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SOME IMPORTANT FACTORS IN CONTROLLING FISHING CAPACITY IN TUNA FISHERIES - Ziro Suzuki, Naozumi Miyabe, Miki Ogura, Hiroshi Shono and Yuji Uozumi[70]

Abstract: Evidence from most international tuna fisheries suggest that they are overexploited, with capacity reduction of the order of 20-30 percent required for sustainable production. Although in many cases fleet numbers have been decreasing, the number of hooks used by individual vessels has increased, resulting in a net increase in fishing capacity. For purse seine vessels, the use of FADs has also resulted in an increase in efficiency and thereby fishing capacity. The change in fishing techniques has had a different impact on the species being caught. In particular, the use of FADs increases the catch of juvenile bigeye. As many of these stocks are already overexploited, the use of these devices may further place pressure on these stocks. Management measures have recently been introduced by the International Commission for the Conservation of Atlantic Tunas (ICCAT) to restrict their use to reduce this problem.


There has been a serious concern about excessive fishing capacity in tuna fisheries, which has led the FAO to take initiative to mediate this problem. One of the tangible actions plans that has emerged from the initiative is the immediate reduction of 20 to 30 percent of fishing capacity of the distant water tuna longline fishery. This reduction is currently being implemented in Japan. However, there is not much action, if any, directed to the reduction of fishing capacity of the tuna purse seine fishery, which dominates the total tuna production. The Inter-American Tropical Tuna Commission (IATTC) has only recently started discussions on fishing capacity of purse seine boats in the Eastern Pacific.

Both the longline and purse seine fisheries, the two major components of the tuna fisheries, are exploiting some tuna species heavily. The size of the tuna taken by the two fisheries tends to differ. The purse seine boats mostly catch juveniles while the longline boats tend to harvest mostly adults. These biological characteristics should be taken into account in assessing overall fishing capacity.

In this paper, important aspects of control of the fishing capacity for tuna fisheries are reviewed with some preliminary analyses for the two major fisheries mentioned above, including trends in fishing capacity, biological and fleet characteristics of tunas and tuna fisheries, estimation of the increase of the fishing efficiency and other relevant subjects to the control of the fishing capacity. The latest information of stock status for some tuna species was used in this study.


A review of the problems regarding fishing capacity measurement methods was made recently by the FAO Technical Working Group on the Management of Fishing Capacity (FAO, 1998). Alternative measurement methods, such as the Date Envelopment Analysis (DEA) and Peak-To-Peak analysis (Kirkley and Squires, 1999), were proposed for future application for fisheries capacity measurement. However, the benefits of such alternative methods seem to require further evaluation before their application to more complicated fisheries. Newton (1999) analyzed the fishing capacity on the high seas using Technological Coefficients, which account for major technological improvement of fishing efficiency. He concluded that a fleet reduction of between 41 and 47 percent was necessary. Suzuki (1999) examined overcapacity specific to world wide distant water tuna fisheries, and, by comparing the stock status and current catch levels, estimated that a reduction in fishing capacity in the distant water longline fishery of between 20 to 30 percent was required to ensure sustainable use of the tuna and tuna-like species.

There are relatively few specific studies on fishing capacity or fishing efficiency. This is mostly due to difficulties to collect quantitative time series information about the factors seemingly related to the fishing efficiency. In addition, as later exemplified, there are inherent difficulties to measure overall change in the fishing efficiency.

Pella and Psaropulos (1975) tried to explicitly include the increase of fishing efficiency of the tuna purse seine fleet in the Eastern Pacific in estimating standardized CPUE based on mathematical representation of purse seine operations during 1960-1971. However, the increase of efficiency or real increase of effective fishing effort per se during the period was not shown.

Gascuel et al. (1993) estimated an increase of overall fishing power, with the use of virtual population analysis (VPA) and general linear modelling (GLM) methods, during the years from 1970 and 1980 for French and Spanish tuna purse seine fleets in the eastern Atlantic. They estimated that fishing powers increased by 17 percent and nine percent on an average for the French and Spanish fleets respectively, and indicated rather complicated pattern of changes in increase or decrease of the fishing power by year or year period and by size of yellowfin. The difference in increased fishing power between the two fleets seemed to reflect fleet-wise difference in operational strategy including target species change.

