measures of effort in tuna longline fisheries: changes at the operational level

Fisheries Research, 12 (1991) 75-87 75 Elsevier Science Publ ishers B.V., Ams te rdam

Measures of effort in tuna longline fisheries: changes at the operational level

T o m P o l a c h e c k 1

Tuna and Billfish Assessment Programme, South Pacific Commission, B.P. D-5, Noumea, New Caledonia

(Accepted 3 December 1990)

ABSTRACT

Polacheck, T., 1991. Measures of effort in tuna longline fisheries: changes at the operational level. Fish. Res., 12: 75-87.

Changes at the operational level of a fishery can affect the efficiency of gear and need to be taken into account when catch per unit effort indices are used to assess the status of fish stocks. Catch per unit of effort indices form the basis for the assessment of most tuna longline fisheries. Effort in these fisheries is measured in terms of the total number of hooks set and differences at the operational level in the number of hooks per longline set are generally not considered. It is shown that the number of hooks per set in the Pacific Japanese longline fishery has increased steadily from 1962 to 1986 by about 40%. The increase does not appear to be related to vessel size.

Changes in the number of hooks per set can potentially affect the average efficiency of individual hooks. Analyses of catch rates (number per hook), based on data from 1979 to 1986 and categorized according to the number of hooks per set, suggest no consistent trend, although the category with the largest number of hooks per set tended to have higher catch rates for yellowfin and slightly lower rates for bigeye. These results suggest that the number of hooks per set does not interact with the efficiency or catchability of an individual hook. Comparison of catch rates, based on the number of hooks and the number of sets, suggests that the choice of an effort measure can substantially affect either observed trends in catch rates over time or the relative changes in their magnitude. In recent years, decreases in the catch per hook have been offset at the set level by increases in the number of hooks per set. Overall, the results suggest that the number of hooks should continue to be used as the best measure of effort for stock assessment purposes, but is not appropriate for economic analyses.

I N T R O D U C T I O N

Indices of abundance derived from catch per unit effort (CPUE) statistics form the basis for most assessments of tuna longline fisheries, with the excep- tion ofbluefin tuna (e.g. Kume and Joseph, 1969; Suzuki et al., 1978; Bartoo

t Present address: Nat iona l Mar ine Fisheries Service, Nor theas t Fisheries Center, Woods Hole, MA, USA.

76 T. POLACHECK

and Shiohama, 1985; Suzuki, 1986; Kume and Miyabe, 1987). A consistent unit of effort over time is essential for any CPUE-based assessment so that changes in abundance are not confounded with changes in efficiency of fishing. Thus, procedures for standardizing effort are employed commonly to take into account factors such as vessel size, horsepower, area and seasonal effects (Beverton and Holt, 1957; Robson, 1966; Allen and Punsly, 1984). As a fishery develops, operational changes often occur which have dramatic effects on the efficiency of a unit of effort (e.g. the use of sonar, changes in net construc- tion, increases in tow times, etc.). The detection, much less the quantifica- tion, of such changes is often difficult. Nonetheless, operational changes must be documented and their effects of CPUE need to be examined if these indices are to be used as measures of relative abundance.

Effort in tuna longline fisheries is almost always measured in terms of the number of hooks set; corresponding catch rates are expressed as the number of fish caught per hundred or thousand hooks. Effort at the operational level of the fishery (i.e. a single set of a longline) is generally not examined. Im- plicit in using hooks as a measure of effort is the assumption that each hook acts as an independent unit. Possible interactions among hooks due to the fact that up to several thousand hooks may be set on a single line have generally not been considered. (Two exceptions are Rothschild (1967) and Au (1985 ). ) Any such interactions would not be important in calculating average catch rates as long as the number of hooks per set remained relatively constant, so that the number of hooks and the number of sets were highly correlated. However, since the number of hooks per set can interact with soak time, hook spacing, hook depth, school structure, school movements and spa- tial coverage, changes in the average number of hooks per set could have potentially large effects on catch rates at the hook level.

