A REVIEW OF THE BIOLOGY AND FISHERIES FOR SKIPJACK TUNA, KATSUWONUS PELAMIS, IN THE PACIFIC OCEAN (contd.)

8. OCEANIC FEATURES ASSOCIATED WITH THE SPECIES

The easterly trade winds drive the principal circulatory pattern in the upper waters of the Pacific (Figure 9), but the pattern is not symmetric with respect to the equator (Pickard, 1968). Of the westward flowing currents, the principal component of the SEC lies between 8°–10°S and 3° N, while the NEC is located between 8°–20° N. The eastward-flowing NECC is situated between these currents, and the generally weaker SECC, normally beneath the surface and under the SEC (IATTC, 1984; Philander, 1990), is also eastwardly directed. This major system of latitudinal flow extends approximately 12,000 km from the Gulf of Panama almost to the Philippines. Of the three major subsurface currents, the core of the Cromwell Current is located in the thermocline during normal, or non-El Niño episodes, and it extends westward from the Galapagos Islands for about 10,000 km. Under normal conditions the sea-surface temperature decreases from west to east, and the depth of the thermocline diminishes from about 150 m in the western Pacific to 40 m or less in the eastern Pacific (Philander, 1990). The distribution of larvae (Section 3), spawning activity (Section 5.2) and migration (Section 6.3) in relationship to these currents have been described earlier.

During El Niño episodes the southeasterly trade winds weaken in the eastern part of the South Pacific, and the tropical currents just described, including the California and Peru (Humboldt) Currents, change their usual positions and strengths (IATTC, 1984). In the eastern Pacific the sea surface rises in response to the decrease in the hydrospheric circulation rate, and coastal and equatorial temperatures also increase. As a result, the thermocline deepens and a flow of warmer surface waters extend over the normally upwelling regions off Ecuador and Peru (Miller and Laurs, 1975; Joseph and Miller, 1988). In addition to a weakening of the SEC, the SECC frequently emerges at the surface between 5°–10°S, and under particularly strong El Niño conditions, such as in the last quarter of 1982 and into 1983, a weakened Cromwell Current may be difficult to locate in transect data at the equator. Schaefer (1961) noted that during the El Niño of 1957–59 skipjack catches declined in the normally productive areas near the Gulf of Guayaquil, Ecuador, and the Costa Rica Dome (centred at 9°N–90°W), but increased substantially off Peru. Warming trends in sea-surface temperatures (SST) were thought to be responsible, but Forsbergh (1989) showed that the SSTs and catches per unit of effort (CPUEs) in the areas were uncorrelated. The intense fishery in the Gulf of Guayaquil lies in the thermal front between the cold, upwelling waters of the Peru Current and the warmer waters north of the equator. The SSTs in the front change rapidly with latitude and position, and the intensity of the front varies with the season and year. Strong upwelling of the Peru Current produces a strong front, and it was thought to induce the formation of concentrations of skipjack forage. However, from 1961 to 1984 the logarithm (log) of CPUE versus SST was significantly correlated only in the first and fourth quarters (Forsbergh, 1989). In the northern part of the Panama Bight the abundance of skipjack appears to be related to upwelling which occurs from January to April; a period of forage production is then followed by a peak in skipjack catch rates in April and July (Forsbergh, 1969).

In terms of vertical distribution, schools of skipjack have been detected at depths of 140 m (Kimura et al., 1952) and 120 m (Yamanaka et al., 1966) by means of echo sounding devices, and at 98–152 m in Hawaiian waters by submarine (Strasburg et al., 1968). Sonic tracking revealed that skipjack move between the surface and 263 m during the day, but remain within 75 m of the surface at night (Dizon et al., 1978). In general, the vertical distribution is limited by the depth profile of the temperature and oxygen concentration with minimum values of 18°C and 3.0–3.5 ml/l, respectively, needed for long-term survival (Forsbergh, 1980). Sharp (1978) considered the lower limits of 15°C and 2.5 ml/l oxygen more appropriate. The upper temperature limit decreases with increasing size from 30°C or more for small fish to as low as 20°C for large fish (Barkley et al., 1978). For these reasons young skipjack can tolerate the conditions in tropical surface waters, but the adult habitat is in the thermocline. As young fish grow they are forced to seek well-oxygenated waters associated with cooler, less stressful temperatures.

