(from "Report of the Scientific Committee")

Agenda

The revised agenda adopted for the meeting was as follows:

1. Re-evaluation of information on different stocks.
2. Review of age-length data and keys.
   (a) Pre-1962.
   (b) Post-1962.
   
3. Estimates of natural mortality rate (M).
4. Review and analysis of information on reproduction and recruitment.
   (a) Age of recruitment and maturity rate of pregnancy.
   (b) Reproduction and recruitment curves.
5. Population estimates based on catch and catch-per-unit-of-effort data.
   (a) Review of effort and catch-per-unit-of-effort.
   (b) Estimates from age-length keys.
   (c) DeLury-type estimates.
   (d) Estimates based on catchability (q) methods.
   (e) Least squares estimates.
   
6. Review and analysis of marking data.
7. Review and analysis of sighting data.
8. Population level to obtain maximum sustainable yield, and maximum sustainable yield.
9. Synthesis of results by several methods, including evaluation of comparative results both between areas and between species.
10. Other items relating to fin whale analysis.
11. Preparation of a report.

Acknowledgements

Particular thanks are due to Mr. R. Shomura, Director, and the secretarial and computing staff of the Bureau of Commercial Fisheries for their hospitality and the provision of the excellent facilities for the meeting, which did much to assist our work.

1. Stock units

The group reviewed information in F/1 on whale movements as indicated by mark recoveries and in F/7 on comparison of growth of fin whales in the several areas. The marking data show, as it has previously, there is a small degree of intermingling between adjacent areas but almost no evidence of large east-west movements. The average body length of whales 25 years and over is greatest in Area II and least in Area VI with a gradation between. Whales of Area I are intermediate in size between those of Areas II and VI.

The meeting also heard from Ivashin (F/13) suggesting a possible classification of different fin whale stocks in the southern oceans. The proposals of Laws (1960) were also recalled. The meeting agreed that the national groups should consider other subdivisions in their future analyses but for the present the traditional subdivisions into Areas I to VI are convenient.

2. Age-Length Keys

Ohsumi made available keys (F/10a and b) by sex and area for pre-1962 and post-1962. It was agreed that comparisons of these keys should be made. Allen indicated that he has used the Japanese key combining pre- and post-1962 data for the whole Antarctic and the Committee of Three key, which was developed from pre-1962 data and found no major differences in resulting recruitment and natural mortality rates. He also used a key for separate areas and with this did find some greater differences. As suggested in F/7, one age key should not be used to estimate the age composition of all whales but an age-length key should be made at least for each area and by sex. Ohsumi emphasized that there are evidences of changes in age and body length at sexual maturity and it was agreed that the present keys should be applied with caution to length data of earlier years. It was suggested that the age-length data based on ovulation counts made available to the Committee of Three should be reviewed and if found reasonable applied to early length data. These data are still retained in Seattle by Chapman.

3. Natural Mortality Rate

The meeting reviewed the estimates of natural mortality coefficient found in F/3 and IWC/21/Sc/18. These estimates are as follows:

It was pointed out that the earplugs of older females are more difficult to read and this may explain part or all of this apparent difference between the sexes. It was agreed that the value of M equal to 0.04 is the best present estimate of the average natural mortality coefficient. However, members should study further the possibility that the indicated difference between the sexes is real. The possibilities of changes of M both with age and as a result of changes in stock condition must also be given further study. In this connection some tentative considerations are reported in F/8.

4. Age of Recruitment

There is substantial agreement on the median age of recruitment (i.e., the age at which 50 per cent of the year-class is recruited). Allen presented data used in his calculations from which the median age of recruitment is estimated at 4.7 years. Shimadzu states that the estimate from his data is about 5 years. Both of these estimates are derived using the age-length key calculated for the period 1957 to the present but applied to length data for all years since 1932. The increase in the proportion of younger animals in the catch in recent years would suggest a reduction in the age of recruitment (F/8).