The Japanese tuna longline fleet has been the largest and most dominant longline fleet operating in the Pacific since its rapid development after World War II. It is also the best documented of the various longline fleets, with a published time series of detailed catch and effort data dating back to 1962. Consequently, CPUE statistics based on these data have been used widely to assess the Pacific tuna stocks affected by longlining. Thus, any systematic changes in the operation of this fishery should be examined for possible effects on CPUE. The purposes of this paper are: ( 1 ) to document that Japa- nese longline vessels at the operational level have steadily increased the number of hooks per set; (2) to examine the effect of such changes on the resultant catch rates.

DATA

Two sources of data, which partially overlap in time but differ in their level of detail, are available for examining these questions. The Fisheries Agency

MEASURES OF EFFORT 1N TUNA LONGLINE FISHERIES 77

of Japan published annual reports from 1962 to 1980 for the Japanese longline fishery. These annual reports were published for each year from 1962 to 1980 with the actual date of publication being one year after the year covered by the report. The reports are titled "Annual report of effort and catch statistics by area on Japanese tuna longline fishery". Statistics for the catch by species and total effort, summarized by month and 5 ° squares, are provided in these reports. Catch is given as the number of fish caught. Effort is given both as the total number of hooks set and the total number of sets (except for 1963 when no statistics for the number of sets are provided). Only a single set is made per day in this fishery. Thus, the number of sets and the number of days fished are equivalent measures of effort. No breakdown is available at the individual vessel or set level. Publication of these annual reports ceased after 1980.

The other available data come from records of daily catch and fishing effort supplied by vessels to individual island states in the western Pacific. These data are provided as part of access arrangements which allow vessels to fish within the 200-mile exclusive economic zone (EEZ) of these states. These catch records are transmitted to the Tuna and Billfish Assessment Pro- gramme of the South Pacific Commission (SPC) for inclusion in the regional data base being compiled by that organization (Polacheck, 1986 ). Vessels are required to report on their activity only within a state's EEZ under these access arrangements. While some vessels do report on their activities outside of EEZs, data are incomplete for areas of international waters. For some states, in past years, adequate data reporting was not included as part of access agree- ments. For this and a variety of other reasons, data in this data base are less complete for the early years. Records of daily fishing activity in the SPC regional data base go back to the second half of 1978 for the Japanese longline fishery. Because of incompleteness, the earliest data need to be interpreted with caution.

Note that while the data from the Japanese Research Agency encompass the complete range of the fishery within the Pacific ocean, the data within the SPC data base cover only the western and central Pacific area, with little data east of 180 ° or outside of the tropics.

RESULTS AND DISCUSSION

Trends in the number of hooks per set

Based on published Japanese data, the average number of hooks used per longline set in the entire Pacific increased steadily from 1962 to 1980, from around 1650 hooks to nearly 2200 (Fig. 1 ). The dotted line in Fig. 1 represents unstratified estimates of the average number of hooks per set (i.e. the ratio of the total number of reported hooks set in a year, divided by the total

7 8 T. POLACHECK

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Fig. 1. The average number of hooks per set from 1962 to 1980 for all Japanese longline vessels operating in the Pacific Ocean based on published data from the Fisheries Agency of Japan. The dashed line represents unstratified average annual estimates and the solid line represents the annual average based on a stratification by 5 ° geographic squares of longitude and latitude.

number of reported longline sets). The solid line represents stratified estimates based on geographic strata of 5 ° of longitude and latitude. These estimates were calculated from the number of hooks divided by the number of sets for each stratum, averaged over all strata in which there was longline effort. The values in Fig. 1 encompass all reported Japanese longline effort in the Pacific (i.e. from 135 ° E to 100 ° W). Stratified estimates are always greater than the unstratified estimates, although the magnitude of the differences decreased from 1962 to 1980. A similar increase in the number of hooks per set is observed if only data from the western tropical Pacific (i.e. from 135 °E to 180°E and from 20°N to 10°S) are used, but the average number of hooks per set for the western tropical Pacific in any year is lower than average for the entire Pacific (Fig. 2 ). For the western tropical Pacific, there is no consistent difference between the stratified and unstratified means.

