Indices of abundance of northern bluefin tuna, Thunnus thynnus, in the eastern Pacific Ocean

William H. Bayliff
Inter-American Tropical Tuna Commission
8604 La Jolla Shores Drive
La Jolla, California 92037-1508 USA

ABSTRACT

Catches, catches per unit of effort (CPUEs), and aerial survey data provide indices of abundance of northern bluefin tuna in the eastern Pacific Ocean. The various indices are described and compared. The habitat index, based on CPUEs of vessels fishing in spatiotemporal strata which are believed to be suitable habitat for bluefin, and the bluefin vessel index, calculated by dividing the annual catch of bluefin by the number of vessels which fish for that species, are believed to provide the most reliable indices of abundance of bluefin in the eastern Pacific Ocean.

1. INTRODUCTION

Larvae of northern bluefin tuna, Thunnus thynnus, in the Pacific Ocean have been found only in the vicinity of Japan, and it is assumed that spawning occurs only in that area. Some fish apparently remain their entire lives in the western Pacific Ocean (WPO). Others migrate to the eastern Pacific Ocean (EPO); these migrations begin mostly, or perhaps entirely, during their first and second years of life. The first-year migrants are exposed to the summer and fall troll fisheries for small bluefin and other species off Japan before beginning their journey to the EPO in the fall or winter. The second-year migrants are also exposed to the winter troll fishery and other fisheries which take place in the vicinity of Japan before beginning their journey to the EPO in the spring, summer or fall. The migrants, after crossing the ocean, are fished by purse seiners off California and Baja California. Eventually, the survivors return to the WPO.

The catches of northern bluefin in the EPO consist mostly of age-1 and age-2 fish. The catches of age-2 fish in the EPO exceed those of age-2 fish in the WPO in most years, whereas the opposite is the case for age-3 fish (Bayliff, 1994a). This probably indicates that the population of age-2 fish is greater in the EPO and that of age-3 fish is greater in the WPO, although it is possible that area- and/or size-related differences in fishing effort and/or vulnerability to capture are responsible for the differences in the catches.

It is obvious that there is extensive interaction between the fisheries of the WPO and the EPO. Such being the case, information on all the fisheries which take northern bluefin in the Pacific Ocean must be taken into account in any attempts to manage these fisheries. It is of fundamental importance, in stock assessment, to have indices of the abundance of the stock which is being studied. This report summarizes the various indices which have been calculated for northern bluefin in the EPO, introduces two new ones, and compares them to one another.

Ahlstrom (1960) defined abundance as “the absolute number of individuals [post-recruits] in a population” and availability as “the portion (a percentage) of the recruited population that is physically within the geographical range of the fishery during the fishing season.” For northern bluefin tuna, only the fish which migrate from the WPO to the EPO are available to the surface fisheries of the EPO. Although the population of bluefin includes individuals in coastal waters of the WPO and the EPO and in offshore waters of the Pacific Ocean, henceforth in this report the word abundance should be taken to mean abundance in coastal waters of the EPO.

The catches of northern bluefin in the EPO during the 1980s and early 1990s have been much less than they were during the 1960s and 1970s. The decreased catches during recent years are believed to be due, at least partly, to lesser abundance of bluefin in the EPO (Bayliff et al., 1991; Bayliff, 1993: 88-90). Bluefin are caught mostly by vessels with carrying capacities of 400 short tons or less (size classes 1-5), and the numbers of vessels in this size range have been decreasing, so it is possible that the declining catches are due to reduced fishing effort, rather than to reduced abundance of fish. (A short ton is about 0.907 metric ton.)

2. INDICES OF ABUNDANCE

Catches, catches per unit of effort (CPUEs), and aerial survey data are used as indices of abundance of bluefin in this report. Several types of each of these are discussed below.

2.1 Catch

In general, the total catch during a year serves as an index of abundance, provided the following requirements are fulfilled:

1. the fishing effort remains constant, i.e., the numbers of vessels, the effectiveness of the gear, and the skill of the fishermen do not change from year to year;

2. the fishing effort is directed primarily at the species in question, i.e., the fishermen do not pass up opportunities to catch the species in question in order to try to catch other species;

3. the range of the species in question is the same each year;

4. the vulnerability to capture of the species in question is the same each year.

2.1.1 Commercial catch

Calkins (1982) stated that, since the area where bluefin are caught is subjected to intensive searching for tunas and other pelagic fish throughout the year, the “total catch may be the best indicator of bluefin abundance.” That statement could be correct, or nearly so, for the 1961-80 period considered by Calkins, but it may not be true for subsequent years, due to decreased fishing effort. Commercial catch data for the EPO for 1959-91 (Bayliff, 1994b) are listed in Tables 1 and 2.

2.1.2 Recreational catch

If commercial catch data provide a realistic index of abundance, then the same may be true of recreational catch data. These vessels operate off California, off northern Baja California north of about 30ºN, and in the vicinity of Guadalupe Island only, however, so they cannot provide realistic estimates of the abundance for the entire area where bluefin occur in the EPO unless the abundance off California and northern Baja California is proportional to that for the entire area. Data for the catches of bluefin aboard commercial passenger-carrying fishing vessels given by Leet et al. (1992: pages 247 and 255) are listed in Table 2. In general, bluefin are difficult to catch with recreational gear, and their vulnerability to capture varies considerably from year to year. In 1956, the year in which the greatest catches of bluefin by recreational gear were made, most of the fish were caught close to shore off Coronado, California. These fish did not form schools, so they could not be caught by purse seine vessels. During the early 1980s recreational-fishing vessels began fishing at Cortes Bank (32º20'N - 119º12'W) for the first time, which increased the catches of bluefin by these vessels (Steven J. Crooke, California Department of Fish and Game, pers. comm.).