5. Age at Sexual Maturity

The analysis of F/2 suggests that the age of sexual maturity in the early 1960's was 6 to 7 years in both sexes. This estimate is lower than the estimate given in IWC/21/Sc/18 which was 10 years. Ohsumi presented data showing a decline in the age at sexual maturity over the last 12 years. Additional data are needed, subdivided by areas. While the evidence is unclear, the age at sexual maturity is probably over 10 in unexploited stocks and may decrease to 6 or 7 in heavily exploited stocks.

6. Pregnancy Rates

Data on pregnancy rates are found in F/8 revised from IWC/21/Sc/18. This shows an increase in the pregnancy rate from around 0.30 in pre-war years to around 0.40 in the 1950's and early 1960's. The meeting also reviewed the possible range of pregnancy rates based on ovulation rates. These suggest a range from a low of 0.33 to a high of 0.50.

7. Reproduction and Recruitment Curves

No data on reproduction curves were presented but recruitment curves were available in IWC/21/Sc/18, F/3 and F/8 and some discussion on recruitment rates in F/4. There was considerable discussion of these recruitment curves; in particular Allen suggested that of his several alternatives he believes that the one given in Table 8 of F/3 is based on the most suitable data for this method. This suggests a net recruitment rate based on the total exploited stock of about 0.035 in the early 1950's and lower rates more recently that are difficult to explain, but may be due to increased selection of larger whales under the present lower quotas. He also explained some improvements that he had developed in his method of calculating recruitment rates. On the other hand Japanese scientists emphasized methods given in IWC/21/Sc/18 which they consider shows sustainable yield rate (corresponding to the net rate of recruitment) equal to 0.032 to 0.047 in early 1950's and 0.057 to 0.064 in the later 1960's. The recent decrease in the age at sexual maturity reported by Ohsumi might lead to increase in the recruitment rate. This needs further study and should be reviewed at the next meeting of the Scientific Committee.

8. Population Estimates Based on Catch and Catch-per-Unit-of-Effort Data

a. Effort and catch-per-unit-of-effort
Ohsumi reviewed his paper F/9 which notes many problems in measuring effort and catch-per-unit-of-effort. It was agreed that these problems deserve serious study and certainly catch-per-unit-of-effort is not a valid index of abundance under all conditions. However, catch-per-unit-of-effort is an important piece of information and must be considered along with other data. Catch-per-unit-of-effort data for fin whales should be least biased for the period from 1953/54 to 1961/62 when fin whales constituted 80 per cent of the total annual baleen whale catch.

b. Estimates from age composition derived from age-length keys
No new estimates of this type were presented. Two such estimates were given in IWC/21/Sc/4 and IWC/21/Sc/18.

c. DeLury-type estimates
Further estimates of this type were available in F/4. To use these estimates it is necessary to have an independent estimate of the net recruitment rate. The value of net recruitment rate used was 0.03 but other alternatives were presented.

d. Estimates based on the catchability coefficient (q) method
No new estimates of this type were available but the estimates in IWC/21/Sc/18 were reviewed.

e. Least squares estimates
New and revised estimates by the method of least squares were presented in F/3 and F/5.

f. Estimates based on recruitment curve
Estimates based on a recruitment curve and iteration method made in IWC/21/Sc/18 were reviewed.

9. Marking Data

Some slight additional analyses were presented in F/4; in general these support the other methods of analysis but because of the variability of the data cannot be considered to be very useful.

10. Sighting Data

In the paper of Doi (F/6) a theoretical model is developed to convert sighting data to estimates of absolute abundance. This is a valuable contribution but more work remains to be done on checking the validity of the model and in obtaining observations to estimate parameters in the model.

11. Maximum Sustainable Yield

Allen estimated in F/3 that the maximum sustainable yield would be 10,000 to 12,000 at an exploited stock level of 250,000 to 300,000. Shimadzu referred to calculations in IWC/21/Sc/18 which showed the maximum sustainable yield is about 9,000-10,000 whales at a stock level of 220,000. The estimates of maximum sustainable yield are close, but there is some difference as to the stock level at which these might be obtained. The present stock level is much below that at which the maximum sustainable yield could be obtained.