The data from the SPC regional data base indicate that the average number of hooks per set has continued to increase since 1980 (Fig. 3 ). For these data, the average number of hooks per set was around 1950 in 1980 and rose to 2350 in 1986. There is no significant difference between the stratified and unstratified means. The period of overlap in the two data sets indicates that the average number of hooks per set is roughly the same from the two sources, although the averages from the SPC data base tend to be slightly lower. Exact agreement between the two data sources would not be expected because of incompleteness in the SPC data noted above.

The increase in the number of hooks per set cannot be attributed to an

MEASURES OF EFFORT IN TUNA LONGLINE FISHERIES 79

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15 ' 1 i , ' i , I ' I 1 9 6 0 1 9 6 4 1 9 6 8 1 9 7 2 1 9 7 6 1 9 8 0

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Fig. 2. The average number of hooks per set for Japanese longline vessels operating in the western tropical Pacific (i.e. from 135°E to 180°E and from 20°N to 10°S) from 1962 to 1980 based on published data from the Fisheries Agency of Japan. The dashed line represents unstratified average annual estimates and the solid line represents the annual average based on a stratification by 5 ° geographic squares of longitude and latitude.

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Fig. 3. The average number of hooks per set from 1978 to 1986 for Japanese longline vessels operating in the western tropical Pacific (i.e. from 135°E to 180°E and from 20°N to 10°S) based on data in the SPC regional data base. The dashed line represents unstratified average annual estimates and the solid line represents the annual average based on a stratification by 5 ° geographic squares of longitude and latitude.

80 T. POLACHECK

increase in the size of average longline vessels (at least since 1978 ). Major changes have not occurred in the relative proportion of the total effort by the different vessel sizes operating in the fishery (Table 1 ). The relative effort in the 100-200 gross registered tonne size class may have increased somewhat, but this is difficult to determine since a significant fraction of the effort from 1978 to 1981 was by vessels for which size was not reported. While vessels in the larger size classes tend to use more hooks per set (Table 2), increases in the number of hooks per set are observed in all size classes. Such increases

TABLE 1

Percentage of the total annual effort (number of sets) by vessel size class for Japanese longline vessels operating in the western tropical Pacific from 1978 to 1986 based on data in the SPC regional data base

Year Vessel size (gross registered tonnage)

< 20 20-50 50-100 100-200 > 200 Unknown

1978 0.0 0.0 53.1 0.0 0.0 46.9 1979 11.9 4.2 68.3 2.2 0.4 12.9 1980 1.5 2.5 61.3 7.9 1.0 25.8 1981 5.3 2.9 67.6 7.6 0.1 16.4 1982 5.5 2.9 69.4 14.1 1.2 6.9 1983 5.2 2.9 72.1 11.8 0.4 7.6 1984 12.0 2.9 57.6 22.8 1.7 2.9 1985 8.1 4.9 57.5 19.2 2.2 8.2 19861 8.0 4.0 68.6 12.8 1.8 4.9

~Based on partial returns for 1986.

TABLE 2

Annual average number of hooks per set for different vessel size classes for Japanese longline vessels operating in the western tropical Pacific from 1978 to 1986 based on data in the SPC regional data base

Year Vessel size (gross registered tonnage)

< 20 20-50 50-100 100-200 > 200 Unknown

1978 - - 1838 - - 1878 1979 1500 1868 1937 1958 2562 1815 1980 1520 1771 1956 2000 2828 1297 1981 1559 1996 1979 2010 2444 1962 1982 1581 1917 2006 1956 2451 1871 1983 1548 1919 2053 2125 2636 2247 1984 1650 2020 2218 2094 2431 2324 1985 1694 1559 2350 2315 2618 2797 19861 1572 2076 2480 2782 2886 2978

JBased on partial returns for 1986.


have been relatively continuous between years and there is no indication that the number of hooks per set has reached a max imum for any size class.

