A Brief History of Age Determination
of Walleye Pollock (Theragra chalcogramma) at the Alaska
Fisheries Science Center
Daniel K. Kimura
Revised February 7, 2008
Like most other groundfish species, walleye pollock are aged by
counting annual growth rings that occur on otoliths (ear bones),
much like growth rings occurring on trees. However, there is a
difference between tree and otolith rings in that otolith rings are
generally not nearly so clear as tree rings, and are therefore
difficult to interpret. Actually otoliths are not true cellular
bone, but are calcium carbonate stones made out of aragonite. These
stones continue to grow through the life of a fish, and usually lay
down an annulus (year mark) once per year. One of the most difficult
aspects of ageing is determining just when the final year mark was
laid down.
The"sagittal" otolith, used for ageing, is the largest of three types
of otoliths that are found in the head of teleost fishes. These
"bones" are part of the vestibular apparatus of fishes that function
as an organ for balance and hearing. Growth marks that are used for
ageing are typically visible on the whole untreated otolith (Fig. 1;
fished aged 3yr). Young clear specimens can be aged from the
surface. However, consistent ages from older specimens requires
breaking the otolith transversely (i.e., in the narrow direction),
and burning the resulting surface in an open alcohol flame (Fig. 2;
fish aged 7yr).
Two of the earliest attempts to age walleye pollock were by Japanese and
U.S. scientists using otoliths (Ishida 1954, Mosher 1954). Mosher,
then of the U.S. Fish and Wildlife Service broke the sagittal
otolith along the transverse axis as was recommended to him by G.
Rollefsen. This was a minor effort on a minor sampling of the virgin
stock of pollock that then existed in the eastern Bering Sea. By the
time J. LaLanne revisited the ageing of pollock (LaLanne 1975,
1979), the pollock fishery had built to 1.4 million metric tons and
age determination of this species was becoming a serious matter.
LaLanne found the whole otolith to be sufficiently clear for age
determination. G. Hirschorn advanced age determination at the AFSC
by bringing on the break and burn method around 1981, and
introducing a quality control system based on comparing repeated
readings from different age readers on the same otolith in 1983. The
break and burn method (i.e., transversely breaking the otolith by
hand and burning the transverse surface over and alcohol flame) was
then being highly touted by Canadian age investigators (Chilton and
Beamish, 1982) and proved to be immensely useful for the age
determination of many Alaska groundfish species. To this day, the
break and burn method is the preferred method for ageing walleye
pollock at the AFSC. Only younger, clear specimens are aged from the
surface. Also, only relatively minor changes in the ageing criteria
of walleye pollock, such as the preferred axis for counting (changed
around 1990), has occurred since 1981.
Several countries which have fished pollock in the international
Donut Hole have attempted to arrive at ageing methodology through
consensus. In September 1989, a "Workshop for Ageing Methodology of
Walleye Pollock" was held in Gdynia, Poland (Kimura 1991). Prior to
this workshop AFSC circulated an exchange sample of 125 broken and
burned pollock otoliths, that was eventually aged by all nations
participating in the workshop (Gdynia Report Table 1). In addition,
Poland prepared 144 otolith sections which were aged by participants
at the Gdynia Workshop (Gdynia Report Table 2). The results from
these exchange samples caused the participating countries (Canada,
Japan, P.R.C., Poland, U.S.) to agree "that under are present
knowledge that the break and burn method provides the best method
for ageing walleye pollock."
A second "Workshop for Ageing Methodology of Walleye Pollock" was held
at the Alaska Fisheries Science Center in Seattle (Anon., 1998).
Results from the reading of reference samples from the Western and
Eastern Bering Sea at this workshop again indicated substantial
agreement between Japanese, Polish, Russian and, and U.S. age
readers.
In 1983 G. Hirschorn adopted a quality control system in an attempt to
assure a consistent quality of ages at the AFSC by randomly
subsampling 20% of otoliths for re-ageing by a second age reader.
Under this system, differences in the ages from the two age readers
are reconciled in attempt to arrive at the best ages possible. When
major discrepancies between the two age readers are detected, large
numbers of otoliths may be re-aged.
Over the years the AFSC and its precursors have aged large numbers
of walleye pollock (Fig. 3). In the earlier years, pollock otoliths
were clear and read from the surface, enabling age readers to age
large numbers in a short time. Gradually, the otoliths became more
difficult, more often requiring the more time consuming break and
burn methodology.
Recently, pollock ageing criteria have been radiometrically validated (Kastelle and Kimura 2006) using Pb-210/Ra-226 methods. Also, pollock ageing criteria have been corroborated (Kimura et al. 2006) by tracking strong year classes, marginal increment analysis, and confirming otolith ages using vertebrae. Taken together these studies give us considerable confidence in the ageing methods we are using for this species.
References
Anon. 1998.
Report from the second workshop on ageing methodology of walleye
pollock (Theragra chalcogramma), held in Seattle, WA, March
17-20, 1998.
Chilton, D.E.,and R.J. Beamish. 1982. Age determination methods for fishes
studied by the groundfish program at the Pacific Biological
Station. Can. Spec. Publ. Fish. Aquat. Sci. 60: 102p.
Ishida, T.
1954. On the age determination and morphometrical differences of
the otoliths of Alaska pollock in the Hokkaido coast. Bull.
Hokkaido Reg. Fish. Res. Lab. 11:36-67. In Japanese (Trans.,
NMFS, Int. Act. Staff, Washington D.C.).
Kastelle, C.R., and D. K. Kimura. 2006. Age validation of walleye pollock (Theragra
chalcogramma) from the Gulf of Alaska using the disequilibrium of Pb-210 and Ra-226.
ICES J. of Marine Science 63:1520-1529.
Kimura, D. K.
1991. Age and Growth, Module D. In Discussion summaries
from the International Workshop on Bering Sea pollock stock
assessment, February 4-8, 1991, compiled by Julie A. Pearce. AFSC
Processed Report 91-06.
Kimura, D.K., C.R. Kastelle , B.J. Goetz, C.M. Gburski, and A.V. Buslov. 2006. Corroborating
ages of walleye pollock (Theragra chalcogramma), Australian J. of Marine and Freshwater
Research 57:323-332.
LaLanne, J.J.
1975. Age determination of walleye pollock (Theragra
chalcogramma) from otoliths. NWAFC Processed Rep. September
1975, 19p. Northwest and Alaska Fisheries Center, National Marine
Fisheries Service, NOAA, 2725 Montlake Boulevard E., Seattle, WA.
98112.
LaLanne, J.J.,
1979. The validity and consistency of age determinations from
otoliths of Pacific pollock (Theragra chalcogramma).
International North Pacific Fisheries Commission, Annual Report.
1977, pp.99-107.
Mosher, K.
1954. Use of otoliths for determining the age of several fishes
from the Bering Sea. Extrait du Journal du Conseil International
pour l'Exploration del la Mer 19:337-344.
Fig. 1. Whole
otolith ageable from the surface. Annular marks, labeled with dots,
are clearly visible in this otolith determined to be from a
3-year-old fish.
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Fig. 2.
Otolith cross-section made by breaking an otolith in half in the
transverse direction, and burning the resulting surface in an
alcohol flame. Annular marks, labeled with dots, indicate this
otolith was taken from a 7-year-old fish.
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Fig. 3. Number
of walleye pollock aged each year from 1979 to 2007. Earlier years
were largely aged from otolith surfaces.
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