12. POPULATION DYNAMICS
Because of the shortage of data essential for the comprehensive analysis of Pacific bigeye tuna, studies done on this subject so far are almost exclusively based on the Japanese longline data.
Table 9. Annual catch (MT) of Pacific bigeye tuna by country.
Year | Bermuda | Cuba | Ecuador | Fiji | Japan | Korea | Other Nei A (Taiwan) | Other Nei B | Solomon Islands | Venezuela | Pacific Total | |||
Panama | Tonga | USA | ||||||||||||
1965 | 66,200 | 700 | 2,000 | 68,900 | ||||||||||
1966 | 70,700 | 2,900 | 3,500 | 77,100 | ||||||||||
1967 | 75,200 | 3,200 | 3,200 | 81,600 | ||||||||||
1968 | 62,400 | 600 | 4,000 | 67,000 | ||||||||||
1969 | 72,600 | 2,500 | 4,600 | 79,700 | ||||||||||
1970 | 71,000 | 5,000 | 0 | 76,000 | ||||||||||
1971 | 100 | 57,900 | 4,700 | 4,300 | 800 | 67,800 | ||||||||
1972 | 77,200 | 7,800 | 3,800 | 100 | 88,900 | |||||||||
1973 | 76,300 | 8,900 | 3,700 | 400 | 89,300 | |||||||||
1974 | 70,392 | 14,444 | 4,420 | 132 | 89,388 | |||||||||
1975 | 85 | 81,170 | 15,484 | 5,348 | 330 | 102,417 | ||||||||
1976 | 0 | 101,040 | 21,395 | 3,078 | 1,039 | 126,552 | ||||||||
1977 | 307 | 120,929 | 17,663 | 4,507 | 581 | 143,987 | ||||||||
1978 | 0 | 104,640 | 8,456 | 4,402 | 423 | 117,921 | ||||||||
1979 | 0 | 107,389 | 12,804 | 4,491 | 1,331 | 126,015 | ||||||||
1980 | 0 | 99,692 | 13,975 | 4,637 | 1,465 | 3,196 | 122,965 | |||||||
1981 | 0 | 83,721 | 10,608 | 3,849 | 770 | 40 | 2,113 | 101,101 | ||||||
1982 | 0 | 1,100 | 8 | 94,113 | 10,050 | 2,111 | 177 | 23 | 1,207 | 108,789 | ||||
1983 | 0 | 1,249 | 14 | 97,224 | 7,706 | 3,477 | 34 | 726 | 400 | 110,830 | ||||
1984 | 0 | 1,814 | 16 | 88,867 | 7,478 | 2,943 | 55 | 696 | 1680 | 103,549 | ||||
1985 | 0 | 2,410 | 133 | 106,486 | 10,898 | 3,031 | 46 | 62 | 820 | 123,886 | ||||
1986 | 0 | 1,116 | 94 | 125,570 | 15,927 | 2,879 | 0 | 101 | 1120 | 146,807 | ||||
1987 | 0 | 240 | 49 | 125,816 | 19,544 | 3,280 | 130 | 259 | 14 | 867 | 260 | 150,459 | ||
1988 | 0 | 240 | 27 | 87,959 | 13,681 | 3,610 | 130 | 1,085 | 7 | 1,956 | 260 | 108,955 |
Data source : FAO (1965–1988).
Table 10. Annual catch (MT) of Pacific bigeye tuna by fishing gear, 1955–1988. LL-longline, BB-baitboat, PS-purse seine.