2.2 Catch Per Unit Effort

Catch per unit of fishing effort is the most widely-used index of the abundance of fish. It is superior to catch because it is not necessary that the numbers of fishing vessels be constant from year to year. It is still necessary that the effectiveness of the gear and the skill of the fishermen be constant from year to year unless information making it possible to adjust for annual changes in these factors is available. The other requirements listed above under Catch must still be satisfied. Bluefin are caught mostly by purse seines in the EPO, but the geographic range of the purse seine fishery for tunas extends far beyond the geographic range in which bluefin occur in the EPO, so CPUE data for the EPO are unlikely to provide realistic estimates of the abundance of bluefin. The indices described below take that fact into account.

2.2.1 Calkins Index

Calkins (1982) attempted to ascertain the fishing effort directed toward bluefin during 1961-80. He adopted the following rules: (1) no effort made south of 23ºN was considered to be bluefin effort; (2) no effort during the November-April period was considered to be bluefin effort; (3) no effort in 1- degree area-month strata in which no sets were made on bluefin was considered to be bluefin effort. The annual sums of the effort for 1959-91 obtained by this method are listed in Table 3 (the values in this table differ slightly from those in Calkins' Table 6 because the IATTC's files have been updated and corrected, as necessary, since 1982). Calkins calculated the CPUEs for 1961- 80 by dividing the sums of the catches of bluefin in the area-time strata in which bluefin effort was assumed to occur by the sums of the effort in those strata. The trends for this index of abundance were similar to those for the catches. When similar calculations were made for 1981-89 (Bayliff, 1991), very high CPUEs were obtained for 1985, 1986, 1987 and 1989, even though the catches were about average during 1985 and 1986 and very low during 1987 and 1989. This was apparently due to violation of Requirement 3 above, as during the years with high CPUEs the fish tended to occur in only a few area-time strata, with high CPUEs in them. In 1989, for example, no bluefin were caught south of 30ºN, and 70 percent of the catch was made during August. The Calkins indices for 1959-91 are listed in Table 2.

2.2.2 Bluefin vessel index

For this method, it is assumed that the fishing effort directed toward bluefin is proportional to the numbers of vessels which direct substantial portions of their effort to fishing for bluefin. Accordingly, each vessel of the purse seine fleet was classified each year as a “bluefin vessel” or a “non-bluefin vessel.” If a vessel caught bluefin in a given year and in two or more of the four closest adjacent years (two years before and two years after the year in question), it was classified as a bluefin vessel for that year. Otherwise it was classified as a non-bluefin vessel. The criteria were relaxed for vessels which entered the EPO tuna fishery less than two years before the year in question or left the fishery less than two years after the year in question. For example, if a vessel entered the EPO tuna fishery in 1971, it was classified as a bluefin vessel for that year if it caught bluefin in 1971 and in either 1972 or 1973. This system of classification is arbitrary, and perhaps some other system would be better. However, it seems to be adequate to give some insight into the value of fleet size as a measure of the effort directed toward bluefin and total catch divided by fleet size as an index of the abundance of bluefin. Data are given in Tables 1 and 3 on the commercial catches of bluefin, the numbers of bluefin vessels in the fleet, and the numbers of purse seine vessels in the fleet. There are almost no data for Class-1 and Class-2 vessels prior to 1971 because these vessels rarely catch yellowfin or skipjack, and the IATTC staff did not begin to collect data on bluefin until 1971.

The coefficient of correlation between the catches and the numbers of bluefin vessels is 0.849 (d.f. = 27, P<0.01). This does not necessarily mean that the amounts of fish caught are determined mostly by the size of the bluefin fleet (or that the size of the fleet is determined mostly by the abundance of bluefin), however. For example, it can be seen that the catches during the 1961-68 period varied by a factor of 2.7 (15,897/5,889), while the number of bluefin vessels varied by a factor of only 1.2 (70/60), and that the catches during the 1981-89 period varied by a factor of 6.0 (5,084/853), while the number of bluefin vessels varied by a factor of only 1.6 (22/14). Accordingly, it appears that the fluctuations in the catches are not due primarily to fluctuations in the numbers of bluefin vessels in the fleet. There is no evidence from these data that indicates that the hypothesis of Bayliff et al. (1991) that decreased abundance of fish in the EPO is an important contributor to the reduced catches of bluefin in that area should be rejected.

Data on the numbers of purse seine vessels in the fleet are also shown in Table 1 to satisfy the curiosity of readers who might think that the comparison might be made between bluefin catches and number of vessels in the purse seine fleet. It can be seen that number of purse seine vessels increased from 1961 to 1979 and then decreased from 1979 to 1989. There appears to be no relationship between bluefin catches and total fleet size. The bluefin vessel index is the total commercial catch for each year divided by the number of bluefin vessels in the fleet during that year. Indices for 1961-89 are shown in Table 2.

2.2.3 Bluefin habitat index

Bluefin are most often caught in the EPO where the sea-surface temperatures (SSTs) are between 17º and 23ºC (Bell, 1963). Accordingly, it was assumed that EPO waters north of 23ºN and west of Baja California and California with SSTs in that range are suitable habitat for bluefin during the period of May through October. The decision to use only data for the area north of 23ºN and only the data for May-October follows the rules of Calkins (1982). Data were assembled on logged purse seine catch and effort and SSTs in the 1-degree area-month strata fitting the above criteria. The logged catch and effort data were taken from files of the IATTC and the SST data were taken from NMFS (1960-80) and NWS (1981-91). The monthly and annual sums of the logged catches of bluefin in the EPO were divided by the monthly and annual sums of the effort in the 1-degree areas which were suitable bluefin habitat (Appendix Table 1) to get monthly and annual CPUEs of bluefin. The annual CPUEs are listed in Table 2 and the monthly and annual CPUEs in Appendix Table 1.

2.3 Aerial Survey Indices

If indices of abundance based upon visual surveys are to be valid the following requirements must be satisfied:

1. the effectiveness of the equipment and the skill of the searchers do not change from year to year;

2. the range of the species in question is the same each year;

3. the visibility of the species in question is the same each year.

Squire (1972, 1983, 1993) calculated indices of abundance for bluefin tuna from data obtained from aeroplane pilots who were searching for tunas and other pelagic fishes off Southern California and northern Baja California for fishing vessels. He calculated two indices for each year, one for “core areas” (areas in which bluefin most commonly occur) and the other for “total areas” (between 27º50'N and 38º10'N). His indices for 1962-90 are listed in Table 2.

