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U.S. DEPARTMENT OF COMMERCE

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION

NATIONAL MARINE FISHERIES SERVICE

I

Marine Biological Laboratory

LIBRARY

JUL 2 11971

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AGE DETERMINATION OF FISHES (REVISED)

Fishery

Leaflet

637

Scale From A 7-Year-Old Haddock

UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
Dr. Robert M. White, Adminishafor

NATIONAL MARINE FISHERIES SERVICE
Philip M. Roedel, Director

Age Determination of Fishes (Revised)

By
FRED E. LUX

Fishery Leaflet 637

Seattle, Washington
June 1971

Age Determination of Fishes (Revised)

By

FRED E. LUX

National Marine Fisheries Service Biological Laboratory
Woods Hole, Massachusetts 02543

Introduction

Span of life in fishes, like size, covers an
extremely wide range, depending upon species.
A tiny European goby that matures at little
over an inch in length is an example of an
"annual" vertebrate, running the course of its
life within a single year. Other fishes are
known to pass the century mark. Canadian
biologists in 1953 determined the age of a 215-
pound sturgeon, caught by a Lake of the Woods
fisherman, to be 152 years.

Aside from its value in filling gaps in our
scientific knowledge of fishes and satisfying
human curiosity concerning them, age and
accompanying growth-rate information is of
vital importance for the management of fishery
resources. For instance, where fish are caught
commercially, growth rate must be known in
order to learn the size and age at which the
fish may be most efficiently harvested. A
further use of age information is in judging
the results of management practices. Knowl-
edge of the average size and age of fish before
and after management measures are put into
efi'ect can sometimes show whether or not such
plans are achieving desired ends.

Age Determination

Three basic methods have been used for age
and growth detei'mination of fishes: (1) ob-
servation of the growth of fishes of known age,
(2) study of fish size-frequencies, and (3)
study of seasonal ring formation in hard body
parts such as scales and bones. The method
used usually depends upon special problems
encountered in age determination of a given
species.

' This publication is a revision of Lux, Fred E.,
Age Determination of Fishes, U.S. Fish Wild. Serv.,
Fish. Leaflet No. 488.

Observation of the Growth of Fish of
Knov^tn Age

Fish of known age are held in a pond or
aquarium for a number of years so that length
for a given age may be determined by simply
catching and measuring the fish periodically.
While the method is direct, it has limited use
since it requires raising fish under artificial
conditions where growth rate may diff'er from
that in their normal surroundings and where
maintaining certain species may be difficult.
The technique probably has its greatest use
in verifying ages that have been determined
by other means that are discussed later.

As an extension of this technique, fish of
known age may be marked and released in
their normal environment and then measured
when they are recaught to learn size for the
known age. Some small fish may be marked
by the removal of certain fins so they can be
identified upon capture. Larger fish usually
are marked with small numbered tags of plastic
or noncorrosive metal. A major drawback to
this method is that the tag or mark may slow
down growth, as has been shown for many
fishes. To use the method eff"ectively, one must
therefore either demonstrate that the tag has
no efi'ect or correct for, its effect. Neither of
these is easily accomplished.

Study of Fish Size-Frequencies

The size-frequency method of age determina-
tion depends on the reproductive and growth
characteristics of fish. Most fishes breed dur-
ing a restricted period and only once a year
so that sizes within a given brood year are
fairly uniform and distinct from sizes from
other brood years. Haddock, for example,

— 1 —

spawn only during the late winter and spring.
Baby fish from this spawning are 4 to 8 inches
long by the summer and fall, and their length
marks them as young of the current year.
Haddock spawned in the previous year would
be 10 or 12 inches long by this time and clearly
separable, by size, from the baby fish.

To illustrate the technique, suppose we use
a net, such as an otter trawl, and obtain a
large catch of one kind of fish in which all
sizes, young to adult, are represented. Let us
measure the lengths of all fish in the catch and
count the number of fish in each length span
of, say, one-half inch for the entire range of
sizes. If we now plot the numbers of fish in
each of these length intervals on graph paper,
it is likely that more than one peak will be
evident in our graph because fish of certain
lengths occur more frequently than others. It
may be suspected that each clear-cut peak rep-
resents the average length in a separate age-
group because of the restricted range of lengths
within each age-group.