The fact that the stratified estimates initially exceeded the unstratified estimates for the entire Pacific (Fig. 1 ) suggests that vessels setting higher numbers of hooks per set tended to be in areas receiving less overall fishing effort (as measured by the number of sets) and may reflect the use of larger vessels in the eastern Pacific. This situation appears to have changed so that vessels setting higher numbers of hooks are distributed more evenly throughout the fishery.

In the early 1970s, Japanese vessels began increasing the distance between floats (i.e. increasing the number of hooks per basket) without altering other aspects of their longline gear (Suzuki et al., 1977; Suzuki and Kume, 1982). This increased distance between floats resulted in an approximate doubling of the depth range over which the gear fished and about 60% of the total Jap- anese effort involved deep longlining in 1979. This shift from 'regular' to 'deep' longline methods does not appear to be related to the increase in the number of hooks per set. The major part of this conversion in gear configuration in the western Pacific occurred over a short t ime span ( 1974-1975 ), while the increase in the number of hooks per set has been more or less continuous since 1962.

Comparison of catch rates

A time series of catch rates based on the number of hooks compared to the number of sets will show a difference in either trend or magnitude when the number of hooks per set is not constant. Thus, a comparison of unstratified catch rates (i.e. total catch divided by total effort) for the Japanese published data from 1962 to 1980 indicates that the magnitude of the changes in catch rates appears greater when number of sets is used as a measure of effort than when number of hooks is used. Overall CPUE between 1962 and 1980, on a per set basis, increased in contrast to no real change based on the number of hooks (Fig. 4 ).

The more recent data from the SPC also demonstrate that the choice of effort measures can make substantial differences (see Tables 6 and 7). For example, when effort is measured in terms of the number of hooks, the CPUE for bigeye suggests little change, while if effort is measured in terms of the number of sets or days fished, then the data indicate an increase of around 25% in the catch rate since 1979. Similarly, yellowfin CPUE shows a stronger decline based on the number of hooks than the number of sets. These comparisons of CPUE are presented to emphasize the importance of the choices of effort measures and the potential for interactions when CPUE is used as a measure of relative abundance. It should be noted that a more comprehensive analysis, which includes factors such as area and season, is needed before these

82 T. POLACHECK

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Fig. 4. Comparison of annual catch rate estimates based on two different effort measures between 1962 and 1980 for Japanese longliners operating in the western and central tropical Pa- cific (i.e. west of 180°E and between 20°N and 10°S). The solid line represents catch rates calculated using the number of sets as an effort measure and the dashed line represents catch rates based on the number of hooks. Catch rates were calculated as the total number of fish caught divided by the total effort expended during any year based on the published report of the Fisheries Agency of Japan. (A) For yellowfin tuna ( Thunnus albacares). (B) For bigeye tuna ( Thunnus obesus).

t i me series o f C P U E shou ld be cons ide r ed as accura te indices o f re la t ive a b u n d a n c e s (see Po lacheck , 1988) .

Catch rates - evidence for interactions?

G i v e n tha t the length o f a t u n a longl ine is on the scale o f t h o u san d s o f me- ters a nd tha t a single set m a y c o v e r 3 5 - 7 5 km, increases in the n u m b e r o f hooks per set might , on first thought , be suspec ted to h av e ins ignif icant effects on the average ca tch rates based on the to ta l n u m b e r o f hooks. H o w e v e r , the re la t ionsh ip be tween the hook ing ra te for a single longl ine an d t u n a abundance is complex . A c o m p r e h e n s i v e mode l wou ld need to take in to accoun t


factors such as soak time, gear saturation and tuna behavior, as well as the number of hooks and the density of tuna (Maeda, 1960; Rothschild, 1967; Au, 1985). The effect of many of these factors on catch rates could be expected to be affected by the number of hooks per set as well as the cumulative total number of hooks. For example, hauling in the longline occupies the greatest proportion of time during a given set and the longline is hauled in the opposite direction to which it is set (Wright, 1980; Farman, 1986 ). Conse- quently, the average soak time for each hook would be expected to change with increases in the number of hooks and the catching efficiency of individual hooks would not be independent of the total number of hooks set.