2.4 Indices for Areas North of 28ºN

The aerial indices apply only to the area north of 27º50'N, and the California-based recreational fishery takes place mostly north of 30ºN. Therefore, the commercial catches, effort, and CPUEs for the area north of 28ºN were calculated so that the CPUEs could be compared with the aerial indices and recreational catches. These data are listed in Tables 2 and 3 and Appendix Table 2.

3. COMPARISON OF MEASURES OF EFFORT AND INDICES OF ABUNDANCE

3.1 Measures of Effort

Five measures of effort are listed in Table 3. No adjustments have been made for differences in efficiencies of vessels of different sizes because sufficient data to perform the necessary calculations are not available. In general, for at least two reasons, larger vessels are more efficient than smaller ones. First, larger vessels are faster, so they can search for fish in greater areas per unit of time than can smaller ones. Second, larger vessels have greater carrying capacities for fish, so when they find an area where fishing is good they can usually remain there for extended periods, whereas smaller vessels often have to return to port to unload their catches while fishing is still good. On the other hand, bluefin are caught mostly by smaller vessels, which may indicate that their captains direct their effort more toward bluefin and are more skilful at catching bluefin than are the captains of larger vessels.

Four of the five measures indicate that the fishing effort for bluefin has been less during the 1980s and early 1990s than during the 1960-79 period. The exception, total number of vessels, is not a meaningful measure of effort directed toward bluefin, as most of these vessels seldom fish in the areas where bluefin occur. The decline in the measure of effort used for calculating the Calkins index is greater than those for the habitat indices. This is because the Calkins index is calculated only with effort in area-time strata where sets on bluefin were made, whereas the habitat indices are calculated with effort in area-time strata where bluefin are likely to be found (assuming that sea-surface temperature is the only factor influencing their distribution). It is the belief of the author of this report that the effort used in calculating the habitat index for the entire area is more realistic than that used in calculating the Calkins index.

3.2 Indices of Abundance

The top panel of Table 4 lists coefficients of correlation for indices of abundance involving the entire area in which bluefin are caught. All but two of these are significant at the 1-percent level, the exceptions being total catch versus the Calkins index (Test 2) and logged catch during May-October versus the Calkins index (Test 7). It was pointed out above that high values of the Calkins index were obtained for 1985, 1986, 1987 and 1989, and that this was probably due to concentration of the fish in a few small areas. When the data for 1981-91 were omitted high coefficients of correlation (Tests 3 and 8) were obtained, confirming Calkins' observation that the total catch and his index for years prior to 1981 are highly correlated. A scatter plot of the data used for Test 13 showed the point for 1989 to be an outlier, so the coefficient of correlation was recalculated without the data for 1989 (Test 14), producing a higher coefficient of correlation.

The middle panel of Table 4 lists coefficients of correlation for indices of abundance involving the area north of 28ºN. Only the r values for logged catch versus habitat index (Tests 19 and 20) and Squire's (1993) core index versus his total index (Test 24) are significant. A scatter plot of the data used for Test 19 showed the point for 1986 to be an outlier, so the coefficient of correlation was recalculated without the data for 1986 (Test 20), producing a higher coefficient of correlation. It is noteworthy that the two aerial indices are correlated significantly only to each other and that the recreational catch index is not correlated significantly with any other index.

The bottom panel of Table 4 lists coefficients of correlation for indices of abundance for the entire area in which bluefin are caught and the area north of 28ºN.

Tests 27 and 28 were performed first to see if indices which were identical except for the areas included were correlated. Both of these tests produced high values of r, which indicates that the recreational catch indices and the aerial indices can be considered to be indices for the entire area of the fishery, rather than just the area north of 28ºN or 30ºN.

None of the original five tests involving recreational catches (Tests 30, 37, 46, 53, and 60 produced significant results. When a single outlier (1986) was removed, a significant relationship was found for the Calkins index- recreational catch comparison (Test 47).

Only one (Test 31) of the original ten tests (Tests 31, 33, 38, 40, 48, 49, 54, 55, 61, and 63) involving the aerial indices produced significant results. When single outliers (1978) were removed, the significance of one relationship increased from 5 percent to 1 percent (Test 32), three relationships which were previously insignificant became significant (Tests 34, 39, and 41), and two relationships remained insignificant (Tests 62 and 64).

Only one (Test 44) of the remaining eight original tests involving catch and/or effort data (Tests 29, 35, 42, 44, 50, 52, 56, and 58), produced insignificant results. When a single outlier (1986) was removed the relationship remained insignificant (Test 45).

For the most part, the indices of abundance derived from catch and effort data are related only to each other, as are the two indices derived from aerial observations. The recreational catch data are not related to any of the other indices. The recreational catches are probably not realistic indices of abundance, for reasons stated above, so they are not considered further. The following possibilities remain: the catch and effort data provide realistic indices of abundance; the aerial observations provide realistic indices of abundance; neither of these provides realistic indices of abundance.

The first possibility appears to be the most likely, for two reasons. First, the catches and CPUEs are highly correlated, as indicated especially by Tests 3, 4, 5 and 19. Second, most of the indices are based upon data for the entire area in which bluefin are caught, rather than only the northern part of that area. (Admittedly, however, the indices of abundance for the entire area and for its northern part are highly correlated (Tests 27 and 28).)

There are seven indices of abundance based on catch and effort data. The three based on catches can be disregarded, as these give biased results when the effort is not constant from year to year. The habitat index for the area north of 28ºN can also be disregarded, as it is presumably inferior to the habitat index for the entire area in which bluefin are caught.

This leaves the Calkins index, the habitat index for the entire area in which bluefin are caught, and the bluefin vessel index. The Calkins index is biased when the range of bluefin contracts, as apparently was the case during 1985-87 and 1989-91. The habitat index may be biased in the same way that the Calkins index is biased but, if so, it is less biased than the Calkins index. The measure of effort from which the bluefin vessel index is calculated is based upon arbitrary classification of vessels as bluefin and non-bluefin vessels, and other classification schemes would undoubtedly produce somewhat different results. There is no objective way to determine which of the two indices, the habitat index or the bluefin vessel index, is better; fortunately, however, the two are highly correlated (Test 15).

