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Report From The Second Workshop On Ageing Methodology Of Walleye Pollock (Theragra Chalcogramma)

Ageing Methodology of Walleye Pollock.

At the "Second Annual Conference of the Parties to the Convention on the Conservation of the Pollock Resources in the Central Bering Sea " held November 5-7 , 1997, in Seattle Washington, a plan for further research on Donut Hole pollock in the Bering Sea was adopted. One of the items in this plan was that a workshop on the ageing of walleye pollock should take place from March 17-20, 1998, at the Alaska Fisheries Science Center (AFSC), in Seattle, Washington.

This age determination workshop follows approximately 7 Yz years after the Workshop on Ageing Methodology of Walleye Pollock (Theragra chalcogramma), held September 10- , 1990 in Gdynia, Poland (see AFSC Processed Report 91- , 1991). The participants at the Gdynia workshop "unanimously agreed that under our present knowledge that the break and burn method provides the best method for ageing walleye pollock." The goal of the present workshop is therefore. to standardize the otolith ageing methodology and criteria for Bering Sea walleye pollock.

Participants at the Seattle workshop were (see Appendix 1):

1. Japan:

  • Dr. Akira Nishimura, National Research Institute of Far Seas Fisheries, Shimizu Japan.
  • Mr. Keizou Yabuki, Hokkaido National Fisheries Research Institute, Hokkaido Japan.

2. Poland, Ms. Magdalena Kowalewska-Pahlke, Sea Fisheries Institute, Gdynia.

3. Russia:

  • Dr. Valery M. Pashchenko, Vladivostok Lab
  • Dr. Elena N. Kusnetsova, Moscow
  • Mr. Alexander V. Buslov, Kamchatka Lab

4. United States, Dr. Daniel K. Kimura, Ms. Betty J. Goetz, Ms. Nancy E. Roberson Mr. Charles E. Hutchinson, Alaska Fisheries Science Center.

Ageing Methodology by Nation

The workshop began with each country reviewing their ageing methodology. When otolith ageing was employed, generally the surface, or break and bum methods were used.


F or the most part, Japan does not age adult pollock from the Bering Sea. Otoliths from adults are collected dry and sent to Poland for ageing. When adult pollock are aged at the Shimizu Lab, otoliths are broken, polished and burned. Hatch date and growth in the first year is analyzed using daily growth ring methods. Fish aged 1-2 Yr are examined using frontal sections.

The Hokkaido Lab uses two age readers , both ageing 100% of the samples. Local pollock are read using sections mounted in black resin.


At the Sea Fisheries Institute, in Gdynia, only otoliths are used. About 2 000 pollock are aged yearly from the Bering and Okhotsk Sea. Ageing methods are very similar to the United States, with the exception that otoliths are stored dry, in envelopes. At the Sea Fisheries Institute there was a group of readers dealing with otoliths of different species other than walleye pollock from the Bering Sea and Okhotsk Sea. This group does not exist anymore due to the small number of otoliths available for reading, so at present there is only one age reader.


At the Vladivostok, TINRO Lab, both otoliths and scales are used. All fish are aged using otoliths and scales. Mostly scales are used, for a maximum age of 15yr. However both otoliths and scales have been collected, and are stored dry or in alcohol. Two age readers are involved in ageing approximately 3 000 pollock annually from all areas of the Bering Sea and Okhotsk Sea. Age-length keys are constructed using 5 or more fish for each 1 cm length groups, by area, sea, and year. Keys are available annually, and by decade for the 1970's, 1980's, and 1990's.

At the Kamchatka Lab, both otoliths and scales are used for age determination. There is one otolith age reader and three scale age readers. Approximately 7 000 pollock are aged mostly from the western Bering Sea and Kamchatka regions using otoliths and scales. Due to the workload, another otolith age reader is needed.

