be-cause many popular game species such aslargemouth bass, smallmouth bass, bluegill,shellcrackers, and trout often inhabit thesetypes of habitats, electrofishing is the fish sam-pling technique most used by fisheries biolo-gists in Virginia and nationwide.

Raising Fish Fry

Once the sampling process is completed andthe data are crunched on the computer back atthe office, a biologist can then begin to outlineremedies that may be needed to restore astream or other body of fresh water to a morebalanced composition, if necessary. The mostcommonly used remedies include seasons,creel limits, fish stocking, and habitat improve-ments. Fish stocking is also often used simply toprovide anglers with a wider variety of speciesto fish for.

There are four warmwater and five coldwa-ter fish hatcheries operated by the Virginia De-partment of Game and Inland Fisheries. In thewestern part of the state alone, over one millioncatchable trout and a quarter of a million fin-gerling trout are produced each year. Thesehatcheries are responsible for supplying the

NettingMost other fish sampling techniques involve

the use of nets. Many are stationary and sus-pended from poles, like the gill net, whichcauses fish to become entangled in the net’smesh when they swim into it. Stationary netsalso include trap nets (which are designed likeconical turtle traps), Fyke nets (which are trapnets with wings that guide fish into the trap),trammel nets (which have large pockets), andpound nets (a series of smaller and smallerimpoundments that corral fish into a concen-trated area).

Other nets, referred to as active nets, requirethe user to move the net through the water, suchas a seine (pronounced “sane”), which isdragged through the water to encircle fish. Afrequently used active net is the trawl, a some-what conical-shaped net that is dragged behinda boat. All of these nets are also used by thecommercial fishing industry to harvest fish forour dinner tables.

FEBRUARY 2002

VIRGINIA WILDLIFE� www.dgif.state.va.us

fish needed to restock a stream, river, or pond. From egg to fry to fingerling, you can see all

stages of fish development at a hatchery. Troutare reared in stacked-tray incubatorsand then in outdoor ponds or race-ways which mimic stream flowby circulating water in a con-

trolled setting. Warmwater fish, like bass,are either spawned naturally in a pond or are

hatched in large jars and thenstocked in ponds when they are

just a few days old. Whenthe time comes to take

the fish by truck to astream or river andstock or releasethem, biologistsinventory the fishand record theexact location,species, andnumber stocked.This information

is then used forcomparison when the

stream is sampled again ata future date.

Biologists gently remove eggsto give future fish generationsa head start in the hatchery.

Fish with Mussels

State fish hatcheries are also playing a role in an-other important project: the restoration of severalfreshwater mussel species that have declined in re-cent decades because of poor water quality andother environmental factors. Mussels, which arebivalves or mollusks like clams, rely on fish tocarry out their life cycle, and each mussel species isassociated with a particular fish species. A develop-ing mussel larva, called a glochidium, attaches it-self as a parasite to the gills of its host fish and ob-tains nutrients from the fish’s tissues. After a fewdays to a few weeks, the glochidium transforms intoa juvenile, drops off the fish, and begins an inde-pendent life. Unfortunately, mussel populations inthe wild are low, and their associations with hostfish are so specific, that the probability of the right

Circulating water provides oxygento fish eggs being hatched in“egg jars” (above). The developingfry (right) are nourished until theyare the proper size for release intoa stream or river, like these cold-water brook trout (below).

mussels coming in contact with the right fish to carry out their unique life cycles is veryslim.

In an attempt to give the wild populations a boost, biologists are raising mussels, like theendangered oyster mussel, with host fish, like the darter, in the Marion state fish hatchery.They collect male and female adult mussels from the Clinch River system in spring and putthem in the hatchery to reproduce. Meanwhile, darters are also collected from the samearea of the river and placed in holding tanks at the hatchery. When the mussels produceyoung, the larvae (glochidia) are removed from the females and transferred to the gills ofthe darter fish. The darters play host to the larvae until the mussels develop into the juvenilestage and drop off. Biologists then siphon the tanks to collect the tiny mussels, which arebetween the size of a dime and a quarter, and then release them back into the river wherethe original adults were collected. The darters are not released but are kept in the hatchery

to host additional generations of mussels. In this way, over 3000 young mussels were re-leased in 2001, about 200 of which were the oyster mussel species.

Did You Know?A fish’s body is protected by overlapping scales,

and each scale contains microscopic rings that canbe read like tree rings to learn the age of the fish (seeillustration). Fish keep scales throughout their lives,except when lost through injury. During winter afish grows slowly; in spring, the rate of growth in-creases. These changing rates of growth affect thedistance between the rings on the scales. Bands ofrings that are close together mark each winter sea-son, whereas widely separated rings indicate fastergrowth that occurs from spring through fall. Biolo-gists can also examine a fish’s ear bones, calledotoliths, to determine its age and learn the nutrition-al status and other clues about its life history.

LearningMore…

Careful Catch is one of many activities avail-able to youth leaders and other outdoor edu-cators through the Sportfishing and AnglingResource Education program of the Vir-ginia Department of Game and InlandFisheries. The activity illustrates the prop-er way to handle and release a fish, suchas using barbless hooks, artificial lures,and landing nets when possible. For in-formation about this or other activitiesfrom the ARE program, contact AnneSkalski, Aquatic Education Coordina-tor, in Richmond at (804) 367-6778.

A small sampling of scales is scraped from a fish(a.) to observe rings of growth on each (b.), muchlike tree rings. The rings on an ear bone or otolith(c.) are more accurate. The spine of a catfish

(d.) can also be cross -sec-tioned (e.) for study.

a.

b.

c.

d.

e.

Frequently Asked Questions, from the NortheastFisheries Science Center, www.nefsc.nmfs.gov.From the main page click on “Fish Facts,” then on thenext page click on “FAQ.”

Freshwater Fish Farming in Virginia, a 1997 publi-cation written by Helfrich, Orth, and Neves at VirginiaTech, can be downloaded fromwww.ext.vt.edu/pubs/fisheries/420-010/420-010.html#L9 You can also see a listing of Hatch-eries in Virginia from their Table of Contents page.

Access the Virginia Chapter of the American Fish-eries Society through the national Web site atwww.fisheries.org

The Field and Stream/Outdoor Life magazineshave numerous articles related to fish and fish-ing. Go to www.fieldandstream.com and clickon “fishing” for a regional fishing guide, the 25 bestfishing books, Ask the Expert, and much more.

Carol Heiser is a Wildlife Habitat EducationCoordinator at the Virginia Department of Gameand Inland Fisheries. Dean Fowler is a fisheriesbiologist with the Department who works out ofthe Williamsburg Regional Office.