10

5-

' k'

1 1 1 1

MALES

( 168FISH)

"

/ ^

/\

t-rEAR OLDS

\

l-YEAR OLDS \

FEMALES

( 144 FISH)

y\

/ i-rEAK \

J OLDS *-

/ £- YEAR \ /-^^
■<K^^ OLDS \/ ^

^ 1 1

j-yearX

OLDS "^^

1 1 i 1 N

9 II 13
TOTAL LENGTH IN INCHES

Figure 1. — The length-frequencies of a catch of yellow-
tail flounder, showing how fish age may be deter-
mined from fish size.

Figure 1 shows the lengths of a catch of
yellowtail flounder graphed in this way. Males
and females have been plotted separately be-
cause females of this species are known to
grow faster than males. One-year-olds out-

number fish of older age-groups because deaths
from various causes take a continuous toll.

There are two peaks in the graph of males,
one at about 8I/2 inches, representing the ap-
proximate average length of 1-year-old fish,
and a second at 12 inches, marking average
length for 2-year-olds. For females, peaks are
shown at 8V2. 13, and 15 1/0 inches, representing
average lengths of 1-, 2-, and 3-year-olds, re-
spectively. The graphs indicate that few fish
older than these age-groups were present on
the fishing ground. Also, no fish in their first
year (0-year-olds) were caught here, for dur-
ing the summer when this catch was made the
baby yellowtail have not yet begun to school
with older ones.

The size-frequency method works best for
young fish, generally less than 3 or 4 years old.
As fish grow older, the spread of sizes within
an age-group becomes more variable. The
peaks representing ages of older fish in the
length-frequency graph tend to blend together,
and it may be impossible to identify them. The
peaks for 2- and ;]-year-old females in Figure
1 already show signs of flattening out and
blending together.

This disappearance of peaks with age is
more apparent in the length-frequency distri-
bution of haddock in Figure 2. This catch was

Figure 2. — The length-frequency distribution of a
catch of haddock, showing the different size groups
of fish caught.

obtained in the fall when baby fish are just
beginning to appear in otter trawl catches.
These averaged about 6 inches in length. One-
year-olds averaged about 11 inches, while 2-
year-olds were slightly over 16 inches in length.
It is clear that the peak for 2-year-olds is not
as sharply defined as that for 1-year-olds. This
is because it undoubtedly also includes some

— 2 —

3-year-olds in the right-hand part. Beyond the
2-year-olds, no peaks are apparent because of
the blending together of age-groups of older
fish.

Study of Seasonal Ring Formation
IN Hard Body Parts of Fishes

Because fish are cold-blooded animals, their
body processes are regulated by the temper-
ature of the water in which they live. Growth
is rapid during the warm season and slows
greatly or stops in winter. The technique of
determining the age of a tree by counting an-
nual rings in a cross-section through the base
of the trunk is a familiar one. As in trees,
seasonal changes in growth rate of fishes are
often reflected in zones or bands in hard body
structures such as scales, otoliths (ear stones),
and bones.

Fish Scales. — Of the hard body parts used
for age determination in fishes, scales are most
useful. They are easy to collect and prepare
for study. Of importance is the fact that a
few can be removed with little or no injury
to the animal since fishes have the ability to
regrow lost scales within a short time. A
further advantage, that of permitting an esti-
mation of the past growi^h history of a fish
from its scale, will be discussed later.

Scales are of value for age determination
in many of our "bony fishes," a broad group-
ing which includes most fishes of importance
for food. Scales are formed when newly
hatched fish complete their larval stages, and
soon cover the entire body, with the exception
of head and fins. In most species they lie in
an overlapping pattern much like shingles on
a roof and serve as a protective coat.

Scale growth begins with the formation of
the scale center or focus, and growth is out-
ward from this focus, though it is greatest
toward the forward margin of the scale. Fine
ridges called circuli are laid down in a circ-
ular pattern around the focus as growth pro-
ceeds. Many circuli are added to the scale each
year.

Most food and game fishes have either
cycloid or ctenoid scales. These two scale
types are illustrated in Figure 3. Cycloid

scales, found on trout, minnows, whitefish, pike,
cod, and most other soft-finned fishes, have
circuli which pass entirely around the scale
margin as growth is added. In ctenoid scales,
found on bass, perch, some flounders, and most
spiny-finned fishes, the focus is near the rear
edge of the scale, and circuli here are obscured
by the tiny spines or ctenii which give these
scales their name. It is the ctenii which give
the bodies of such fish a rough sandpapery
feeling.