Recently, Shono and Ogura (1999)[71] analyzed changes in fishing efficiency for skipjack of the Japanese pole and line fishery, by use of the GLM and explicitly accounting effect of use of auxiliary fishing devices such as low temperature bait tank, bird radar, sonar, etc. Although this preliminary study showed relatively small increase in the use of these devices (in the order of ten to 20 percent), the complicated nature of the change in fishing efficiency was revealed. There appeared to be several factors that interacted with the change of fishing efficiency - the efficiency changed with time, area and shift of target species between skipjack and albacore.

Fitzpatrick (1996) estimated technology coefficients by major vessel types by decade from 1965 to 1995. For tuna purse seiners and longliners of 65m vessel length, these coefficients increased from 1.6 in the 1980 period (1976 to 1985) to 2.3 in the 1995 period. However, it was not explained in details how those coefficients were estimated. Three or five percent of annual increase in fishing efficiency due to technological improvement of fishing gears and associated devices has been assumed for French and Spanish Atlantic tuna purse seine fishery although the derivation of these specific values has not been well documented (ICCAT, 1999a). As for the tuna surface fisheries, especially purse seining, rapid and extensive use of artificial fish aggregating devices (FAD), which appears to contribute substantially to increases in fishing power, causes a serious problem in reliable estimate of the fishing effort of the surface gears (ICCAT, 1998).

A cursory review indicates that it is necessary to conduct basic studies to identify factors affecting the increase of fishing power by major tuna fisheries before development of methods to measure fishing capacity. Other important factors such as multispecies, multigear and international nature of the tuna fisheries should be recognized and somehow included in comprehensive methods for the measurement of fishing capacity.


3.1 Longline fishery

3.1.1 Carrying capacity vs number of hooks

In spite of the voluntary reduction of the number of the Japanese distant water longline boats that took place in the early 1980s, increases in the number of hooks used by each operation (on average a 20 to 30 percent increase) resulted in a net increase in the total number of hooks used by this fleet segment (Suzuki, 1999). In fact, although the total number and total carrying capacity of the Japanese distant water longline boats has shown a decreasing trend, the total amount of hooks used by those boats has shown an increasing trend.

For Taiwanese distant water longline fishery, the number of boats and carrying capacity has been increasing but the increase rate of total number of hooks is much more rapid than that of the number of the boats or carrying capacity (Dr. S. K. Chang, personal comm.). This implies that an increase of the number of hooks per operation has occurred also for the Taiwanese boats. Therefore, the total number of hooks used is a better index of fishing capacity for this type of fishery.

3.1.2 Improvement of gear technology

According to the technology coefficient reported by Fitzpatrick (1996), large sized longline boats have increased their fishing power by more than a factor of two during the past 10 years. Since detail of the derivation of this value is not explained, it is impossible to use this value for any specific use. There are no analyses available explaining the technological improvement of fishing efficiency on tuna longline fisheries.

Although, generally speaking, increases in fishing efficiency may occur with the tuna longline fisheries, it is likely that the rate of increase may not be as great as for active fishing gears such as purse seine gear, for example, as the longline method is, by comparison, passive. At any rate, it is recommended that relevant studies of fishing efficiency be undertaken for the longline fishery, considering such technologies as the age of the boats, satellite information on sea and weather conditions and GPS. Until such time as when the more relevant information become available, total number of hooks appears, by default, to be the best indicator of fishing capacity for this fishery.

3.1.3 Multispecific nature

The longline fishery is essentially a multispecies fishery. This makes measurement of species-specific fishing capacity difficult because the efficiency of longline gear is different depending on the species targeted. For example, Japanese distant water longline boats targeting bigeye in the tropical water especially use so called deep longline to set hooks deeper for the purpose of taking more efficiently the deep swimming bigeye. This commonly used method has opposite effect in fishing efficiency for surface fishing species such as marlins because the hooks are placed mostly out of their vertical habitat. Standardization or measurement of fishing capacity of longline fishing effort should be made species specific to avoid capturing possible false signal from stocks utilized.

3.2 Purse seine fishery

3.2.1 Factor affecting fishing efficiency

In the Workshop on Abundance Indices from Tropical Tuna Surface Fisheries (ICCAT, 1998), various factors affecting fishing efficiency were discussed. Although a summary table of the various factors with time series information was shown, no follow-up studies to take these factors into account in the analysis of abundance indices have been made yet.