4. CONCLUDING REMARKS

The habitat indices indicate that the abundance of bluefin in the EPO has been low during the 1977-91 period, except for 1985 and 1986. The bluefin vessel indices also indicate that the abundance of bluefin was greatest during 1985 and 1986, but they indicate that it was also greater than average during 1979 and 1982. Even though both indices indicate high abundance of bluefin in 1985 and 1986, the catches were only average because the fishing effort was so low. Unless the fishing effort increases, it appears unlikely that catches greater than those of 1985 and 1986 will be taken in the future.

5. ACKNOWLEDGMENTS

Dr. Richard B. Deriso suggested the use of the habitat index. Ms. Gayle Ver Steeg extracted the data necessary for performing many of the calculations from the IATTC's computerized catch and effort data base.

6. REFERENCES CITED

Ahlstrom, E.H. 1960. Fluctuations and fishing. Proceedings of the World Scientific Meeting on the Biology of Sardines and Related Species, FAO, Rome, Vol. 3: 1353-1371.

Bayliff, W.H. 1991. Status of northern bluefin tuna in the Pacific Ocean. Spec. Rep. IATTC 7: 29-88.

Bayliff, W.H. (ed.). 1993. Annual Report of the Inter-American Tropical Tuna Commission, 1992. I-ATTC: 315 p.

Bayliff, W.H. 1994a. Interactions among fisheries for northern bluefin tuna, Thunnus thynnus, in the Pacific Ocean. In: Shomura, R.S., J. Majkowski and S. Langi (eds.). Interactions of Pacific tuna fisheries. Proceedings of the First FAO Expert Consultation on Interactions of Pacific Tuna Fisheries, 3-11 December 1991, Noumea, New Caledonia. Vol. 1: Summary report and papers on interaction. FAO Fish. Tech. Pap. (336/1): 249-263.

Bayliff, W.H. 1994b. A review of the biology and fisheries for northern bluefin tuna, Thunnus thynnus, in the Pacific Ocean. In: Shomura, R.S., J. Majkowski and S. Langi (eds.). Interactions of Pacific tuna fisheries. Proceedings of the First FAO Expert Consultation on Interactions of Pacific Tuna Fisheries, 3-11 December 1991, Noumea, New Caledonia. Vol. 2: Papers on biology and fisheries. FAO Fish. Tech. Pap. (336/2): 244-295.

Bayliff, W.H., Y. Ishizuka and R.B. Deriso. 1991. Growth, movement, and attrition of northern bluefin tuna, Thunnus thynnus, in the Pacific Ocean, as determined by tagging. Bull. IATTC 20(1): 1-94.

Bell, R.R. 1963. Synopsis of biological data on California bluefin tuna Thunnus saliens Jordan and Evermann 1926. FAO Fish. Rep. 6(2): 380- 421.

Calkins, T.P. 1982. Observations on the purse seine fishery for northern bluefin tuna (Thunnus thynnus) in the eastern Pacific Ocean. Bull. IATTC 18(2): 121-225.

Leet, W.S., C.M. Dewees and C.W. Haugen (eds.). 1992. California's Living Marine Resources and their Utilization. California Department of Fish and Game: iv, 257 p.

NMFS. 1960-80. Calif. Fish. Market News Monthly Summary, Part 2, Fishing Information. US Nat. Mar. Fish. Serv., Southwest Fish. Sci. Center.

NWS (National Weather Service). 1981-91. Oceanographic Monthly Summary. US Dept. Commerce, Nat. Ocean. Atmos. Admin., Nat. Weather Serv.

Squire, J.L. 1972. Apparent abundance of some pelagic marine fishes off the southern and central California coast as surveyed by an airborne monitoring program. Fish. Bull., NOAA-NMFS 70(3): 1005-1019.

Squire, J.L. 1983. Abundance of pelagic resources off California, 1963-78, as measured by an airborne fish monitoring program. NOAA Tech. Rep. NMFS SSRF-762: v, 75 p.

Squire, J.L. 1993. Relative abundance of pelagic resources utilized by the California purse seine fishery: results of an airborne monitoring program, 1962-90. Fish. Bull. NOAA-NMFS 93(2): 348-361.

Table 1. Commercial catches (in metric tons) of bluefin in the eastern Pacific Ocean, numbers of bluefin vessels (defined in the text) in the fleet, and numbers of purse seine vessels in the fleet. The numbers 1 through 6 refer to the size classes, based on carrying capacities of the vessels in short tons (1 = 50 or less; 2 = 51-100; 3 = 101-200; 4 = 201-300; 5 = 301-400; 6 = greater than 400).

