A REVIEW OF THE BIOLOGY AND FISHERIES FOR NORTHERN BLUEFIN TUNA, THUNNUS THYNNUS, IN THE PACIFIC OCEAN (contd.)

11.2 Western Pacific Ocean

11.2.1 Annual catches

The total annual catches of northern bluefin in the western Pacific Ocean by vessels of Japan, the Republic of China, and the Republic of Korea are shown in Table 6. The data for Japan are from the National Research Institute of Far Seas Fisheries of Japan. They include small amounts of bluefin caught in the central and eastern Pacific Ocean. The data for 1951–1965 include only large fish (greater than about 15 kg or 33 pounds), while those for 1966–1992 include both small and large fish. The data for the small fish were estimated from the catches of meji (small bluefin, yellowfin, and bigeye) by a proration process based upon the catches of large bluefin, yellowfin, and bigeye. The data for the Republic of China are from the yearbooks of fisheries statistics of the Taiwan Fisheries Bureau and those for the Republic of Korea are from the yearbooks of fisheries statistics of the Food and Agriculture Organization of the United Nations. Data on the Japanese catches by gear are shown in Table 9.

11.2.2 Monthly catches

No information on this subject, other than that given in Section 10.2, is available.

11.2.3 Effort and catch per unit of effort

Except for data on the numbers of Japanese purse-seine vessels and traps (Skillman and Shingu, 1980), no information on this subject is available.

FIGURE 8. Total catch, logged fishing effort, and logged catch per unit of fishing effort for northern bluefin in the eastern Pacific Ocean (after Bayliff, 1993b).

TABLE 9. Catches, in metric tons, of bluefin in the Pacific Ocean by Japanese vessels (after Ishizuka, 1989).

	Small fish (less than about 15 kg)								Large fish (more than about 15 kg)								Grand total
	Purse seine	Long-line	Bait-boat	Gillnet	Troll	Trap	Other	Total	Purse seine	Long-long	Bait-boat	Gillnet	Troll	Trap	Other	Total	Grand total
1966	315	107	0	37	1028	493	23	2004	8517	1311	437	2	392	803	20	11482	13486
1967	1380	104	0	10	1742	1468	193	4897	4742	1193	1011	29	1016	937	67	8995	13892
1968	2705	134	0	3	1341	1810	7	6000	6178	1283	850	2	947	1300	31	10591	16591
1969	202	103	0	13	1261	1237	4	2820	2504	1005	377	5	1285	1015	11	6202	9022
1970	556	62	0	0	1545	684	10	2856	1812	800	378	0	813	1046	10	4859	7715
1971	897	48	0	15	2826	684	12	4482	2665	643	740	1	579	934	2	5564	10046
1972	366	96	0	131	1217	565	9	2383	3445	637	472	3	331	559	1	5448	7831
1973	1032	179	0	160	2928	1251	28	5579	1618	1077	180	115	903	1083	7	4983	10562
1974	927	695	0	165	2439	3955	127	8309	3012	3448	929	70	963	2207	13	10642	18951
1975	1321	241	0	195	1137	1346	28	4267	3505	1219	659	68	307	1138	11	6907	11174
1976	139	115	0	481	860	1185	0	2780	1785	391	565	352	308	2027	0	5428	8208
1977	746	111	0	399	1741	858	12	3868	3751	483	1971	402	469	1475	8	8559	12427
1978	559	139	0	551	3915	1453	40	6657	8526	753	986	1344	752	1308	15	13684	20341
1979	483	313	0	564	2028	2742	35	6164	11854	808	1144	756	672	2084	24	17342	23506
1980	1886	318	0	668	1735	1280	29	5918	9205	838	1279	909	494	1464	42	14231	20149
1981	1089	118	0	970	1757	970	8	4912	21476	619	675	772	363	1233	7	25145	30057
1982	555	143	0	240	834	552	2	2326	17272	756	1710	1337	177	1147	1	22400	24726
1983	1227	84	0	28	1040	427	0	2806	12388	286	320	786	146	546	0	14472	17278
1984	384	77	0	78	1340	1642	25	3546	4021	162	456	430	295	846	2	6212	9758
1985	1522	57	0	228	1332	1318	0	4457	3302	324	1700	332	324	1380	0	7362	11819
1986	1341	21	0	128	808	1911	0	4209	6367	142	1022	141	182	1120	0	8974	13183

11.3 Central Pacific Ocean

The catch data in Tables 6 and 9 include catches made by Japanese vessels in the central Pacific. No further information on this subject, other than that given in Section 10.3, is available.