As stated earlier, fish growth is reflected in
scale growth. Circuli are widely spaced in
warm seasons whe" fish growth is rapid and
closely spaced in cold seasons when it is slow.
In some northern climates, especially in ice-
covered lakes, fish growth stops in winter. The
growth of a fish during 1 year, therefore, is
shown on its scale as a series of widely spaced
spring and summer circuli followed by a series
of closely spaced fall and winter circuli. Since
fishes continue to grow throughout their lives,
this pattern is repeated each year. The outer
edge of a series of closely spaced circuli is
generally taken to be the end of growth for
that year, and this point is referred to as the
year-mark or annulus (see whitefish and had-
dock scales in Figures 3 and 5). The age of
a fish is determined by counting the number of
annuli or year-marks.

In some cycloid scales, such as those on trout,
there may be no clear seasonal diflTerence in
spacing between circuli. On these scales the
year-mark is sometimes shown as a discontin-
uous or broken circulus following a series of
complete circuli.

In ctenoid scales, like those of sunfish and
bass, there is often no detectable diff'erence in
the seasonal spacing of circuli. Here another
feature of scale circuli is relied upon to identify
the end of growth for a given year. On these
scales the last few circuli laid down in a year
are often incomplete in that they do not con-
tinue all the way around to the spiny area of
the scale. When fast growth resumes in spring,
the circuli are again complete and cut across
the ends of the incomplete circuli inside of
them. The first complete circulus of the growth
for the new year is considered to be the year-
mark. This cutting over of circuli is illustrated
in the sunfish scale in Figure 3.

— 3

ANTERIOR SCALE MARGIN

C IRCULI

ANNUL

FOCUS

r.»'/

CTENII-

-■■'..^ » /x^-^^^x^A-^-^i^^^r^z

Fi^re 3. — The cycloid scale of a whitefish (left) and the ctenoid scale of a sunfish,
showing year-marks (annuli) and general scale features. Both fish were 2 years old.

Figure 4. — A microprojector designed for the study of fish scales. The magnified scale
image is projected on the frosted glass screen.

Scales may be prepared for study by mount-
ing the whole scales on glass slides or, more
commonly, by pressing imprints of the scale
circuli into transparent plastic. The scale
photographs shown are of these plastic im-
pressions. Scales or scale impressions are ex-

amined under a low-power microscope or by
use of a microprojector like the one shown in
Figure 4.

As fish grow older and growth slows down,
with a consequent narrowing of the band of
circuli added to the scale each year, it becomes

— 4 —

increasingly difficult to identify year-marks.
This causes a greater amount of error in the
determination of age of older fish.

A slowdown in fish growth may occur dur-
ing a growing season, and the resulting check
in scale growth sometimes resembles a year-
mark. Where such checks are counted as year-
marks, the age determined from the scale will
be greater than the true age. These false year-
marks are sometimes associated with reduced
growth-rate at spawning time or with shock,
such as injury or disease.

Growth in length of a fish scale is propor-
tional to the growth in length of the fish itself.
Because this is true, the past growth history
of a fish can often be worked out from its scale
through a technique called back-calculation.
If the length of a fish at capture is known, it
is possible to calculate length at earlier ages
from measurements of the scale at each year-
mark. The increase in length of a haddock
in each year, in relation to increase in scale
length, is illustrated in Figure 5. Lengths at
each of the three year-marks were determined
from back-calculations.

1 YEAR

YEARS

2.0

YEARS - I 7.0'

Figure 5. — Illustration showing how the past growth
history of a fish may be determined from its scale.
The 18%-inch haddock was in its fourth season
since it has three year-marks on its scale and the
beginnings of fourth-year growth on the scale edge.

The back-calculation method is of importance
in fisheries studies since it permits an evalu-
ation of growth rate of fish in all years of life.