To get an indication of whether the increase in the number of hooks per set has affected the resulting catch rate, the catch and effort data from SPC were categorized according to the number of hooks per set, and catch rates were calculated for yellowfin ( Thunnus albacares) and bigeye ( Thunnus obesus) tuna. Four categories were chosen: ( 1 ) < 1500 hooks per set; (2) 1500-2000 hooks per set; (3) 2000-2500 hooks per set; (4) > 2500 hooks per set. Table 3 provides a breakdown of the amount of annual effort in the four categories. Initially, most of the effort was concentrated in the second one (i.e. most vessels were setting between 1500 and 2000 hooks per set). The relative proportion of the effort in this category has decreased steadily. By 1985, more effort was expended in the third category and, by 1986, few sets were made with < 2000 hooks. Note, a similar categorization from the published Japa- nese data cannot be made for earlier years as the published data have been pooled across these categories and are not available on an individual set basis.

These four categories yielded no consistent patterns in their catch rates when using the number of hooks as the effort measure (Table 4 ). Ranking the catch

TABLE 3

The total annual effort (number of sets) categorized by the number of hooks set per set for Japanese longline vessels operating in the western tropical Pacific based on data in the SPC regional data base

Year < 1500 1501-2000 2001-2500 > 2500 Total

1978 0 309 24 0 333 1979 1161 10707 1765 85 13718 1980 555 15889 4820 0 21264 1981 921 16493 7212 40 24666 1982 762 16879 5089 560 23290 1983 524 9557 4740 494 15315 1984 1307 7431 9935 3309 21982 1985 704 4878 8942 6382 20906 19861 160 910 2535 1235 4840

~Data incomplete for 1986.

84 T. POLACHECK

TABLE 4

Annual catch rates (number per thousand hooks) for yellowfin and bigeye tuna when the data have been categorized by the number of hooks per set. Catch rates were calculated as the annual number of fish caught divided by the total number of hooks based on the data in the SPC regional data base

Year Yellowfin Bigeye

<1500 1501-2000 2001-2500 >2500 < 1500 1501-2000 2001-2500 >2500

1978 - 16.41 15.57 - - 4.70 4.68 - 1979 11.87 16.81 16.64 15.02 3.75 5.27 4.64 4.40 1980 21.55 19.26 18.58 - 5.98 4.68 3.81 - 1981 13.43 15.59 15.92 38.67 2.90 3.19 2.92 0.30 1982 8.06 15.29 14.16 19.78 4.04 3.52 3.98 3.21 1983 18.81 19.63 17.49 22.28 4.63 3.72 3.80 3.14 1984 14.28 11.72 11.30 14.70 4.71 4.32 4.33 3.85 1985 9.12 8.90 9.77 14.55 4.51 4.77 4.86 4.43 1986 t 13.97 14.14 10.17 10.71 5.52 6.21 5.00 4.30


TABLE 5

Correlation coefficient between the catch for yellowfin and bigeye based on the catch rates in Table 4

Number of hooks per set

Number of hooks per set

< 1500 1501-2000 2001-2500 > 2500

> 1500 0.55 1501-2000 0.03 - 0 . 1 8 2001-2500 - 0 . 3 2 - 0 . 3 8 - 0 . 5 8

>2500 - 0 . 1 8 - 0 . 3 2 - 0 . 4 7 - 0 . 9 7

rates within year among the four categories for the 7 years with catch rates in each category yields average ranks of 3.0, 2.3, 3.0 and 1.7 for yellowfin, and average ranks of 2.1, 2.1, 2.1 and 3.9 for bigeye. Based on Friedman's non- parametric two-way analysis of variance (Sokal and Rohlf, 1969), these differences in mean rank are not significant for yellowfin ( P > 0.10, g 2 = 4.9 with three degrees of freedom), but are significant for bigeye ( P < 0.025, Z2= 10.5 with three degrees of freedom). The significant difference in the ranking for bigeye is due to sets with > 2500 hooks. There is no indication of a consistent difference in catch rates among the other three categories.