At the Russian Federal Research Institute, in Moscow, a 500 pollock sample, collected from the northwest Bering Sea was aged using scales, otoliths, and finrays. For fish up to 30 cm, otoliths and scales compare well, with finrays giving older ages. For fish up to between 30 cm and 46 cm, there is good agreement between otoliths, scales, and finrays. For fish over 46 cm, scales age low, but otoliths and finrays compare well. The conclusion is that break and bum otoliths work well for all lengths. A similar study is being carried out for the Okhotsk Sea and northern Kuril Islands.

United States

At the Alaska Fisheries Science Center approximately 12 000 pollock are aged annually from otoliths. Collection methods are similar for fishery observer samples and survey vessels. Fish sex, length and weight are recorded, and both sagitta are removed, cleaned and placed in a buffered solution of 70% ethanol. Readers are provided with cruise number, specimen number, length, weight, date of collection, and collection area. Readers are provided with the option of assigning surface ages when the surface pattern is clear. If the surface pattern is not clear, the otolith is sawed using either an Isomet low speed saw, or Tyslide diamond tool cutting machine. The cut surface is often polished with a fine grit sandpaper. Otoliths are burned over an alcohol flame, and coated with either cedar or mineral oil. The whole otolith is viewed in a water filled petri dish with a dark velvet background. Broken and burned otoliths are inserted in modeling clay. Both are viewed under a dissecting microscope, and surface lit with a fiber optic lighting. The reader assigns a raw age and a readability code, edge code and appropriate coded comments. The data is entered into a computer database and summary data, mainly size or weight at age are generated. A precision test sample is selected (20% of the sample) for independent viewing by a second age reader. The second age reader ages the test sample without knowledge of the age assigned by the first age reader.

Edge Interpretation

The interpretation of growth on the edge of the otolith is a very difficult part of standardizing ageing methods. It is the desire of the Alaska Fisheries Science Center to standardize the so called January 1 st international birthday. Under this system, all fish are aged 1 yr older following January 1 st (i. , they have a birthday). Therefore, otoliths that have nearly a year s growth in Oct.-Dec, would not have their edge counted, but the same fish collected in Jan. Feb, would have its edge counted. Each nation was asked if it used the international birthday ageing convention of January 1 st. All nations agreed that they apply the international birthday when they age Bering Sea pollock. Japan noted that in local fisheries outside the Bering Sea an April 1 st convention is used. The Alaska Fisheries Science Center reference sample described below was selected year around so that edge interpretation would be necessary.

Reported Research

Dr. Kevin Bailey, of the AFSC Bering Sea FOCI group, gave a presentation concerning stock structure and spawning times for pollock in the Bering Sea. He noted that mitochondrial DNA studies can generally distinguish between the Gulf of Alaska, the Bering Sea , and the Western Pacific. Within the Bering Sea, spawning is believed to occur from Jan. Feb in the Aleutian Basin, March-April around Unimak Pass, May-June northward, and March I- 10th in Bogoslof. However, the April-May spawning period was thought to encompass perhaps 90% of the spawning. Larvae from later spawning fish were thought to have poor prospects of survival due to the unavailability of food.

Dr. Akira Nishimura presented his paper titled "False ring observed in the otolith of age walleye pollock collected in the Bering Sea." This paper was previously presented at the Meeting of the Science Group of the Scientific and Technical Committee for the Convention on the Conservation and Management of Pollock Resources in the Central Bering Sea" held in Gdynia, Poland, September 3- , 1997. The problem posed is an important one since if false rings are counted, fish will obviously be over-aged. The author analyzed 130 age 1 walleye pollock collected in the summer of 1989. Examining the frontal section of the otoliths, 3 different growth patterns were recognized: Type A which had a single annulus, Type B, which had two marks, and Type C which had three marks. By counting daily growth rings, the author was able to conclude that the inner rings on Type Band C otoliths were checks and should not be counted as annual rings. Also, Dr. Nishimura noted that true annuli and false rings have different microstructure. Finally, Dr. Nishimura concluded that false rings could be excluded from annual ring counts on the long axis of the frontal section, by excluding rings whose long axis measured to the outer diameter was less than about 4.0 mm.

