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FISHERIES


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TECHNICAL REPORT
Interpretation of the Age and Growth of
Anadromous Salmonids Using Scale Analysis




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Number 84-5
September 25, 1984


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Michigan Department of
Natural Resources




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MICHIGAN DEPARTMENT OF NATURAL RESOURCES
FISHERIES DIVISION
Fisheries Technical Report No. 84-5
September 25, 1984
INTERPRETATION OF THE AGE AND GROWTH OF ANADROMOUS
SALMONIDS USING SCALE ANALYSIS
Paul W. Seelbach
and


George B. Beyerle




2
Introduction
Scale reading is a widely used  and valid   method  for
aging salmonids (Havey 1959). Information on age and growth
derived by scale reading provides the basis for a large part
of   the   biological understanding   of fish populations.
Fishery biologists cannot begin to  assess  the  effects  of
management programs   or environmental influences on a population without  accurate  age determination   of  individual
fish.
Michigan's  anadromous  salmonids  spend  part of their
lives in nursery streams and/or hatcheries and part  in  the
Great Lakes.    The  fish in one year class of salmonids may
vary considerably in both age at smolting  (migration  to  a
lake) and   age  at sexual maturity    (migration  back to a
stream).   The  size of   adult anadromous    salmonids   is
determined  primarily by time spent in the Great Lakes, not
by age  (Maher and   Larkin  1954; Michigan Department of
Natural Resources, unpublished    data).   Thus, when aging
anadromous salmonids, it is  important to   separate stream
years  from  lake years   in order to    account for all the
variations in growth patterns which may occur. Proper aging
allows us to place fish into biologically meaningful groups
and  avoid  misinterpretation  of data. For example, age-4
steelhead (Salmo qairdneri) may have spent 1, 2, 3 years  in
a  lake.   To  lump  all these steelhead together as 4 years
old, and determine an average length and weight, does not
really  explain what happened biologically. However, -if we
separate the steelhead into their various stream-age and
lake-age groups, then we can look at what is happening to
each group at specific points in time.
Scale Analysis
Nomenclature
Stream  annuli are designated by arabic       numerals,


followed  by  a period or slash (virgule), followed by lake




3


annuli in arabic numerals. The two   sets  of annuli added
together  represent total age (Godfrey et al. 1968; Hartman
1959; Narver 1969; Shapovalov and Taft 1954; Withler 1968).
A steelhead which has spent 2 years in a stream and 2 years
in a lake would be age 2.2 or 2/2 and would be total age 4.
Criteria for determining stream age and lake age
Anadromous trout and   salmon  scales  usually  show  a
pronounced  and  easily  detectable difference between lake
growth and stream growth (LaLanne and  Safsten  1969, Maher
and  Larkin  1954).   One year of stream growth typically
consists of a period   of  fast growth   (relatively widely
spaced  circuli), followed   by  a period   of slower growth
(closely spaced circuli). The annulus is   detectable  as  a
distinct   "cutting  over" of   incomplete, closely   spaced
circuli, by a complete cirulus which marks the  commencement
of fast growth in spring (Fig. 1).
Lake growth is characterized by circuli being much more
widely spaced than during fast stream growth (Fig. 1) and by
the formation of an annual ring (Fig. 2). An annual ring is
a crowding together of a circuli in a noticeable dark band.
The annulus may form at the inner edge, anywhere within, or
at the outer edge of the annual ring. Variability occurs in
the width   of  the  annual ring   and in the clarity of the
annulus, but aging is easier if the scale reader has   these
two features in mind.
Supplemental growth "checks" are similar to annuli and
annual rings. No definite criteria can   be  given  for  the
detection   of   supplemental   checks.   They  are usually
recognized by their unusual position in the pattern of scale
growth  and  often  cannot be traced    around  the   entire
sculptured portion.    Annuli and   annual rings occur at
regularly spaced intervals on the scale; they usually can be
traced around the entire sculptured portion. In smolts of
steelhead, brown trout    (Salmo  trutta), Atlantic salmon
(Salmo salar), and coho salmon  (Oncorhynchus kisutch) the