As indicated by the difference in the average rank, sets in which the number of hooks per set exceeded 2500 tended to catch more yellowfin per hook and less bigeye per hook than the other strata. The two intermediate categories tended to have similar catch rates. Differences among catch rates which exist may reflect variability in targeting for bigeye or yellowfin, rather than inher-

MEASURES OF EFFORT IN TUNA LONGLINE FISHERIES 8 5

ent species-specific differences in efficiency or catchability at the hook level as the result of setting more hooks in an individual set. Thus, yellowfin catch rates tend to be negatively correlated with bigeye catch rates, particularly for categories with the greater number of hooks per set (Table 5 ). Overall, the comparisons of catch rates suggest that the total number of hooks fished has not interacted with the efficiency or catchability of an individual hook.

Economic and management considerations

The increase in the number of hooks per set has implications for economic assessments of the fishery and possible management measures. The marginal cost of increasing the number of hooks per set is most likely substantially less than increasing the number of sets; operational costs are more closely tied to the number of sets (i.e. the number of days fishing) than to the number of hooks per set. Thus, catch rates based on the number of hooks would be a poorer economic indicator.

For example, the combined yellowfin and bigeye catch rate in 1986 from the SPC data, based on the number of hooks, was 32% less than the catch rate in 1979, while the catch rate at the operational level of a set declined only 4% (Tables 6 and 7). Since these two species dominate the longline catches in the tropical western Pacific and are similar in value (although bigeye usually command a somewhat higher price), the results suggest that individual vessels have been able to compensate for decreases in the size or availability of the resource by increasing the number of hooks. The bioeconomic equilib- rium, or the effort level at which the economic yield equals the total costs (Clark, 1985 ), appears to have shifted continually towards higher effort lev- els in terms of the number of hooks. The upper limit to the max imum number

TABLE 6

Comparison of annual catch rates for yellowfin tuna based on the number of hooks and the number of sets for Japanese Iongline vessels operating in the western tropical Pacific from 1978 to 1986 based on data in the SPC regional data base

Year Number per 1000 hooks Number per set

1978 16.42 30.59 1979 16.32 30.44 1980 19.29 37.58 1981 15.67 30.70 1982 14.72 29.18 1983 19.31 39.05 1984 12.01 25.69 1985 11.27 25.78 19861 11.47 26.63


86 T. POLACHECK

TABLE 7

Comparison of annual catch rates for bigeye tuna based on the number of hooks and the number of sets for Japanese longline vessels operating in the western Pacific from 1978 to 1986 based on data in the SPC regional data base

Year Number per 1000 hooks Number per set

1978 4.69 8.74 1979 5.02 9.36 1980 4.54 8.85 1981 3.07 6.01 1982 3.67 7.28 1983 3.81 7.71 1984 4.30 9.14 1985 4.74 10.84 19861 5.06 11.76

IBased on partial returns for 1986.

of hooks per set a vessel can deploy either physically or without a decrease in catch rate is not clear. The average number of hooks per set can be expected to increase since in 1986 only about 25% of the sets were made wi th> 2500 hooks.

For most western Pacific states, regulation of the longline fisheries within their EEZs is based on licensing individual vessels on a trip-by-trip basis. The underlying criterion for setting license fees has generally been a fixed percentage of the expected revenue. Estimation of the expected revenue has been based on a running average of the catch per trip within a state's EEZ by similar sized vessels during the previous 3 years. If all factors remained constant, except for the increasing trend in the number of hooks per set, the catch per trip based on this running average would tend to underestimate future performance. Moreover, if management of this fishery with direct effort controls was deemed desirable, limitations on the number of vessels, number of trips or number of days could be expected to be circumvented to some extent by increasing the number of hooks per set.