Year	Catch	Bluefin vessels							Total purse-seine vessels
		1	2	3	4	5	6	Total	1	2	3	4	5	6	Total
1961	8,136	0	1	38	16	7	0	62	0	3	52	40	20	9	124
1962	11,268	0	0	34	22	9	0	65	0	2	46	41	29	12	130
1963	12,271	0	0	26	27	15	0	68	1	7	43	41	31	18	141
1964	9,218	0	0	27	30	13	0	70	0	1	39	45	29	20	134
1965	6,887	0	0	27	23	12	0	62	1	12	39	43	29	22	146
1966	15,897	0	0	23	27	14	0	64	0	0	34	39	32	21	126
1967	5,889	0	0	21	24	16	0	61	0	2	29	37	30	24	122
1968	5,976	0	0	21	25	12	2	60	1	11	26	34	30	37	139
1969	6,926	0	0	18	20	7	1	46	0	7	29	35	29	49	149
1970	3,966	0	0	15	17	12	3	47	0	13	24	33	27	65	162
1971	8,360	0	9	17	17	13	5	61	0	16	26	32	27	84	185
1972	13,348	0	10	16	13	12	10	61	1	22	22	34	25	102	206
1973	10,746	0	10	16	18	8	11	63	1	16	24	39	16	120	216
1974	5,617	0	14	12	15	5	7	53	0	19	27	37	15	132	230
1975	9,583	0	14	14	13	9	9	59	4	24	33	25	17	146	249
1976	10,646	0	15	11	11	11	12	60	0	24	27	25	16	158	250
1977	5,473	0	5	10	8	12	15	50	0	20	32	19	18	161	250
1978	5,396	0	9	6	6	9	16	46	0	24	37	25	20	156	262
1979	6,118	0	9	6	4	9	14	42	2	27	34	25	22	158	268
1980	2,938	0	9	4	4	7	11	35	0	17	32	25	20	164	258
1981	1,090	0	9	3	3	2	3	20	1	20	25	20	16	166	248
1982	3,150	0	10	2	2	2	4	20	1	22	16	16	13	153	221
1983	853	0	9	3	0	1	2	15	7	28	21	14	10	123	203
1984	882	0	11	1	2	0	3	17	3	27	16	9	6	105	166
1985	4,054	0	11	3	2	1	3	20	4	22	18	10	6	117	177
1986	5,084	2	14	4	0	1	1	22	5	19	19	9	5	109	166
1987	995	0	9	4	0	0	2	15	2	16	18	8	5	128	177
1988	1,423	0	10	4	0	0	0	14	2	23	17	7	6	132	187
1989	1,169	0	10	4	0	0	2	16	5	19	17	7	6	122	176

Table 2. Indices of abundance for bluefin tuna. The units are as follows: commercial catch, metric tons; recreational catch, numbers of fish; Calkins index, metric tons per day; aerial indices, metric tons per block-area flight; bluefin vessel, metric tons per year; habitat, metric tons per day.

Year	Catch				Calkins index	Aerial indices		Present study
	Commercial			Recreational		Core	Total	Bluefin vessel index	Habitat indices
	Jan.-Dec.	May-Oct.	May-Oct. N of 28ºN						Total N of 28ºN
1959	6,914	3,091	1,060	1,330	3.67
1960	5,422	4,466	3,152	97	3.03				2.37	3.38
1961	8,136	7,934	6,760	2,268	3.81			131	2.91	5.34
1962	11,268	9,952	7,829	2,453	3.47	5.96	3.99	173	3.45	4.98
1963	12,271	12,115	8,675	737	4.50	6.32	6.18	181	3.86	4.44
1964	9,218	7,915	4,755	693	2.66	6.32	6.18	132	2.44	4.02
1965	6,887	5,892	3,711	92	2.38	1.16	0.82	111	2.30	4.31
1966	15,897	14,544	3,605	1,998	5.86	11.82	8.58	249	5.33	3.27
1967	5,889	5,356	559	3,166	1.88	4.09	3.10	96	1.54	0.56
1968	5,976	5,005	4,218	1,231	2.52	8.57	5.47	100	2.14	3.29
1969	6,926	5,836	2,618	1,470	2.57	0.54	0.35	151	2.09	2.39
1970	3,966	3,992	662	1,833	2.25	<0.01	0.02	84	1.51	0.73
1971	8,360	7,478	2,561	749	2.87	0.79	0.38	137	2.54	2.14
1972	13,348	11,978	7,308	1,470	3.70	6.30	2.49	219	3.31	4.23
1973	10,746	9,004	5,303	5,347	3.33	15.77	14.92	171	2.75	3.63
1974	5,617	4,057	1,463	5,765	1.97	16.62	15.68	106	1.92	2.10
1975	9,583	6,579	2,316	3,348	3.87	2.36	2.30	162	3.63	4.78
1976	10,646	8,957	7,003	2,040	3.80	12.46	11.97	178	3.55	4.15
1977	5,473	4,773	2,672	1,838	2.40	7.90	6.46	110	1.75	2.13
1978	5,396	3,922	3,312	479	2.10	31.32	27.28	117	1.66	2.74
1979	6,118	4,728	2,861	1,087	2.65	11.68	11.18	146	1.44	2.83
1980	2,938	2,413	754	729	1.79	0.81	0.63	84	0.97	1.25
1981	1,090	702	629	542	1.03	0.86	0.73	54	0.46	0.93
1982	3,150	2,424	1,689	665	2.20	1.24	1.15	158	0.85	1.34
1983	853	549	534	1,912	0.75	0.05	0.18	57	0.23	0.44
1984	882	726	726	2,834	0.70	0.56	0.60	52	0.34	0.50
1985	4,054	3,358	1,891	4,980	5.99	2.10	2.33	202	3.67	3.22
1986	5,084	4,277	4,073	693	10.95	1.80	2.75	231	8.19	11.01
1987	995	772	728	1,859	4.04	0.70	0.50	66	1.54	3.00
1988	1,423	1,242	592	321	1.53	1.12	0.51	102	0.73	1.20
1989	1,169	986	986	6,519	7.05	0.93	0.44	73	1.22	3.37
1990	1,541	1,373	1,373	3,756	5.52	0.66	0.32	1.80	3.67
1991	462	415	309	5.37	1.24	2.06

Table 3. Measures of fishing effort for bluefin in the eastern Pacific Ocean.