12. POPULATION DYNAMICS

12.1 Predator-prey Relationships

Doi (1960) used data on the catches of northern bluefin and of squid and five species of smaller fishes as indices of their abundance, and then applied Volterra's predator-prey equations to the data to compare the observed and theoretical abundances. The actual and expected results were fairly close for the bluefin-squid model for 1951–1955 and for the bluefin-squid plus smaller fishes model for 1951–1956. Additional information on predator-prey relationships is given in Section 9.1.

12.2 Yield-per-recruit Relationship

A yield-per-recruit analysis has been performed for northern bluefin, using the data in Table 10 and Figure 9. The following assumptions were made: (1) the fish are hatched on May 16;(2) they are recruited to the fishery on July 1 of the same year at a length of 15 cm; (3) they attain a length of 223 cm on their 15th birthday, at which time they disappear from the fishery. (The 223-cm value comes from an estimate of the asymptotic length given by Bayliff et al. (1991).) The annual coefficient of natural mortality was set at 0.2 and 0.3 because those values bracket the estimate of 0.276 calculated in Section 7. Actually, M probably varies among age groups. The annual coefficients of fishing mortality were set at 1.0 for the periods of substantial catches, 0.0 for the periods of zero or very low catches, and 0.5 for the periods of transition. The data in Figure 9 are based upon what are believed to be three of the most common scenarios for individual fish: remaining throughout life in the western Pacific Ocean (Option 0); beginning a west-east migration during the first year of life and then beginning an east-west migration during the third year of life (Option 1); and beginning a west-east migration during the second year of life and then beginning an east-west migration during the third year of life (Option 2). Graphs showing the yields per recruit possible with different ages of entry into the fishery, and with three multipliers of the vector of fishing effort, are shown in Figure 10. Because the values of the natural and fishing mortality, especially the latter, are little more than guesses, the results of these analyses should not be taken literally. Nevertheless, it appears that increasing the age at entry into the fishery to about 2 ½ years (about 90–100 cm) would maximize the overall yield per recruit of fish which migrate to the eastern Pacific Ocean, and increasing the age at entry to about 4 years (about 130–140 cm) would maximize the yield per recruit of fish which remain in the western Pacific.

12.3 Cohort Analysis

Cohort analyses were run, using the data in Tables 11 and 12 and the methods described by Tomlinson (1970). In these tables all fish caught during the calendar year in which they were hatched are referred to as age-0 fish, all fish caught during the calendar year after that in which they were hatched are referred to as age-1 fish, and so on. The cohort analyses were begun at the end of the 12th year in the fishery (December 31). Initiating rates of F of 0.5 and 0.4 were used with M values of 0.2 and 0.3, respectively (see Section 7). The results are shown in Table 13. It can be seen that the catches in the western Pacific usually exceed those in the eastern Pacific for all ages except the 2-year olds. The 1969 year class, which was about average (Table 13, second column), produced very poor catches of age-2 fish in the western Pacific and excellent catches of age-2 fish in the eastern Pacific. The 1972 year class, which was a little below average, produced very poor catches of age-0 and -1 fish in the western Pacific and excellent catches of age-1 and -2 fish in the eastern Pacific. The 1981 year class, which was about average, produced above-average catches of age -2 fish in the western Pacific and very poor catches of age-2 fish in the eastern Pacific.

TABLE 10. Growth and weight-length data used for estimation of yields per recruit of northern bluefin.