Longline | Surface fishery | Grand Total | % of Japan LL | ||||||||||
Year | Japan | Korea | Taiwan | Sub-Total | % of Japan LL | Japan | Solomon PS+BB | IATTC PS+BB | Sub-Total | ||||
BB | Trop. PS | Others | |||||||||||
1955 | 39,200 | 800 | 40,000 | 98 | 4,009 | 342 | 117 | 4,468 | 44,468 | 88 | |||
1956 | 30,700 | 900 | 31,600 | 97 | 4,373 | 957 | 40 | 5,370 | 36,970 | 83 | |||
1957 | 64,400 | 900 | 65,300 | 99 | 5,198 | 435 | 68 | 5,701 | 71,001 | 91 | |||
1958 | 86,500 | 1,000 | 87,500 | 99 | 4,196 | 114 | 232 | 4,542 | 92,042 | 94 | |||
1959 | 79,300 | 800 | 80,100 | 99 | 1,729 | 74 | 150 | 1,953 | 82,053 | 97 | |||
1960 | 87,600 | 700 | 88,300 | 99 | 1,524 | 152 | 183 | 1,859 | 90,159 | 97 | |||
1961 | 132,200 | 1,500 | 133,700 | 99 | 1,837 | 111 | 213 | 2,161 | 135,861 | 97 | |||
1962 | 119,800 | 3,400 | 123,200 | 97 | 824 | 213 | 328 | 1,365 | 124,565 | 96 | |||
1963 | 144,400 | 3,600 | 148,000 | 98 | 1,822 | 39 | 75 | 1,936 | 149,936 | 96 | |||
1964 | 99,500 | 3,500 | 103,000 | 97 | 1,142 | 260 | 68 | 1,470 | 104,470 | 95 | |||
1965 | 73,500 | 700 | 2,000 | 76,200 | 96 | 1,254 | 231 | 118 | 1,603 | 77,803 | 94 | ||
1966 | 76,900 | 2,900 | 3,500 | 83,300 | 92 | 1,108 | 96 | 267 | 1,471 | 84,771 | 91 | ||
1967 | 77,700 | 3,200 | 3,200 | 84,100 | 92 | 2,803 | 314 | 1,663 | 4,780 | 88,880 | 87 | ||
1968 | 63,939 | 600 | 4,000 | 68,539 | 93 | 2,272 | 250 | 2,559 | 5,081 | 73,620 | 87 | ||
1969 | 91,885 | 2,500 | 4,600 | 98,985 | 93 | 1,679 | 158 | 576 | 2,413 | 101,398 | 91 | ||
1970 | 71,165 | 5,000 | 76,165 | 93 | 1,579 | 247 | 1,332 | 3,158 | 79,323 | 90 | |||
1971 | 65,059 | 4,700 | 4,300 | 74,059 | 88 | 931 | 218 | 2,567 | 3,716 | 77,775 | 84 | ||
1972 | 82,632 | 7,800 | 3,800 | 94,232 | 88 | 2,364 | 781 | 2,238 | 5,383 | 99,615 | 83 | ||
1973 | 90,313 | 8,900 | 3,700 | 102,913 | 88 | 852 | 251 | 1,978 | 3,081 | 105,994 | 85 | ||
1974 | 68,730 | 14,444 | 4,420 | 87,594 | 78 | 729 | 456 | 889 | 2,074 | 89,668 | 77 | ||
1975 | 76,913 | 15,484 | 5,348 | 97,745 | 79 | 3,522 | 743 | 3,722 | 7,987 | 105,732 | 73 | ||
1976 | 96,816 | 21,395 | 3,078 | 121,289 | 80 | 7,982 | 889 | 10,185 | 19,056 | 140,345 | 69 | ||
1977 | 115,833 | 17,663 | 4,507 | 138,003 | 84 | 5,096 | 970 | 7,054 | 13,120 | 151,123 | 77 | ||
1978 | 100,557 | 8,456 | 4,402 | 113,415 | 89 | 3,330 | 1,987 | 11,710 | 17,027 | 130,442 | 77 | ||
1979 | 104,776 | 12,804 | 4,491 | 122,071 | 86 | 1,967 | 1,239 | 7,530 | 10,736 | 132,807 | 79 | ||
1980 | 96,637 | 13,975 | 4,637 | 115,249 | 84 | 2,205 | 1,021 | 15,417 | 18,643 | 133,892 | 72 | ||
1981 | 78,630 | 10,608 | 3,849 | 93,087 | 84 | 2,357 | 1,733 | 40 | 10,089 | 14,219 | 107,306 | 73 | |
1982 | 87,571 | 10,050 | 2,111 | 99,732 | 88 | 4,057 | 2,516 | 23 | 4,103 | 10,699 | 110,431 | 79 | |
1983 | 91,200 | 7,706 | 3,477 | 102,383 | 89 | 3,847 | 1,559 | 645 | 34 | 3,260 | 9,345 | 111,728 | 82 |
1984 | 83,504 | 7,478 | 2,943 | 93,925 | 89 | 3,447 | 884 | 745 | 55 | 5,853 | 10,984 | 104,909 | 80 |
1985 | 104,208 | 10,898 | 3,031 | 118,137 | 88 | 2,895 | 1,163 | 1,382 | 46 | 4,531 | 10,017 | 128,154 | 81 |
1986 | 123,103 | 15,927 | 2,879 | 141,909 | 87 | 2,227 | 1,672 | 1,009 | 0 | 1,979 | 6,887 | 148,796 | 83 |
1987 | 121,386 | 19,544 | 3,280 | 144,210 | 84 | 1,834 | 1,765 | 1,002 | 259 | 771 | 5,631 | 149,841 | 81 |
1988 | 94,666 | 13,681 | 3,610 | 111,957 | 85 | 2,900 | 1,044 | 953 | 1,085 | 1,053 | 7,035 | 118,992 | 80 |
1989 | 103,326 | 2,472 | 1,741 | 1,317 | 1,807 |
Data source :
Japan LL >20 GT for 1955–1973 : Kume (1979b) and FAO (1974–1988).