A study[72] was initiated to apply the GLM method to the Japanese tuna purse seine boats operating in the western tropical Pacific. The aim of the project was to consider several of the factors in abundance indices that appear to have significant effect on fishing efficiency of the tuna purse seine fishing. Major factors selected for the study include bird radar, sonar, school type, net size, power block, purse winch, age of ship, GPS, etc., along with the usual factors such as time, area and year effects. Preliminary results from the study suggest that several factors have highly significant effects on CPUE, although these results are not definitive due to the complicated nature of the analysis and the use of only Japanese boat data. Among the significant factors, it was noted that type of schools (schools associated with floating objects vs free swimming schools) was one of the highest factors that affect CPUE both for yellowfin and skipjack, with higher CPUE being found for sets on schools associated with floating objects. This has a significant implication regarding recent development of the FAD (fish aggregation devices) operations in measuring fishing capacity of the purse seine fleets, as will be further discussed later in this paper.

3.2.2 Use of FADs

The use of FADs has had a dramatic effect on the fishing efficiency of the purse seine fleet. As will be mentioned later, this practice has implications not only for the purse seine fishery but also for the longline fishery. FADs were generally introduced into purse seine fisheries around the start of the 1990s. The introduction took place on a worldwide scale with only a minor difference in the starting year and magnitude of deployment by the different fishing nations.

There are two major advantages of using FADs - creation of new fishing grounds where no opportunities of successful fishing existed in the past; and increases in catch rates within the current fishing grounds due to high successful set rates compared to that for free swimming schools[73]. New FAD fishing grounds, formed outside of the current fishing grounds in the tropical waters, usually produce congregations of juvenile tunas, i.e. skipjack, yellowfin and bigeye. Therefore, since the introduction of FAD fishing, catches of these three species in the Atlantic and Indian Ocean tuna purse seine fishery has increased by a factor of three for bigeye and 1.5 for the other two species (ICCAT, 1999a; IOTC, 1998) despite of relatively stable carrying capacity after mid 1980s (Suzuki, 1999). In the eastern Pacific IATTC area, the overall carrying capacity of purse seine boats has been more changeable in the past decade. However, catches per ton by species show similar magnitude of increase for bigeye and skipjack (IATTC, 1998). As for yellowfin, however, there appears to be no appreciable change in catches per ton in the IATTC area before and after the FAD operations. Although the reason for this is unknown, this might be related to the dolphin regulations in that area.

In the western Pacific, the FAD operation by the purse seine boats has not been as widespread as in other Oceans. However, the FAD operations have increased substantially from 1996, especially for the US boats, which increased the bigeye catch by purse seiners to a record high in 1997. The use of FADs was maintained in 1988, although the bigeye catch declined (Hampton et al., 1999).

3.3.3 Multispecies nature

How to manage mixed species with different stock exploitation conditions is a common problem in fisheries. Bigeye is by-catch for purse seine boats, and has only minor share in the total purse seine catch, which is dominated by yellowfin and skipjack. However, the use of FADs caused concerns for management of world bigeye stocks that have already been overfished. On the other hand, skipjack stocks appear to be either underexploited or moderately exploited and yellowfin stocks either moderately exploited or fully exploited except for western Pacific stock (Suzuki, 1999). As far as the FAD operations are concerned, it is not possible at present to avoid bigeye catch.


There is no substantial update for stock status of the tuna and tuna-like fish from the summary given by Suzuki (1999). However, a few new management measures have been introduced recently. Quotas on yellowfin tuna were resumed in 1998. In 1999, IATTC introduced regulations to prohibit the use of FADs by purse seine boats after 40 000 tonnes of bigeye had been caught. In addition, the previously voluntary time/area closure by purse seiners for the use of FADs became mandatory for the contracting countries. Prohibition of FAD fishing was proposed by the tropical tuna group of the IOTC to reduce exploitation on bigeye stock in the Indian Ocean.

The western and central Pacific yellowfin and skipjack stocks are considered to be underexploited. However, some concern has been expressed about rapid increase of exploitation rate (up to about 0.4) in nursery ground of yellowfin in the Philippines water (Hampton et al., 1999). In the Atlantic, some concern was raised for possible local overexploitation of the skipjack (ICCAT, 1999b).