Ms. Betty Goetz noted that following a reading of Dr. Nishimura s paper, age readers at the AFSC searched for otoliths that were aged 1 or 2 Yr-old and had small first years. Generally, what AFSC age readers were calling first years, were large enough to fit Dr. Nishimura s measurement criterion for a true annulus. However, there were a few otoliths that did appear to have small first years. Consulting with colleagues at the AFSC suggested that there was a possibility that some fish identified as walleye pollock might actually be arctic cod (Boreogadus saida). Special collections of small walleye pollock and other gadid species (less than 20 cm) will be collected, and positively identified so that this problem can be evaluated. AFSC age readers noted that age 1 or 2 yr fish are usually aged from the surface where the checks are less visible. Originally, age readers at the AFSC rationalized the small size of some first years as being due to late spawning times, but this interpretation may be incorrect according to Dr. K. Bailey's presentation.

Dr. Nishimura (co-authored with T. Yanagimoto from the Shimizu Lab, Japan, and J. Janusz, from the Sea Fisheries Institute, Poland) also presented a paper titled "Ageing results of Aleutian basin pollock collected in 1993 and 1996." All age data in this report was from otoliths collected by the Japanese and aged later than 1990 in Poland, using the break and bum method. An interesting figure in this report compares size at age 9- 10 yr for fish aged in 1978 and 1987, with those collected in 1990, 1991 , 1993 , and 1996. A large increase in size at age was reported in the 1990's. These results are consistent with density dependent growth for pollock in the Donut Hole.

Mr. A. Buslov presented a paper titled "The analysis of the ages estimated from scale and otolith samples and general population parameters of walleye pollock from the western Bering Sea." The author s results show that otoliths and scales give nearly the same age for fish having otolith ages of 9 yr or less. However, scales give statistically significant younger ages for fish aged with otoliths at 11yr and older. This is consistent with previous studies. The author concluded that since growth parameters are not strongly affected by the method of ageing, "that general insights to the size-age structure and the abundance of mentioned population wouldn't be extensively transformed when changing the method of age estimation." However, Dr. D. Kimura noted that the implied natural mortality rate from using otoliths rather than scales would suggest a much more conservative harvest strategy.

Dr. V. Paschenko reported on "The morphological characteristics of otoliths from the Bering Sea and the Okhotsk Sea." Bering Sea samples were available from 1996- , and Okhotsk Sea samples were available from 1996. Otolith mass, width, and length were significantly greater for a given length fish for Okhotsk Sea pollock, compared with Bering Sea pollock. F or the Bering Sea, the relationship between fish length and otolith dimensions was highly consistent between 1996 and 1997. Small samples of fish size and otolith measurements is all that is needed to distinguish samples of fish from the two seas.

Otolith Reference Samples

Representatives from different nations brought samples they wished to have examined or aged at the workshop.


  • Dr. A. Nishimura brought 10-20 age 1 yr frontal sections, and 20-30 age 1 yr whole otoliths. These were from the eastern Bering Sea.
  • Mr. K. Yabuki brought 48 transverse cut otolith halves mounted in resin. Some of these were burned whole before mounting in resin.


  • Ms. M. Kowalewska-Pahlke brought 10 otolith samples from the Cape Navarin shelf, and 10 otolith samples from the Okhotsk Sea collected from August to October, 1997.


  • Mr. A. Buslov brought 100 otolith samples, collected in October, 1995- , from the western Bering Sea.

United States

The Alaska Fisheries Science Center reference sample was selected from 1996 survey and fishery samples from the Bering Sea. The reference sample attempted to cover the complete age range, and three time periods from throughout the year (January-February (20), June-July (20), September-November (27)). This collection was designed so that the age readers must take into account the time of collection, so that fish age can be assigned according to the January 1 st birthday.


The samples from Dr. A. Nishimura allowed a viewing of the materials used in his paper on false rings. He described how daily rings were evaluated to distinguish false rings from true annuli. Twenty four whole otoliths were hydrated in order to examine growth patterns for false rings. All showed clear first years. The otoliths mounted in resin by Mr. Yabuki for Hokkaido pollock were very fine preparations that showed the usefulness of black resin for mounting whole otoliths. His method of burning whole otolith before mounting in resin drew considerable interest. However, the preferred preparation now is not to burn the otoliths, because burning can cause brittleness and breakage.