Age in years	Equation	Units of measurement	Source
Growth
0.125–0.917	L_t²^0.001 = 581^0.001 + (L_t¹^0.001 - 581^0.001)e^{0.01184(t₂ - t₁)}	millimeters, days	this report, Section 5.1
0.917–4.500	L_t = 564 + 0.709δt	millimeters, days	this report, Section 5.1
5.500–13.500	L_t = 1482 + 105.9δt	millimeters, years	linear interpolation
5.00–15.00	L_t = 320.5(1 - e^{-0.1035(t + 0.7034)}	centimeters, years	this report, Table 1
Weight-length
0.125–1.042	w = (1.82335 × 10^-9)l^3.39759	millimeters, kilograms	Bayliff, 1991, Table 7
1.042–14.500	w = (2.01711 × 10^-7)l^2.79130	millimeters, pounds	Bayliff, 1991, Table 7

FIGURE 9. Fishing mortality rates used for estimation of yield-per-recruit curves for northern bluefin. WPO and EPO stand for western Pacific Ocean and eastern Pacific Ocean, respectively. The heavy and light segments of the lines indicate annual instantaneous fishing mortality rates of 1.0 and 0.5, respectively, and the blank areas between segments of the lines indicate no fishing. The options are described in the text.

FIGURE 10. Yield-per-recruit curves for northern bluefin, based on the data in Table 10 and Figure 9 (after Bayliff, 1993b). The three values, 0.5, 1.0, and 2.0, represent multipliers of the vectors of fishing mortality shown in Figure 9. The horizontal portions of the curves are caused by lack of fishing effort during these periods.

TABLE 11. Estimated numbers of bluefin, in thousands, caught in the eastern Pacific Ocean (after Bayliff, 1993a: Appendix 1).

Year caught
Age	1959	1960	1961	1962	1963	1964	1965	1966	1967	1968	1969	1970
1	12	58	524	829	526	972	327	622	97	416	290	14
2	269	66	347	608	521	256	305	715	308	155	311	258
3	109	33	6	9	80	29	16	35	34	5	32	35
4	4	69	<1	1	5	4	1	0	1	0	1	1
>4	<1	4	0	1	3	0	1	0	<1	0	1	<1
Year caught
Age	1971	1972	1973	1974	1975	1976	1977	1978	1979	1980	1981	1982
1	467	-	609	51	260	488	55	508	78	48	4	249
2	711	-	589	440	722	150	250	81	539	284	70	120
3	7	-	2	1	2	172	21	7	<1	1	11	2
4	0	-	0	0	<1	24	21	8	<1	0	<1	1
>4	0	-	0	0	0	3	11	12	<1	0	0	<1
Year caught
Age	1983	1984	1985	1986	1987	1988	1989	1990	1991
1	12	33	76	22	7	34	45	5	52
2	62	44	315	388	74	45	40	78	8
3	6	6	<1	5	6	21	6	8	1
4	<1	<1	0	<1	<1	<1	<1	1	<1
>4	<1	<1	0	<1	<1	1	<1	<1	0

The cohort analysis is highly speculative. The errors in assigning the fish to ages 0, 1, 2, and 3 are probably minimal, but those in assigning them to greater ages are almost certainly greater. In addition, the fact that many fish migrate back and forth across the Pacific Ocean probably causes the results to be less accurate than they would be otherwise.

TABLE 12. Estimated numbers of bluefin, in thousands, caught in the western Pacific Ocean (after Ishizuka, 1989).