4


change from stream to   lake  growth  often  occurs   in  late
spring,   long  after  the annulus has formed.  The resulting
supplemental "smolt check" can be   confused  with  a  stream
annulus (Fig. 3).
Hatchery growth may add confusion to the determination
of stream age.   An annulus formed while  a  fish   is  living
under  hatchery conditions is often hard to distinguish. On
a hatchery fish "winter" and   "spring"  growth  are  not  as
distinguishable   as  on  a wild   fish and this can make the
annulus hard to locate (Havey 1959). The scale reader must
look  carefully   for the "cutting over" described previously
(Fig. 4).
Diagrams of typical scales of anadromous salmonids are
shown in Figure 5.
Age of fish collected at various times of the year
The   arabic    numerals   used  in  age  classification
represent annuli, not growth seasons. This presents    little
problem  when   aging  spring steelhead, where most fish have
recently laid down an annulus or annual ring. All fish    are
considered to have a birthday on January 1. Therefore, fish
collected   between   January  1  and  the  time  of  annulus
formation in spring are aged as 1 year older than the number
of visible annuli (or annular rings). The presence    of  the
unseen   (or  virtual)  annulus  is  recorded  by  adding  an
asterisk (*) to the age (Chapman 1958; Hartman   1959; Merna
et  al.  1981).    A steelhead showing two stream annuli, two
Lake Michigan annuli, and which was   collected  in  January,
before  forming   an  annulus, would   be  age 2.3*.  In fall
collections of trout and salmon, only the annuli are counted
and recorded (Fig. 5c).
Table  1  shows   the  poss ible  ages  for   anadromous
salmonids collected in Michigan.




5


La ke
Sgrowth
Stream
growth
Figure 1. Stream and lake growth on an anadromous salmonid
scale.
Annuol
ring
Figure 2. Annual ring during lake growth on an anadromous
salmonid scale.




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Smolt
check
Annulus I
OD        1(stream)
Annulus 2
(stream)
Figure 3. Scale of anadromous salmonid showing smolt check.


Annulus 2


Figure 4. Comparison of annulus 1 in wild and hatcheryraised anadromous salmonids.




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Smolt check


Figure 5a. Age 1.1 steelhead
with smolt check.


Figure 5b. Age 2.0 steelhead
with smolt check.
Annulus 1
Stream growth
Age 0.1 chinook salmon.


Figure 5c.


Figure 5.


Diagrams of some typical scales of anadromous
salmonids.




8


Table 1.  Ages of anadromous salmonids in Michigan.
= possible ages   ** - most common ages,
- age of normal jack salmon.)
Species
Steelhead, brown trOut,           Pacific salmon
and Atlantic salmon
Chinook     Coho
Age         Spring         Fall              Fall      Fall


0.1
0.2
0.3
1.0
1.2
1,3
1,4
1,5
2.0
2.1
2,2
2.3
2.4
2,5
3.0
3.1
3.2
3.3
3o4
3.5


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9


Literature Cited
Chapman, D. W. 1958. Studies on the life history    of the
Alsea   River   steelhead.   Journal   of Wildlife
Management 22:123-134.
Godfrey, H., D. D. Worlund, and H. T. Bilton. 1968.   Tests
on    the   accuracy   of  aging   chinook   salmon
(Oncorhynchus tshawytscha)   from   their   scales.
Journal of the Fisheries Research Board of Canada
25:1971-1982.
Hartman, W. L.   1959.  Biology  and  vital statistics of
rainbow trout in the Finger Lakes Region, New York.
New York Fish and Game Journal 6:121-177.
Harvey, K. 1959. Validity of the scale method    for aging
hatchery-reared Atlantic salmon.    Transactions of
the American Fisheries Society 88:193-196.
LaLanne, J. J., and G. Safsten.  1969.   Age determination
from  scales of chum   salmon   (Oncorhynchus keta).
Journal of the Fisheries Research  Board  of Canada
26:671-681.
Maher, F. P., and P. A. Larkin. 1954. Life history of the
steelhead trout of the Chilliwack River, British
Columbia.   Transactions  of the American Fisheries
Society 84:27-38.
Marver, D. W. 1969. Age and size of steelhead     trout  in
Babine River, British Columbia.     Journal of the
Fisheries Research Board of Canada 26:2754-2760.
Merna, J. W., J. C. Schneider, G. R. Alexander, W. D.
Alward, and R. L. Eshenroder.      1981. Manual of
fisheries survey methods.  Michigan Department of
Natural   Resources,   Fisheries Management Report
No. 9, Ann Arbor, Michigan, USA.
Shapovalov, L., and A. C. Taft. 1954. The   life histories
of the steelhead rainbow trout (Salmo gairdneri) and
silver    salmon-   (Oncorhynchus   kisutch),   and
recommendations    regarding   their    management.
California Fish and Game, Fisheries Bulletin 98.
Withler, I. L. 1966. Variability in life history characteristics of steelhead trout (Salmo gairdneri) along
the Pacific Coast of North America. Journal of the
Fisheries Research Board of Canada 23:365-392.




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