Otoliths. — In fishes that do not have scales,
or where annual zones are not clearly shown on
scales, it is often possible to determine age from
seasonal bands laid down in otoliths. Otoliths,
or earstones, structures formed of calcium in
the heads of bony fishes, function as organs
of balance. Although there are three pairs
of otoliths altogether, only one pair is large
enough to be of use in age determination.
Otolith form varies in diflferent species from a
flat oval to spindle shape. Growth, as in the
scale, is concentric around a central kernel or
nucleus (Figure 6). Factors, such as water

Figure 6. — An otolith from a 4-year-old yellowtail
flounder. The fish grew most rapidly during its
second year, and the otolith band in that year is
therefore the broadest one.

temperature, that affect fish growth cause
seasonal changes in the density of layers laid
down in otoliths, and in some cases it is pos-
sible to determine fish age from the banding
that results. When otoliths are viewed under
a low-power microscope, the layers making up
spring and summer growth appear as a white,
opaque band. Layers laid down in the fall, and
also in the winter in some fishes, appear as a
dark translucent band. A light and a dark

5 —

band together make up the annual growth,
and age in years is determined by counting the
number of dark bands. The otolith from a
yellowtail flounder shown in Figure 6 illus-
trates the features described.

For many fishes otoliths show age more
clearly than scales. They are often considered
better than scales for determining age of older
fish.

Preparation of otoliths for examination var-
ies with the fish species under study. In some
cases bands show clearly in dried otoliths,
which may be examined whole or sectioned and
polished. For other species, banding remains
clear only if the otoliths are stored in some
fluid such as glycerine or alcohol, upon removal
from the fish.

Determination of fish age from otoliths is
generally not difficult since little preparation
time is required. Otolith extraction requires
killing the fish, however, a disadvantage for
some studies.

Bones. — Cross sections through the bases of
fin rays, the thin bones that support fins, often
show concentric banding that is related to fish
age. The age of the 152-year-old sturgeon,
mentioned earlier, was determined from a
cross-section of the thick, spiny fin ray found
at the base of one of the paired breast fins of
this fish. Fin rays have been used for age
determination in a number of species besides
the sturgeon, such as catfish, bullheads, and
suckers. Zonation is similar to that found in
otoliths with a light band forming in the early
part of the growing season followed by a nar-
row, dark band in fall and winter. The had-
dock fin ray cross-section shown in Figure 7
shows seven pairs of such bands. Age in years
is determined by counting the dark bands.

Certain other bones of fishes may show
seasonal zonation associated with age. Indi-
vidual vertebrae and opercular bones (gill cov-
ers) are often well suited for age determination
although other bones have been used in some
cases. Annual bands, where present, are ident-
ified by diff'erences in color shade between
early and late season growth, as in fin rays.
Zonation in vertebrae is concentric around the
center line of the backbone. Opercular bones
show banding along the growing edge.

Figure 7. — The cross-section of a fin ray from a 23-
inch haddock, showing seven growth zones.

Bones may be cleaned by boiling in water.
Most bones are then air dried to ready them
for study, but fin rays must be cut into thin
sections with a jeweler's saw or other suitable
instrument. The sections are sometimes
ground and polished for examination under a
microscope.

References

The foregoing treatment of fish age deter-
mination is necessarily brief. For the reader
who is interested in further pursuit of the
subject, a short list of reference books con-
taining sections on fish age and growth is given
below. Some of the larger public libraries may
be consulted for these as well as additional
works.

CARLANDER, KENNETH D.

1950. Handbook of freshwater fishery biology.

Wm. C. Brown Co., Dubuque, 281 pp.

Summaries of age and growth data for

most North American freshwater species,

extracted from published reports.

CURTIS, BRIAN.

1948. The life story of the fish. Harcourt,
Brace and Company, New York, 284 pp.
A general work on fishes.

6 —

LAGLER, KARL F. ROUNSEFELL, GEORGE A., and W. HARRY

1952. Freshwater fishery biology. Wm. C. EVERHART.

Brown Co., Dubuque, 360 pp. 1953. Fishery science, its methods and appli-

Good technical treatment of age and cations. John Wiley & Sons, Inc., New

growth studies in fishes. Contains ex- York, 444 pp.

tensive bibliography of technical papers Good technical treatment of age and

on this subject. growth studies in fishes.

NORMAN, J. R.

1951. A history of fishes. A. A. Wyn, Inc.,
New York, 463 pp. (This is a 1951 re-
print of the 1931 edition.) ,
Extensive work on the natural history
of fishes of the world.

GPO 998-093

7 —

1©^1

1971

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