Japan LL <20 GT for all years : MAFFJ (1955–1990).
Japan surface catch : MAFFJ (1955–1990).
Korea LL : FAO (1965–1988). All are assumed by LL.
Taiwan LL : FAO (1965–1988). other nei A. All are assumed by LL. Before 1965 data are taken from Kume (1979b).
Solomon : FAO (1981–1988).
IATTC : IATTC (1989).
12.1 Production Model Analysis
It may not be appropriate to apply the method of production model analysis if the assumptions that (a) the rate of natural increase of the stock responds immediately to changes in population density, and (b) the rate of natural increase of the stock at a given level of biomass is independent of the age (or size) composition of the stock, are not satisfied. In the case of Pacific bigeye tuna it is considered that it takes 4 years for the fish to recruit into the longline fishery (Suda, 1970b) so that the first assumption is not matched satisfactorily. Furthermore, the second assumption also appears seldom satisfied for long-lived species such as tunas. On the other hand, the fact that the fishing condition and the fishery have been stable, and that the changes in the age (or size) composition of the catch seem to be smaller than for other tunas, may provide a basis for the application of this method. Irrespective of the above-mentioned drawbacks, production model analysis might give a general picture of stock status, MSY, etc., for the species under consideration, especially since there is a lack of complete and detailed data for more sophisticated analysis such as cohort analysis.
Fig. 14. Relationship between estimated Japanese longline catch and effort for bigeye tuna in the eastern tropical Pacific for 1957–1980. The fine lines and figures in two digits denote hook rates in numbers of fish per 100 hooks and year, respectively. After Miyabe and Bayliff (1987).
Production model analysis requires catch and fishing effort or CPUE. Since complete fishing effort data are not available, Kume (1979b) and Miyabe (1989; 1991) used the Japanese longline effort as basic data. The effective fishing effort, standardized by the Honma method (Honma 1974) of the Japanese longline fishery, was raised to the total effective effort using the proportion of the Japanese longline catch relative to the total catch. As stated above, the proportion of the Japanese longline catch has been very high (70–90%). The programme “PRODFIT” of Fox (1975) is used, applying the number of year classes that contributes significantly in the catch set at four. The results are shown in Figure 17 and Table 11. Kume (1979b) estimated the MSY between 100,000–106,000 mt with best fit at shape parameter m = 0.0. Miyabe (1991) estimated the MSY between 130,000–167,000 mt with best fit at m = 0.0. Although the shape of the production curve is not known, and the current level of fishing effort is the highest recorded to date, the fishing effort does not appear to exceed considerably the level that gives the MSY.
Fig. 15. Annual trend in the Japanese longline CPUE standardized by Honma method. After Miyabe (1991).