Age	Year caught
Age	1966	1967	1968	1969	1970	1971	1972	1973	1974	1975	1976	1977
0	1270	3607	2300	2970	1938	3316	498	4875	3953	1277	1784	2542
1	266	461	964	371	378	443	682	124	1403	676	222	698
2	91	3	1	78	48	2	15	20	28	46	96	61
3	62	5	52	13	23	14	18	4	12	115	61	44
4	30	8	17	1	17	24	18	16	3	29	26	95
5	25	22	41	5	5	3	34	15	11	10	8	10
6	63	7	28	19	16	7	7	19	27	10	6	14
7	1	22	22	9	13	15	6	8	30	9	2	2
8	2	21	7	2	1	11	4	5	13	6	4	1
9	4	4	4	1	<1	2	1	2	7	2	4	1
10	3	2	3	1	<1	1	<1	1	5	1	1	1
11	1	1	2	1	<1	<1	<1	1	2	<1	<1	<1
12	<1	<1	1	1	<1	<1	<1	<1	<1	<1	<1	<1
13	<1	<1	<1	<1	<1	<1	0	<1	<1	<1	<1	<1
14	<1	<1	0	0	0	0	<1	<1	<1	<1	0	<1
15	0	<1	0	0	0	<1	0	<1	0	0	0	<1
16	0	<1	0	0	0	0	0	<1	0	0	<1	0
Age	Year caught
Age	1978	1979	1980	1981	1982	1983	1984	1985	1986
0	5091	2088	2810	1975	665	1362	2417	2046	1470
1	478	1452	611	605	785	213	421	757	760
2	151	98	119	180	200	139	44	49	61
3	14	38	76	584	64	54	21	86	123
4	10	28	7	84	99	56	15	75	30
5	87	15	11	32	73	71	15	4	24
6	20	72	3	26	22	43	4	3	3
7	31	61	27	5	19	13	4	2	2
8	5	4	51	10	11	4	5	2	3
9	2	1	7	15	10	3	3	2	4
10	2	1	3	2	13	2	3	2	4
11	1	1	2	1	3	2	4	2	4
12	1	<1	1	<1	1	1	1	1	2
13	<1	<1	<1	<1	<1	<1	<1	<1	1
14	<1	<1	<1	<1	<1	<1	<1	<1	<1
15	<1	<1	<1	0	<1	<1	<1	<1	<1
16	<1	<1	<1	0	<1	0	<1	0	<1

TABLE 13a. Results of the cohort analysis described in the text, with an annual coefficient of natural mortality of 0.2 (from Bayliff, 1993a). IPS, WPO, EPO, F, and Mort. stand for initial population size, catch in western Pacific Ocean, catch in eastern Pacific Ocean, annual coefficient of fishing mortality, and natural mortality, respectively. All the values except those for F are in thousands of fish.

Year class	Age 0					Age 1
Year class	IPS	WPO	EPO	F	Mort.	IPS	WPO	EPO	F	Mort.
1966	2692	1270	0	0.72	351	1071	461	97	0.84	133
1967	6959	3607	0	0.83	869	2483	964	416	0.93	298
1968	4221	2300	0	0.90	512	1409	371	290	0.71	184
1969	5156	2970	0	0.98	604	1582	378	14	0.32	247
1970	4012	1938	0	0.75	519	1555	443	467	1.01	181
1971	6217	3316	0	0.87	764	2137	682	292	0.59	190
1972	3532	498	0	0.17	591	2443	124	609	0.40	369
1973	10139	4875	0	0.74	1314	3950	1403	51	0.52	565
1974	6331	3953	0	1.13	700	1678	676	260	0.93	201
1975	3331	1277	0	0.54	469	1585	222	488	0.67	212
1976	3750	1784	0	0.73	489	1477	698	55	0.81	186
1977	5950	2542	0	0.63	809	2599	478	508	0.54	368
1978	9958	5091	0	0.81	1252	3615	1452	78	0.62	495
1979	3927	2088	0	0.86	483	1356	611	48	0.75	175
1980	4601	2810	0	1.09	519	1272	605	4	0.74	165
1981	4510	1975	0	0.65	607	1928	785	249	0.88	236
1982	1685	665	0	0.56	235	785	213	12	0.38	120
Average	5116	2527	0	0.76	652	1937	622	232	0.68	254