While the purse seine vessels target less heavily exploited stocks such as skipjack and yellowfin, it should be noted that the FAD operation per se could give much higher potential to purse seine fishing capacity than previously thought, as was demonstrated in the various part of the Oceans. In addition, an urgent problem that needs to be addressed is the assessment of the impact of juvenile bigeye catch on the stock and on the longline fishery targeting mostly adult bigeye.

As purse seine and longline take different size of bigeye, the impact of the two fisheries on the stock will be different. Some conversion factors are needed to calculate the impact to be used as a single value. One way is to calculate the impact of the respective catches on the spawning biomass. In this case, it is obvious that taking juvenile by purse seine boats has higher impact than by longline boats. The magnitude of that differential impact depends on value of age specific natural mortality (M) and ages to reach maturity. Unfortunately, no reliable estimates of age specific M is available. Therefore, it is urgent to address this deficiency.

Furthermore, highly mobile purse seine fleet leave the fully exploited Atlantic, Indian and Eastern Pacific and migrate to the western and central Pacific where the stock status of target species is healthy. The MHLC, an international negotiation body for establishing management measures for highly migratory species in the central and western Pacific by 2000, issued a resolution urging several actions to be taken. Above all, they request that all states and other entities refrain from increasing fishing effort and capacity within that region.

Therefore, it is recommended that the current fishing capacity of the distant water purse seiners should not be increased as a whole and specifically for bigeye, reducing or at least capping juvenile bigeye catch by the use of FADs is desirable.


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Fitzpatrick, J., 1996. Technology and Fisheries Legislation, In Precautionary approach to fisheries, Part 2: Scientific papers, FAO Fisheries Technical Paper 350/2. Rome, FAO. pp. 191-199.

Gascuel, D., Fonteneau, A. & Foucher, E. 1993: Analyse de l'évolution des puissances de pêche par l'analyse des cohortes: application aux senneurs exploitant l'albacore (Thunnus albacares) dans l'Atlantique Est. Aquatic Living Resources, 6(1): pp.15-30.

Hampton, J., Lewis, A. & Williams, P. 1999. The Western and Central Pacific Tuna Fishery 1998, Overview and status of stocks, Oceanic Fisheries Programme, Tuna Assessment Report No.1, 39 pp.

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ICCAT, 1999b. Report of the ICCAT SCRS skipjack stock assessment session, Funchal, Madeira, Portugal, 28 June to 2 July 1999, ICAT Report SCRS/99/21, 52 pp.

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Kirkley, J.E. & Squire, D. 1999. Measuring Capacity and Capacity Utilization in Fisheries. In: Gréboval, D (Ed). Managing Fishing Capacity: Selected Papers on Underlying Concepts and Issues. FAO Fisheries Technical Paper No 386. Rome, FAO. pp. 75-200.

Newton, C. 1999. Review of issues for the control and reduction of fishing capacity on the high seas. In: Gréboval, D (Ed). Managing Fishing Capacity: Selected Papers on Underlying Concepts and Issues. FAO Fisheries Technical Paper No 386. Rome, FAO. pp. 49-74.

Pella, J.J. & Psaropulos, C.T. 1975. Measure of tuna abundance from purse-seine operations in the eastern Pacific Ocean adjusted of fleet-wide evolution of increased fishing power, 1960-1971. Inter-American Tropical Tuna Commission Bulletin, 16: pp. 281-400.

Shono, H. & Ogura, M. 1999. The standardized skipjack CPUE, including the effect of searching devices, of the Japanese distant water pole and line fishery in the Western Central Pacific Ocean, document presented to the ICCAT Skipjack Assessment Meeting, 18 pp.

Suzuki, Z. 1999. Review of fishing capacity deployed on tuna and tuna-like fish fisheries, particularly for tuna longline fishery, National Research Institute of Far Seas Fisheries Bulletin, 36: pp. 33-45.

Uozumi, Y. 1999. Review of problems on stock assessment of marlins laying stress on the coverage of landing and catch and effort information in the Pacific Ocean. SCTB12 BBRG-6, 11 pp.

[70] National Research Institute of Far Seas Fisheries.
[71] See also ICCAT (1999).
[72] Undertaken by scientists from the National Research Institute of Far Seas Fisheries, Japan.
[73] No substantial difference were noticed in catch rates between the two types of schools

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