Year	Calkins index	Bluefin vessels	Total vessels	Habitat index	Habitat north of 28ºN index
1959	824.0		87
1960	1453.0		112	1889.5	932.0
1961	2051.0	62	124	2721.5	1265.5
1962	2886.5	65	130	2890.5	1572.0
1963	2677.0	68	141	3131.5	1957.5
1964	2941.0	70	134	3240.0	1182.0
1965	2370.0	62	146	2569.5	861.0
1966	2422.5	64	126	2727.0	1102.5
1967	2812.0	61	122	3483.0	998.0
1968	1968.0	60	139	2336.0	1279.5
1969	2258.0	46	149	2801.0	1092.0
1970	1682.5	47	162	2658.5	916.0
1971	2595.5	61	185	2944.5	1196.0
1972	3220.0	61	206	3613.0	1727.5
1973	2664.5	63	216	3271.0	1461.0
1974	1974.0	53	230	2105.5	699.5
1975	1673.0	59	249	1811.0	484.5
1976	2322.0	60	250	2524.5	1685.5
1977	1948.0	50	250	2723.0	1251.0
1978	1841.0	46	262	2356.5	1211.5
1979	1771.0	42	268	3284.0	1010.0
1980	1329.5	35	258	2481.0	600.0
1981	664.5	20	248	1515.5	673.5
1982	1060.0	20	221	2845.5	1260.5
1983	651.0	15	203	2430.0	1215.0
1984	1022.0	17	166	2159.5	1463.5
1985	566.0	20	177	916.5	587.5
1986	384.0	22	166	522.0	370.0
1987	174.5	15	177	500.0	242.5
1988	755.5	14	187	1705.5	493.0
1989	140.0	16	176	808.0	292.5
1990	234.0			764.5	375.0
1991	75.0			335.0	150.5

Table 4. Correlations between different indices of availability of bluefin tuna in the eastern Pacific Ocean.

Comparison	Coefficient of correlation		Degrees of freedom	Probability
Entire area
1. Total catch versus Logged catch, May-October	0.981		31	**
2. Total catch versus Calkins index	0.151		31	n.s.
3. same with 1981-1991 data omitted	0.867		20	**
4. Total catch versus Vessel index	0.767		27	**
5. Total catch versus Habitat index	0.616		30	**
6. same with 1986 data omitted	0.872		29	**
7. Logged catch, May-October, versus Calkins index	0.163		31	n.s.
8. same with 1981-1991 data omitted	0.812		20	**
9. Logged catch, May-October, versus Vessel index	0.751		27	**
10. Logged catch, May-October, versus Habitat index	0.611		30	**
11. same with 1986 data omitted	0.867		29	**
12. Calkins index versus Vessel index	0.624		27	**
13. Calkins index versus Habitat index	0.763		30	**
14. same with 1989 data omitted	0.854		29	**
15. Vessel index versus Habitat index	0.836		27	**
North of 28°N
16. Logged catch, May-October, versus Sport catch	-0.132		30	n.s.
17. Logged catch, May-October, versus Aerial core index	0.363		27	n.s.
18. Logged catch, May-October, versus Aerial total index	0.333		27	n.s.
19. Logged catch, May-October, versus Habitat index	0.594		30	**
20. same with 1986 data omitted	0.773		29	**
21. Sport catch versus Aerial core index	0.078		27	n.s.
22. Sport catch versus Aerial total index	0.102		27	n.s.
23. Sport catch versus Habitat index	-0.032		29	n.s.
24. Aerial core index versus Aerial total index	0.988		27	**
25. Aerial core index versus Habitat index	0.090		27	n.s.
26. Aerial total index versus Habitat index	0.106		27	n.s.
Total area versus North of 28ºN
27. Logged catch, May-October, versus Logged catch north of 28ºN	0.818	31		**
28. Habitat index versus Habitat index north of 28ºN	0.866	31		**
29. Total catch versus Logged catch north of 28ºN	0.787	31		**
30. Total catch versus sport catch	-0.058	30		n.s.
31. Total catch versus Aerial core index	0.391	27		*
32. same with 1978 data omitted	0.586	26		**
33. Total catch versus Aerial total index	0.345	27		n.s.
34. same with 1978 data omitted	0.508	26		**
35. Total catch versus Habitat index north of 28ºN	0.410	30		*
36. same with 1986 data omitted	0.631	29		**
37. Logged catch, May-October, versus Sport catch	-0.071	30		n.s.
38. Logged catch, May-October, versus Aerial core index	0.342	27		n.s.
39. same with 1978 data omitted	0.550	26		**
40. Logged catch, May-October, versus Aerial total index	0.292	27		n.s.
41. same with 1978 data omitted	0.466	26		*
42. Logged catch, May-October, versus Habitat index north of 28ºN	0.396	30		*
43. same with 1986 data omitted	0.613	29		**
44. Calkins index versus Logged catch north of 28ºN	0.211	31		n.s.
45. same with 1986 data omitted	0.236	30		n.s.
46. Calkins index versus Sport catch	0.245	30		n.s.
47. same with 1986 data omitted	0.470	29		**
48. Calkins index versus Aerial core index	-0.071	27		n.s.
49. Calkins index versus Aerial total index	-0.056	27		n.s.
50. Calkins index versus Habitat index north of 28ºN	0.777	30		**
51. same with 1986 data omitted	0.575	29		**
52. Vessel index versus Logged catch north of 28ºN	0.628	27		**
53. Vessel index versus Sport catch	-0.013	27		n.s.
54. Vessel index versus Aerial core index	0.231	26		n.s.
55. Vessel index versus Aerial total index	0.213	26		n.s.
56. Vessel index versus Habitat index north of 28ºN	0.640	27		**
57. same with 1986 data omitted	0.596	26		**
58. Habitat index versus Logged catch north of 28ºN	0.566	30		**
59. same with 1986 data omitted	0.696	29		**
60. Habitat index versus Sport catch	-0.012	29		n.s.
61. Habitat index versus Aerial core index	0.129	27		n.s.
62. same with 1978 data omitted	0.260	28		n.s.
63. Habitat index versus Aerial total index	0.132	27		n.s.
64. same with 1978 data omitted	0.256	26		n.s.