Year class	Age 3					Age 4					Age 5
Year class	IPS	WPO	EPO	F	Mort.	IPS	WPO	EPO	F	Mort.	IPS	WPO	EPO	F	Mort.
1966	380	1	155	0.59	52	172	13	32	0.34	27	100	17	1	0.21	15
1967	805	78	311	0.75	104	312	23	35	0.23	50	204	24	0	0.14	35
1968	564	48	258	0.89	68	190	14	7	0.13	33	136	18	6	0.19	19
1969	943	2	711	1.67	84	146	18	24	0.31	16	88	16	0	0.22	15
1970	464	15	269	1.43	89	91	4	2	0.08	16	69	3	0	0.05	12
1971	973	20	589	1.13	108	256	12	1	0.06	45	198	29	0	0.18	33
1972	1341	28	440	0.48	194	679	115	2	0.21	112	450	26	24	0.13	76
1973	1931	46	722	0.57	269	894	61	172	0.34	139	522	95	21	0.28	83
1974	541	96	150	0.68	71	224	44	21	0.38	34	125	10	8	0.17	21
1975	663	61	250	0.72	87	265	14	7	0.09	46	198	28	0	0.18	34
1976	538	151	81	0.64	73	233	38	0	0.20	40	155	7	0	0.05	27
1977	1245	98	539	0.81	155	453	76	1	0.20	74	302	84	0	0.36	46
1978	1590	119	284	0.33	247	940	584	11	1.16	104	241	99	1	0.60	33
1979	522	180	70	0.74	68	204	64	2	0.44	30	108	56	0	0.83	13
1980	498	200	120	1.18	53	125	54	6	0.74	16	49	15	1	0.41	6
1981	658	139	62	0.41	99	358	21	6	0.09	63	268	75	0	0.37	41
1982	440	44	44	0.24	70	282	86	0	0.41	42	154	30	0	0.24	25
Average	829	78	297	0.78	111	343	73	19	0.32	52	198	37	4	0.27	31

TABLE 13b. Results of the cohort analysis described in the text, with an annual coefficient of natural mortality of 0.3.

Year class	Age 0					Age 1
Year class	IPS	WPO	EPO	F	Mort.	IPS	WPO	EPO	F	Mort.
1966	3271	1270	0	0.59	651	1350	461	97	0.64	263
1967	8132	3607	0	0.70	1541	2984	964	416	0.75	555
1968	4965	2300	0	0.75	923	1742	371	290	0.57	348
1969	6004	2970	0	0.82	1082	1952	378	14	0.26	448
1970	4656	1938	0	0.64	904	1814	443	467	0.84	325
1971	7428	3316	0	0.71	1404	2708	682	292	0.46	464
1972	5336	498	0	0.11	1311	3527	124	609	0.27	806
1973	12453	4875	0	0.59	2472	5106	1403	51	0.40	1105
1974	7285	3953	0	0.95	1246	2086	676	260	0.71	393
1975	4316	1277	0	0.41	925	2114	222	488	0.48	440
1976	4539	1784	0	0.59	900	1855	698	55	0.62	364
1977	7224	2542	0	0.51	1482	3200	478	508	0.44	680
1978	11622	5091	0	0.69	2212	4319	1452	78	0.52	885
1979	4532	2088	0	0.74	845	1599	611	48	0.63	312
1980	5168	2810	0	0.95	884	1474	605	4	0.64	287
1981	5610	1975	0	0.52	1152	2483	785	249	0.64	482
1982	2285	665	0	0.40	491	1129	213	12	0.26	260
Average	6166	2527	0	0.63	1201	2438	622	232	0.54	495