In conclusion, there has been a large and steady increase in the average number of hooks per set by Japanese longliners operating in the Pacific. Re- sults based on the SPC data from 1978 to 1986 indicate that catch rates on a per hook basis are not greatly affected by the number of hooks set per operation. In the case of this tuna longline fishery, a major change at the operational level appears not to have had an effect on the efficiency of the basic unit of effort. Thus, the total number of hooks continues to be the best measure of effort to use for this fishery for stock assessment purposes, whereas for measuring economic performance, the number of hooks per set needs to be taken into account since the marginal cost of increasing the number of hooks


per set is l ikely to be substant ia l ly different from that o f increas ing the n u m - ber o f sets.

REFERENCES

Allen, R. and Punsly, R., 1984. Catch rates as indices of abundance of yellowfin tuna, Thunnus albacares, in the eastern Paci fc Ocean. IATTC Bull., 18: 303-379.

Au, D., 1985. Interpretation oflongline hook rates. ICCAT, Coll. Vol. Sci. Pap., 25: 377-385. Bartoo, N. and Shiohama, T., 1985. A production model analysis of North Pacific albacore

population including estimates of the sensitivity of the results to measurement errors in the input data. Bull. Far Seas Res. Lab., 22:109-116.

Beverton, R.J.H. and Holt, S.J., 1957. On the Dynamics of Exploited Fish Populations. Fishery Investigation, Series 2, Vol. 19, HMSO, London, 533 pp.

Clark, C.W., 1985. Bioeconomic Modelling and Fisheries Management. Wiley, New York, 291 PP.

Farman, R.S., 1986. An investigation of longline activities in the waters of Tonga (24 April-19 May 1985). South Pacific Commission. Tuna Billfish Assess. Prog. Tech. Rep. No. 17, 20 pp.

Kume, S. and Joseph, J., 1969. The Japanese longline fishery for tunas and billfishes in the eastern Pacific ocean east of 130W, 1964-1966. IATTC Bull., 13:277-418.

Kume, S. and Miyabe, N., 1987. An updated stock assessment on Atlantic bigeye tuna by cpue and production model. ICCAT SCRS/86/37:111-115.

Maeda, H., 1960. A tentative analysis of the distribution pattern of tuna projected on the longline. J. Shimonoseki Coll. Fish., 9: 89-397.

Polacheck, T., 1986. The Statistical System of the South Pacific Commission. Appendix 1V in FAO, Report of the Ad Hoc Consultation on Global Tuna Statistics, 6-7 December 1985, at Colombo, Sri Lanka. FAO Fish. Rep., No. 365, 91 pp.

Polacheck, T., 1988. Yellowfin tuna, Thunnus albacares, catch rates in the western Pacific. Fish. Bull., 87: 123-144.

Robson, D.S., 1966. Estimation of the relative fishing power of individual ships. ICNAF Res. Bull., 3: 5-14.

Rothschild, B.J., 1967. Competition for gear in a multiple-species fishery. J. Cons. Int. Explor. Mer., 31:102-110.

Sokal, R.R. and Rohlf, F.J., 1969. Biometry. Freeman, San Francisco, 776 pp. Suzuki, Z., 1986. Interaction study on yellowfin stock between longline and purse seine fisheries

in the western Pacific Ocean. Indo-Pacific Tuna Development Programme, Colombo, Sri- Lanka Col. Vol. Working Documents, pp. 175-190.

Suzuki, Z. and Kume, S., 1982. Fishing efficiency of deep longline for bigeye tuna in the Atlantic as inferred from the operations in the Pacific and Indian Oceans. 1CCAT Col. Vol. Sci. Pap., 17:471-486,

Suzuki, Z., Warashina, Y. and Kishida, M., 1977. The comparison of catches by regular and deep tuna longline gear in the western and central equatorial Pacific. Bull. Far Seas Fish. Res. Lab., 15:51-72.

Suzuki, Z., Tomlinson, P.K. and Honma, M., 1978. Population structure of Pacific Yellowfin Tuna. IATTC Bull., 17: 277-441.

Wright, A., 1980. An investigation of Japanese longline tuna fishing operations in the region of Papua New Guinea. Department of Primary Industry, Port Moresby, Papua New Guinea, Res. Bull. No. 23, 49 pp.