Readers from all nations aged the reference samples, using equipment supplied by the AFSC, and interpreting edge growth using the January 1 st international birthday.

The samples brought by Ms. M. Kowalewska-Pahlke were chosen for their difficulty. This is evident in the variability of ages provided from age readers (Table 1).

Due to time constraints, only half of the sample supplied by Mr. A. Buslov was aged at the workshop. Although the results from ageing this sample appeared to be generally good, several of the specimens appeared to be difficult, resulting in a considerable range of ages (Table 2).

The samples supplied by the AFSC, were from winter, summer, and fall, and were designed to test the ability of age readers to interpret edge growth when assigning a final age (Table 3). Generally, the results from reading this sample appears to be quite good.


Generally, the results from ageing reference samples from this workshop did not differ greatly from that of the first workshop held in Gdynia, September, 1990. In both workshops, age readers from distant labs, and varying experience levels ageing walleye pollock otoliths, were able to age reference samples and arrive at quite similar ages. indicated from the Polish sample, when specimens are difficult to age, there will be considerable variability in the assigned ages. However, ages will become more similar with increased experience levels, and the ability of age readers to work together under a double scope to compare their ages.

Although differences in ageing criteria can be uncovered by mailing exchanges of reference samples, the resolution of discrepancies can be difficult without the age readers traveling to the same geographic location for face to face discussions.

At the first workshop on ageing walleye pollock, held in Gdynia, two research issues were described for the ageing of walleye pollock: identifying the first annulus, and validating break and bum ages. To a significant extent, the paper by Dr. Nishimura addresses identifying the first year in walleye pollock. He validates the first year using daily growth techniques, and documents how counting checks can be avoided. A brief examination of AFSC age data indicates that miss-identifying the first year does not appear to be a significant problem. This may be due to the practice at the AFSC to age 2 yr olds from the surface, or break and burns, where checks are less visible. Older fish also need to be examined to further assure that checks are not being counted. Also, there may have been some miss-identification of another cod species for juvenile pollock in the AFSC collections.

Further Research

  • As a follow up to the present meeting, the AFSC asked Dr. Nishimura to provide samples of frontal sections of 1 yr old walleye pollock showing "false rings " along with the corresponding whole otolith pair. Just a few samples of this type might give us more insight into whether the AFSC is counting false rings. This verification would assume that the same checky pattern appears on both otoliths.
  • The break and burn method has yet to be fully validated for pollock. One of the difficulties for age validation is that pollock have poor survivability following tagging.

Table 1. Results from age readers reading the Polish sample collected from Cape Navarin and the Sea of Okhotsk (August-October). Sample was selected to be difficult. Readers are identified by initials, Japan (A.N., K.Y.); Poland (M.P.); Russia (V.P., E.K., A.B.); and the U.S. (B.G., N.R., C.H.).

Age and Growth Table Information

Table 2. Results from ageing the Russian sample from the western Bering Sea, October 1997. Readers are identified by initials, Japan (A.N., K.Y.); Poland (M.P.); Russia (V.P., E.K., A.B.); and the U.S. (B.G., N.R., C.H.).

Age and Growth Table Information
Age and Growth Table Information

Table 3. Results from ageing the reference sample prepared by the AFSC. Collections were from three time periods of the year (Jan. Feb), (June-July), and (Sept.-Nov.), so that age readers made an interpretation of age according to the January 1 st international birthday. Readers are identified by initials, Japan (A. , K.); Poland (M.); Russia (V., E., A.); and the U. S. (B., N., C. H.).

Age and Growth Table Information
Age and Growth Table Information

Appendix 1

Second Workshop on Ageing Methodology of Walleye Pollock

Held in Seattle, WA, U. , March 17- 1997n Seattle, WA, U. , March 17- 1997