Year class	Age 3					Age 4					Age 5
Year class	IPS	WPO	EPO	F	Mort.	IPS	WPO	EPO	F	Mort.	IPS	WPO	EPO	F	Mort.
1966	529	1	155	0.41	113	260	13	32	0.22	61	154	17	1	0.14	36
1967	1049	78	311	0.55	212	448	23	35	0.16	108	282	24	0	0.10	69
1968	733	48	258	0.65	142	285	14	7	0.09	71	193	18	6	0.13	44
1969	1112	2	711	1.27	167	232	18	24	0.19	48	142	16	0	0.14	35
1970	579	15	269	1.02	140	155	4	2	0.05	39	110	3	0	0.03	28
1971	1270	20	589	0.79	232	429	12	1	0.04	110	306	29	0	0.12	75
1972	1988	28	440	0.32	445	1075	115	2	0.13	262	696	26	24	0.09	173
1973	2547	46	722	0.42	544	1235	61	172	0.25	286	716	95	21	0.21	169
1974	757	96	150	0.47	159	352	44	21	0.24	82	205	10	8	0.11	50
1975	964	61	250	0.46	202	451	14	7	0.06	114	316	28	0	0.11	79
1976	738	151	81	0.45	156	350	38	0	0.14	86	226	7	0	0.04	58
1977	1534	98	539	0.64	297	600	76	1	0.16	144	379	84	0	0.29	85
1978	1904	119	284	0.28	434	1067	584	11	1.00	180	292	99	1	0.50	61
1979	628	180	70	0.60	124	254	64	2	0.35	56	132	56	0	0.66	25
1980	578	200	120	0.98	197	161	54	6	0.55	32	69	15	1	0.29	15
1981	967	139	62	0.27	222	544	21	6	0.06	137	380	75	0	0.26	87
1982	644	44	44	0.17	153	403	86	0	0.28	92	225	30	0	0.17	54
Average	1089	78	297	0.57	232	488	73	19	0.23	112	284	37	4	0.20	67

13. INTERACTIONS AMONG FISHERIES

There are two basic types of interactions among the fisheries which take northern bluefin, interactions between ocean areas (eastern, central, or western Pacific) and interactions within ocean areas.

Northern bluefin are recruited to the troll fishery for small fish in the western Pacific. Some of them remain in the western Pacific and others, after exposure to the troll fishery and other fisheries in the western Pacific, migrate to the eastern Pacific. In the eastern Pacific they are exploited by the purse-seine fishery. After a sojourn in the eastern Pacific, those which are not caught or succumb to natural mortality return to the western Pacific, where they are exposed to further exploitation. Accordingly, there is important interaction between the fisheries of the western and eastern Pacific. It is known that bluefin are caught by the gillnet and baitboat fisheries of the central Pacific (Bayliff et al., 1991: Table 6) and by longlining, but reasonably complete estimates of the amounts of bluefin caught are not available, so the importance of the interactions between the western and central Pacific or the eastern and central Pacific are not known.

Northern bluefin are caught almost exclusively by purse seining in the eastern Pacific, so the principal interaction is that between Mexican- and U.S.-flag vessels. The fish usually appear first off Mexico in about May or June, and later are taken in waters off the United States in July, August, September, and October (Figure 6).

Northern bluefin are taken by trolling vessels, traps, purse seines, baitboats, gillnets, handlines, and longlines in the western Pacific (Table 9), and the activities of each type of vessel affect the catches of those which take fish of the same size or larger fish.

14. ACKNOWLEDGEMENTS

The cohort analyses were performed by Mr. Patrick K. Tomlinson of the Inter-American Tropical Tuna Commission. Suggestions for improvement of the manuscript were received from Drs. Richard B. Deriso, Ziro Suzuki, and Alexander Wild, and Messrs. Doyle A. Hanan and Richard S. Shomura.

15. REFERENCES CITED

Aikawa, H., and M. Kato. 1938. Age determinations of fish. I. Bull.Japan.Soc.Sci.Fish., 1 (7): 79–88 [translation by W. G. Van Campen, 1950, Spec.Sci.Rep.U.S.Fish Wild.Serv. (Fish.), (21):22 p.].

Bayliff, W.H. 1980. Synopsis of biological data on the northern bluefin tuna, Thunnus thynnus (Linnaeus, 1758), in the Pacific Ocean. Spec.Rep.I-ATTC, (2):261–93.

Bayliff, W.H. (ed.) 1986. Annual report of the Inter-American Tropical Tuna Commission 1985. Annu.Rep.I-ATTC, (1985):248 p.

Bayliff, W.H. 1991. Status of northern bluefin tuna in the Pacific Ocean. Spec.Rep.I-ATTC, (7):29–88.

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