** significant at the 1-percent level
* significant at the 5-percent level
n.s. not significant at the 5-percent level

Year	May			June			July			August			September			October			Total
	C	f	C/f	C	f	C/f	C	f	C/f	C	f	C/f	C	f	C/f	C	f	C/f	C	f	C/f
1960	0	5.0	0.0	1612	366.0	4.4	1788	997.0	1.8	877	499.5	1.7	189	18.0	10.5	0	4.0	0.0	4466	1889.5	2.37
1961	30	104.0	0.3	1139	564.5	2.0	1078	726.0	1.5	4384	806.5	5.4	1166	434.0	2.7	138	86.5	1.6	7934	2721.5	2.91
1962	308	253.5	1.2	1887	556.0	3.4	3117	945.5	3.3	4274	954.0	4.4	367	160.5	2.3	0	21.0	0.0	9952	2890.5	3.45
1963	89	88.0	1.0	2167	356.0	6.1	1773	568.0	3.1	4710	1065.5	4.4	3369	921.5	3.6	7	132.5	0.1	12115	3131.5	3.86
1964	18	18.0	1.0	1461	552.5	2.6	21061	215.0	1.7	3553	1009.5	3.5	773	430.0	1.8	5	15.0	0.3	7915	3240.0	2.44
1965	3	77.0	0.0	526	452.0	1.2	816	754.5	1.1	2282	373.5	6.1	1383	474.0	2.9	883	438.5	2.0	5892	2569.5	2.30
1966	210	136.5	1.5	4588	496.0	9.3	6412	1051.0	6.1	3186	782.5	4.1	149	236.5	0.6	0	24.5	0.0	14544	2727.0	5.33
1967	47	448.5	0.1	2806	636.0	4.4	2182	1161.0	1.9	83	425.0	0.2	222	537.0	0.5	16	275.5	0.1	5356	3483.0	1.54
1968	0	58.5	0.0	738	475.5	1.5	2042	761.5	2.7	1349	704.0	1.9	728	272.0	2.7	148	64.5	2.3	5005	2336.0	2.14
1969	505	309.0	1.6	689	489.5	1.5	2263	878.0	2.5	1744	554.5	3.2	635	352.0	1.8	0	218.0	0.0	5836	2801.0	2.09
1970	0	264.5	0.0	1461	478.5	3.1	2166	980.5	2.2	299	671.5	0.5	64	208.5	0.3	0	55.0	0.0	3992	2658.5	1.51
1971	1937	456.0	4.3	1746	304.5	5.7	1293	944.0	1.4	708	441.5	1.6	999	341.5	2.9	796	457.0	1.7	7478	2944.5	2.54
1972	341	201.0	1.7	2236	627.0	3.5	2367	1033.5	2.3	4864	896.5	5.4	695	566.0	1.3	1474	289.0	5.1	11978	3613.0	3.31
1973	16	107.5	0.2	1745	559.0	3.1	4735	1153.0	4.1	2293	961.0	2.4	215	324.0	0.6	0	166.5	0.0	9004	3271.0	2.75
1974	0	35.5	0.0	1194	413.0	2.9	1420	709.5	2.0	627	377.5	1.6	756	394.5	1.9	61	175.5	0.4	4057	2105.5	1.92
1975	53	5.0	10.5	2489	177.0	14.1	1522	630.5	2.4	364	463.0	0.8	1581	288.0	5.5	570	247.5	2.3	6579	1811.0	3.63
1976	241	64.0	3.8	1650	551.5	3.0	279	327.5	0.8	3391	500.0	6.8	3084	937.0	3.3	311	144.5	2.2	8957	2524.5	3.55
1977	1383	355.0	3.9	391	607.5	0.6	62	559.5	0.1	1956	605.0	3.3	649	512.5	1.3	332	83.5	4.0	4773	2723.0	1.75
1978	0	172.0	0.0	588	609.5	1.0	1341	601.0	2.3	1982	752.0	2.6	0	155.0	0.0	11	67.0	0.2	3922	2356.5	1.66
1979	1500	383.5	3.9	357	986.5	0.4	694	794.0	0.9	1851	660.5	2.8	214	339.5	0.6	112	120.0	0.9	4728	3284.0	1.44
1980	304	350.0	0.9	1100	825.5	1.4	239	466.0	0.5	501	282.5	1.8	269	303.5	0.9	0	253.5	0.0	2413	2481.0	0.97
1981	0	222.5	0.0	262	283.0	0.9	80	223.5	0.4	255	282.0	0.9	105	199.0	0.5	0	305.5	0.0	702	1515.5	0.46
1982	0	328.5	0.0	0	380.0	0.0	852	526.5	1.6	659	609.5	1.1	581	500.0	1.2	333	501.0	0.6	2424	2845.5	0.85
1983	0	235.0	0.0	0	646.0	0.0	250	478.5	0.5	163	606.0	0.3	98	247.5	0.4	37	217.0	0.2	549	2430.0	0.23
1984	23	256.0	0.1	81	446.5	0.2	316	910.5	0.4	125	336.5	0.4	122	60.0	2.0	59	150.0	0.4	726	2159.5	0.34
1985	27	23.0	1.2	1548	161.0	9.6	322	292.5	1.1	762	170.0	4.4	130	145.0	0.9	570	125.0	4.5	3358	916.5	3.67
1986	15	13.0	1.1	307	49.5	6.2	1551	181.0	8.5	1560	130.5	12.0	363	57.0	6.4	482	91.0	5.3	4277	522.0	8.19
1987	0	13.5	0.0	33	38.0	0.8	261	132.5	2.0	245	154.5	1.5	188	89.5	2.1	45	72.0	0.6	772	500.0	1.54
1988	0	5.0	0.0	215	167.0	1.3	852	660.0	1.3	99	504.0	0.2	16	172.0	0.1	60	197.5	0.3	1242	1705.5	0.73
1989	0	15.0	0.0	0	83.0	0.0	6	127.0	0.1	732	221.0	3.4	212	254.5	0.8	35	107.5	0.4	986	808.0	1.22
1990	0	100.5	0.0	0	202.5	0.0	470	141.0	3.4	288	178.0	1.6	536	79.0	6.8	79	63.5	1.3	1373	764.5	1.80
1991	0	1.0	0.0	98	6.0	16.3	16	55.0	0.3	301	160.5	1.9	0	75.5	0.0	0	37.0	0.0	415	335.0	1.24

Appendix Table 2. Catches (C), in metric tons, effort (f), in days of fishing by purse seine vessels in “bluefin habitat” north of 28ºN, and catch per unit of effort (C/f) for bluefin tuna in the eastern Pacific Ocean.

