'itII!I

iil'IIIi

II

I

IiIi,

.'

1

Ii

i

I

III

Why it's Part of the Answer to "How are

My Fish Doing?"With increasing interest in ponds across this

country, more pond owners and managers are

seeking detailed information about "how to" or

"hands on" pond managementpractices. This ismost evident from the nature of the questions

sent to Pond Boss Magazine and asked on the

Pond Boss Forum. A critical question often

at the core of most pondmeister's thoughts

is "How are my fish doing?" To answer

this question, pondmeisters, with their ever-

increasing knowledge and sophistication,

began to use the relative weight concept to help

provide an answer. However, relative weights

provide part of the answer. Professional fisherybiologists would prefer to have information

on fish age and growth to truly understand a

population.Perhapsyou read the article titled Trade-Offs

in Pond Management in the January/Februaryissue of Pond Boss and looked at the charts

regarding the status of a small Missouri

impoundment. Largemouth bass and bluegill

growth were critical components of thatassessment.

In the big picture of determining "How are

my fish doing?" age and growth information

together provide a combined assessment that

actually indicative of other factors such as

water quality, prey base,habitat suitability, and

genetics. All these factors are reflected in the

quality of your fish community.So, let's look into someof the aging structures

commonly usedfor fish. Then, we'll come back

to how you can use this information.

I

I!I

I

Common Fish Aging Structures

Biologists can estimate the age of individual

fishes using a variety of aging structures. For

years, many fishes have been aged from their

scales. Perhapsyou have heard the analogy that

scaleshave annual rings much like a tree? The

concept is that fish lay down an annual mark

during periods of slow growth during winter.

It's quite distinct from the warm part of the year,when fish accomplish most of their growth.

26 POND BOSS

On NorthernPond

FishAgingb~Dr. DaveweLLisan,dtreeWest

Biologists actually remove scales fromdifferent locations on different types of fishes.

For some of the spiny-rayed fishes such as

largemouth bass and bluegill, scales often aretaken from the tip of the pectoral fin, and justbelow the lateral line. Look at the bluegill

picture that accompanies this article. The finon the side of the fish that originates just behind

the gill flap is called the pectoral fin. Scalesaresometimes agedsimply by placing between two

glass slides and viewing under a microscope.However, as fish get older the scales tend to

become thicker and more opaque. Such scales

can be pressed into acetate (i.e., plastic) slides,which greatly improves viewing.

Scales are not always a reliable agingstructure. The picture of the white crappie scale

that accompanies this article shows a scale atits best. Others can be much more difficult

to interpret, with extra marks and lines on the

scale. Biologists in the southern part of thecountry are to the point where few actually usescales to age fish. The shorter, warmer wintermakes the annual marks less distinct than in

the northern states. Even in the north country,

ages of older fish are unreliable from scales.Generally, we can age fish from scales overtheir few years of life. However, a 15-year-oldwalleye scale would typically be under-aged by

5 years or more.Today, many fishery biologists rely more

on otoliths than scales for fish aging. The ageestimates from otoliths tend to be much more

reliable, even for older fish. Otoliths are alsoknown as "earstones," and they are part of the

inner ear of a fish. They are part of the balanceand hearing systems for fish.

For some fishes, especially at youngerages, we often can age the otoliths whole.For example,in our recentworkwith walleyeand bluegillotoliths,we generallyused wholeotoliths through about age 4 or 5. If you lookat the bluegill otolith that accompaniesthisarticle, the annual marks (laid down duringwinter) are distinct even in the whole otolith.The picture was taken under low magnificationwith a microscope. For older fishes, we often

cracktheotolithsin half,sandthema little, and

then age them. Most often, we'll put them in asmall base of play-dough, and put a drop of oilon the otolith to make it easier to see the annual

marks. Then, we use a small, fiber-optic lightheld just to the side and below the otolith. Onceyou get the technique down, the annual marksalmost jump out at you. Until then, it can be a

little frustrating.

Many other structures have been used toage fish. Fin rays are used for some fish. Weincluded a picture of a dorsal spine aged fora walleye. This structure has to be sectioned

with a fine, diamond-edged waver blade on a

special saw.Some marine fish are aged by sectioning

their vertebrae. Soft fin rays can be aged for

fishes such as suckers and other species. Inthe paddlefish, which is a cartilaginous fish

rather than a bony fish, we can age thin sections(slices)takenfrom the bottomjaw of the fish.In a current study with common carp, wefirst aged them with otoliths, which of courserequires that we kill the fish. However,thehard, spiny, serrated pectoral fin rays turned outto be reliable through age 13. Thus, biologistscan at times find nonlethalalternativeagingstructures. For years, catfish were aged bysectioning their spines, just as you can see forthe bullhead spine section with this article.Recent work within the profession indicates that

otoliths may be a more reliable aging structurethan spines for many catfish species.

It's important that we stress one thought.Aging fishes from these various structurescan at times be very easy, but many times isdifficult. The mostusefulaging structurewillvary by geographiclocation,by fish species,andat timesevenfrom laketo lake. Generally,a little help gettingstarted,followedby somepractice, makes a big difference.

How To Use This Information

Why age fishes? Well, this informationis used to determinematurity,growth rates,survival or mortality rates, and longevity. Inaddition, this information can provide biologists

MarchiApril 2006

Common CarpPectoral fin raysection, age 5

White CrappieScale collected in fall, age 2,

would be 3 on Jan 1of next yr

Black BullheadPectoral spineremoved and

sectioned, age 6

White BassCracked (halt)otolith, age 12

with information on year-class strength of a fishpopulation. Fish are "assigned" a birthday bybiologists. In the northern hemisphere, it'sJanuary I each year. Actually, it's not reallya birthday for most fish, other than guppiesand mosquitofish. It's really a hatch-day,as most fishes are hatched rather than born.

However, you probably catch our point. As aresult of these age assignments, we can thentell in which year a fish was hatched. A fishhatched in 2005 then becomes age I on JanuaryI of 2006. This is the source of the term "yearclass," which refers to the year in which a fishwas hatched. The 200I year classes of walleye,bluegill, yellow perch, and white bass were allquite strong in eastern South Dakota. Weatherconditions were good that year.

One of the important jobs of a fishery

www.pondboss.com

BluegillWholeview-otolith,age 4

WalleyeCracked otolith (right),age 5;

(below)dorsal spine, age 7

biologist, either an agency biologist or a private

consultant, is to know the growth characteristics

of fish at their geographic location. They either

know typical growth rates for their location, or

they know were to find such information. Most

state agencies have growth summaries showing

averages and ranges for sport fish in their state.

So, let's take this back to your pond.Whether electrofishingis used to sample afish community,or a pondmeistermonitorsangler catches, trends over time are animportantsource of information. Trends inabundance, fish sizes, and growth tell thefinalstoryon what is happeningwithina pondfish community. If all is well - then handsoff. However,if managementstrategiesareimplementedto make a change,then changesshould be evident in abundance, size, condition

and growth information. If largemouth bassare too dense, a culling program might beinstituted. If so, and sufficient small bass areculled, then abundance (catch rates) shoulddecline, and size, growth, and condition shouldincrease. In reality, growth information is moredifficult and time-consuming to obtain thanother population information. However, it'svalue is correspondingly higher as well!

Dr. David W. Willis is a professor in theDepartment of Wildlife and Fisheries Sciences,

South Dakota State University in Brookings.

~

POND BOSS 27