12.2 Virtual Population Analysis (VPA)
Kume (1979b) estimated the catch-at-age for fish caught by the Japanese longline fishery for 1957–1975 using the length-frequency samples and the growth equation by Suda and Kume (1967). Then, assuming that the Taiwanese and Korean longline fleets caught the same size of fish in a given area, catch-at-age was prorated to include the catch by Taiwan and Korea. Minimum stock size analysis (Honma 1978), which is one of the variety of VPA analyses, was applied in order to estimate the recruitment. Natural mortality rate (0.361) was employed from Suda and Kume (1967). The estimated recruitment (age 1 fish) was about 9 millions for 1956–57 cohorts and 6 to 6.5 millions for 1964–66 cohorts. The recruitment at age 1 was also estimated assuming the constant recruitment number using the relationship between fishing effort and reciprocal of CPUE (Suda 1970a). The estimate was 7.4 million and very close to Kume's (1979b) results.
Similarly, Miyabe (1989) constructed the catch-at-age for 1965–1987 but solely for the fish caught by the Japanese longline fishery. Miyabe (1989) tuned the VPA with standardized CPUE from the Japanese longline fishery in a way described by Parrack (1986). A value of 0.4 was used for M. The objective function to be minimized is:
SSQ = ∑ (CPUEcal - CPUEobs)2
where SSQ = sum of squares, CPUEcal = calculated CPUE by VPA, and CPUEobs = observed CPUE. CPUEcal can be calculated by regressing population number (N) from the VPA to observed CPUE applying the equation CPUEcal = q.N. Here q is catchability coefficient.
The estimated population number at age 1 ranges between 11 to 13 million with smaller fluctuations (10–20%) among years. This is similar to the findings of Kume (1979b) although the level of recruitment is different.
It should be noted that data, in particular length samples, are often less than the desired level, and the assumptions used may not be appropriate since they cannot be proved practically. Because of this, the results should be interpreted with caution.
Fig. 16. Changes in CPUE by age of bigeye tuna in the equatorial Pacific, shown by 5-year intervals. Numbers in the panels are total CPUE. After Kume (1979a).
Fig. 17. Annual catch against annual fishing effort and estimated production curves for Pacific bigeye tuna. After Miyabe (1991).
Table 11. Results of production model analysis on Pacific bigeye tuna. After Miyabe (1991).
m (shape parameter) | MSY (1,000 MT) | Fopt (million hooks) |
0.0 | 167 | ∞ |
1.001 | 130 | 500 |
2.0 | 130 | 550 |
12.3 Yield-per-Recruit (Y/R) Analysis
Suda (1970b) presented the results of Y/R analysis incorporating the Ricker-type stock-recruitment relationship, which was estimated from the spawning potential. Effective fishing effort and M (0.361) were taken from Suda and Kume (1967) for 1957 1964. The estimated equilibrium yield curve is shown in Figure 18. The MSY is about 90,000 mt when F is 0.5–0.6 assuming the knife-edge-type recruitment at age 4. He also presented the results of several combinations of input parameters, such as recruitment age and F.
Thompson-and-Bell-type Y/R analysis was done by Miyabe (1991). The inputs are M, weight-at-age and selectivity-at-age. Ages 1 through 7 are included in the calculation. Selectivity-at-age in the most recent year was estimated by the Pope and Shepherd's (1982) separable VPA applying the recent catch-at-age for the Japanese longline catch. The estimated Y/R, which is shown in Figure 19, increases to about 8 kg as F becomes larger up until approximately 0.8 and then levels off thereafter. Judging from the current information on the average size of bigeye (40–45 kg) caught by longline gear, it appears F is in moderate range (0.2–0.4) for fully-recruited ages.
Fig. 18. The estimated equilibrium curve for Pacific bigeye tuna. After Suda and Kume 1967.
Fig. 19. Thompson and Bell type Y/R curve estimated for the Pacific bigeye tuna. After Miyabe (1991).
13. INTERACTIONS
Among tunas, bigeye seems to be one of the species with the lowest level of gear interaction. There are several fishing gears which harvest bigeye tuna, such as longline, purse seine, baitboat and other miscellaneous gears (trolling, hand-lining, ring net, gill net, etc.). In Table 10, the catch was shown divided into surface and longline catch, which consists of smaller to medium and medium to large fish, respectively. It indicated that the longline catch accounted for more than 85% of the total catch. This means there is less within-generation interaction between fisheries. In addition to this, the major distributional pattern of catch by two gear categories differs geographically. The greater catch occurs in coastal or island areas for the surface fishery but in high seas for the longline fishery.
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