Year	May			June			July			August			September			October			Total
	C	f	C/f	C	f	C/f	C	f	C/f	C	f	C/f	C	f	C/f	C	f	C/f	C	f	C/f
1960	0	0.0	0.0	548	118.0	4.6	1539	353.0	4.4	877	440.0	2.0	189	18.0	10.5	0	3.0	0.0	3152	932.0	3.38
1961	30	0.0	-	153	33.0	4.6	889	232.5	3.8	4384	707.0	6.2	1166	274.5	4.3	138	18.5	7.4	6760	1265.5	5.34
1962	26	5.0	5.3	204	0.0	-	2958	667.0	4.4	4274	771.0	5.5	367	124.0	3.0	0	5.0	0.0	7829	1572.0	4.98
1963	89	19.0	4.7	24	4.0	6.2	535	50.5	10.6	4650	870.5	5.4	3369	906.0	3.7	7	107.5	0.1	8675	1957.5	4.44
1964	18	0.0	-	54	1.0	54.4	440	111.0	4.0	3553	887.5	4.0	686	174.5	3.9	5	8.0	0.5	4755	1182.0	4.02
1965	3	0.0	-	0	6.0	0.0	3	22.5	0.1	1523	291.5	5.3	1377	369.0	3.7	806	172.0	4.7	3711	861.0	4.31
1966	7	1.0	7.3	0	11.5	0.0	284	100.0	2.7	3186	764.5	4.2	101	201.0	0.5	0	24.5	0.0	3605	1102.5	3.27
1967	0	0.0	0.0	91	12.0	7.5	211	180.0	1.2	18	144.0	0.1	222	537.0	0.5	16	125.0	0.1	559	998.0	0.56
1968	0	0.0	0.0	3	12.0	0.2	1989	383.5	5.2	1349	601.5	2.3	728	245.5	3.0	148	37.0	4.0	4218	1279.5	3.29
1969	36	0.0	-	46	41.5	1.2	171	294.0	0.5	1731	532.5	3.3	634	188.0	3.4	0	36.0	0.0	2618	1092.0	2.39
1970	0	0.0	0.0	0	21.5	0.0	298	286.5	1.0	299	507.5	0.6	64	89.5	0.7	0	11.0	0.0	662	916.0	0.73
1971	2	9.0	0.2	0	0.0	0.0	57	84.0	0.7	708	402.0	1.7	999	341.5	2.9	796	359.5	2.2	2561	1196.0	2.14
1972	0	0.0	0.0	8	12.0	0.7	503	121.0	4.2	4657	774.5	6.0	670	531.0	1.3	1470	289.0	5.1	7308	1727.5	4.23
1973	16	1.0	16.3	0	7.0	0.0	2844	526.5	5.4	2228	746.5	3.0	215	157.0	1.4	0	23.0	0.0	5303	1461.0	3.63
1974	0	0.0	0.0	0	6.0	0.0	232	105.5	2.2	507	265.0	1.9	701	295.0	2.4	23	28.0	0.8	1463	699.5	2.10
1975	0	0.0	0.0	6	0.0	-	275	98.5	2.8	306	107.0	2.8	1273	177.0	7.2	456	102.0	4.4	2316	484.5	4.78
1976	0	0.0	0.0	4	18.5	0.2	279	154.5	1.8	3336	438.5	7.6	3079	932.0	3.3	306	142.0	2.2	7003	1685.5	4.15
1977	0	0.0	0.0	14	53.0	0.3	62	170.5	0.4	1948	551.0	3.5	649	423.5	1.5	0	53.0	0.0	2672	1251.0	2.13
1978	0	2.0	0.0	91	24.0	3.8	1265	372.5	3.4	1946	661.0	2.9	0	134.0	0.0	11	18.0	0.6	3312	1211.5	2.74
1979	0	0.0	0.0	0	10.0	0.0	685	145.5	4.7	1851	479.5	3.9	214	304.0	0.7	112	71.0	1.5	2861	1010.0	2.83
1980	0	3.5	0.0	0	12.0	0.0	14	35.0	0.4	471	226.0	2.1	269	275.5	1.0	0	48.0	0.0	754	600.0	1.25
1981	0	5.0	0.0	262	50.0	5.3	70	163.0	0.5	210	254.5	0.8	87	178.5	0.5	0	22.5	0.0	629	673.5	0.93
1982	0	0.0	0.0	0	8.0	0.0	117	64.5	1.8	659	403.0	1.6	581	453.0	1.3	333	332.0	1.0	1689	1260.5	1.34
1983	0	4.0	0.0	0	5.0	0.0	250	196.5	1.3	149	545.0	0.3	98	247.5	0.4	37	217.0	0.2	534	1215.0	0.44
1984	23	4.0	5.6	81	155.5	0.5	316	828.5	0.4	125	304.0	0.5	122	60.0	2.0	59	111.5	0.5	726	1463.5	0.50
1985	0	0.0	0.0	268	18.5	14.4	206	144.0	1.5	718	161.0	4.4	130	141.0	0.9	570	123.0	4.6	1891	587.5	3.22
1986	15	1.0	14.5	216	16.5	13.1	1438	138.5	10.3	1560	121.0	12.9	363	52.0	7.0	482	41.0	11.8	4073	370.0	11.01
1987	0	2.0	0.0	8	5.0	1.6	241	46.0	5.3	245	72.0	3.4	188	57.5	3.3	45	60.0	0.7	728	242.5	3.00
1988	0	1.0	0.0	1	12.0	0.7	416	185.5	2.3	99	184.0	0.5	16	55.5	0.3	60	55.0	1.1	592	493.0	1.20
1989	0	0.0	0.0	0	3.0	0.0	6	7.5	0.8	732	117.0	6.3	212	145.5	1.5	35	19.5	1.8	986	292.5	3.37
1990	0	2.5	0.0	0	3.0	0.0	470	123.0	3.8	288	105.0	2.7	536	79.0	6.8	79	62.5	1.3	1373	375.0	3.67
1991	0	0.0	0.0	0	0.0	0.0	10	10.0	1.0	299	80.0	3.7	0	55.5	0.0	0	5.0	0.0	309	150.5	2.06