12. Synthesis of Different Estimates

It is agreed that the CPUE is least biased by extraneous factors for the period 1954 to 1962 because throughout this period 80 per cent of the catch was in fin whales. Also the estimates by several different methods agree reasonably closely through much of this period. In view of the great effect that the recruitment rate has on estimates both of the present stock size and of the present sustainable yield it is clearly very important that more precise estimates of the present net recruitment rate be obtained as soon as possible. The several estimates are given in Table 1.

To extend these estimates forward it is necessary to use information on the rate of recruitment. As pointed out in the section on reproduction and recruitment curves, the rate of net recruitment (based on the total exploited stock) is calculated by Allen to be 0.035 in the early 1950's decreasing to 0.02 or less in recent years; on the other hand the Japanese scientists assumed reproduction curves with rates of net recruitment increasing from 0.04 to 0.06 in the 1950's and 1960's. If the average figures given in Table 1 are accepted, they imply a net recruitment rate of 0.04 approximately during the period 1958 to 1962. The following tables show calculations of present stock level and sustainable yield using these three sets of values for r - M, the net recruitment rate. More specifically the calculations detailed in Table 2 use the following parameter values in addition to r; M = 0.04; age of 50% recruitment = 5; 1957 population level 187 thousand, 1962 population 100 thousand.

TABLE 1
Estimates of Total Exploitable Population Size of Fin Whales (thousands)

	1958	1962
Least squares (F/3, Table 8)	171	93
Modified DeLury (F/4 recalculated with r - M = 0.04)	176	108
q (IWC/21/Sc/18)	194	110
Reproduction curve (IWC/21/Sc/18) (Average of Cases 2 and 4)	146	91
Average	171.8	100.5

TABLE 2

Year	Population size at beginning of season	Catch	Survival from catch and natural mortality	Recruitment	Population size at beginning of next season
(All figures in thousands) (a) Calculation of 1970 Fin Whale Population (r - M = 0.02)*1
1962	100.0	26.4	70.7	11.2	81.9
1963	81.9	18.6	60.8	10.3	71.1
1964	71.1	13.6	55.2	9.2	64.4
1965	64.4	7.3	54.8	8.2	63.0
1966	63.0	2.3	58.3	7.1	65.4
1967	65.4	2.9	62.5	6.0	68.5
1968	68.5	2.1	63.7	4.9	68.6
1969	68.6	3.0	63.0	4.3	67.3
1970	67.3
(b) Calculation of 1970 Fin Whale Population (r - M = 0.04)*2
1962	100.0	26.4	70.7	15.0	85.7
1963	85.7	18.6	64.4	13.6	78.0
1964	78.0	13.6	61.8	12.2	74.0
1965	74.0	7.3	64.0	10.8	74.8
1966	74.8	2.3	69.6	9.4	79.0
1967	79.0	2.9	73.1	8.0	81.1
1968	81.1	2.1	75.8	6.9	82.7
1969	82.7	3.0	76.5	6.2	82.7
1970	82.7
(c) Calculation of 1970 Fin Whale Population (r - M = 0.049 in 1962 increasing to 0.057 in 1970)*3
1962	100.0	26.4	70.7	16.9	87.6
1963	87.6	18.6	66.2	15.5	81.7
1964	81.7	13.6	65.4	14.0	79.4
1965	79.4	7.3	69.2	12.5	81.7
1966	81.7	2.3	76.2	11.0	87.2
1967	87.2	2.9	80.9	9.4	90.3
1968	90.3	2.1	84.7	8.3	93.0
1969	93.0	3.0	86.4	7.8	94.2
1970	94.2

*1 On the basis of these calculations, with r - M = 0.02 the present sustainable yield is about 1.3 thousand (67.3 thousand x 0.02).
*2 On the basis of these calculations, with r - M = 0.04 the present sustainable yield is about 3.3 thousand (8.27 thousand x 0.04).
*3 On the basis of these calculations, with r - M = 0.057 the present sustainable yield is 5.4 thousand (94.2 thousand x 0.057).

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