It is currently believed that the skipjack fishery in the EPO is supported principally by the immigration of recruits spawned earlier in the central Pacific and, possibly, in the western Pacific. The SST in this spawning area apparently has an effect on the catches in the EPO. Forsbergh (1989) found a positive correlation between the SST 18 months earlier and logCPUE for baitboats during the 1934–1960 period, and the logarithm of the catch rate of recruits (numbers per day) for purse seiners from 1961–1984. A wind-mixing index calculated as the cube of the wind speed 18 months earlier in the spawning area was also inversely correlated with logCPUE for purse seiners for the same period. The hypothesis supporting this negative relationship is that increased wind conditions can disrupt the stability of the mixed layer, thereby dispersing the food aggregations that support survival (Lasker, 1975; Owen, 1981a, 1981b). On the whole, though, attempts to relate the apparent abundance of skipjack with environmental conditions suggest that the relationship is complex and not obvious (Forsbergh, 1989). Some of the problems may be resolved if studies could be carried out on all population units that contribute to the fishery in the EPO rather than the portions which contribute only for part of their life span (IATTC, 1991).

9. INTERACTION WITH OTHER SPECIES

For purposes of potential management information; Orange et al. (1957) and Broadhead and Orange (1960) studied the frequency with which skipjack were caught together with yellowfin in mixed schools in the EPO. From baitboat data it appeared that about 51 percent of the yellowfin and 29 percent of the skipjack tonnages were caught in mixed schools, and for purse seiners the percentages were 6 and 16, respectively. These last figures are probably more representative of the fishery in the EPO today since it is now dominated by purse seiners. On average, Broadhead and Orange (1960) found that the sizes of skipjack from pure and mixed schools were similar. On the other hand, although yellowfin and skipjack in mixed schools were also alike, yellowfin in pure schools tended to be larger and more variable in size.

In the WPO, both skipjack and yellowfin are caught by purse seiners more frequently in mixed schools than is the case in the EPO. From all purse-seine data held by SPC, 37 percent of yellowfin and 47 percent of skipjack tonnages have been caught in mixed schools (SPC, unpubl. data). This is probably because of the greater emphasis placed on fishing schools associated with logs, or floating objects, and the virtual absence of dolphin-associated schools in the WPO.

The implication of “mixed” schools is that yellowfin and skipjack exist in composite aggregations. Yuen (1963) examined this concept by means of underwater observations and photography in a baitboat fishery. He found that the species maintained a significant degree of separation into schools or species groupings, and the random response of individuals from either group to the “chum”, or bait, created the illusion of complete mixing in the catch. While skipjack and yellowfin may therefore prefer to school by species, their co-occurrence is probably the result of a mutual attraction to an external stimulus, such as food.

10. GENERAL DESCRIPTION OF THE FISHERIES

10.1 Eastern Pacific

During the early part of the 1950s, the surface fishery for tunas in the eastern Pacific was conducted primarily by baitboats in coastal waters and near offshore islands. Following the conversion of most of the larger vessels to purse seiners in the later part of the decade and early 1960s, the fishery began to expand offshore. By 1968 the fleet had penetrated the eastern Pacific as far west as 115°W, and by 1974 tuna were being caught near 150°W (Calkins, 1975; Peterson and Bayliff, 1985). Together with this expansion, the number of baitboats declined from 204 to 36 during the 1950–1988 period, and their capacity decreased from about 36,300 to 2,800 mt. At the same time the number of purse seiners increased from about 68 to 182 together with a capacity increase from 7,200 to 121,800 mt. With the exception of the region surrounding French Polynesia, the fishery in the EPO occupies the area to the east of 150° W between the latitudes of 39°N and 35°S. Besides baitboats and purse seiners, smaller amounts of skipjack are also caught near the equator and south of it by coastal dayboat operations involving jigboats and bolicheras. By 1990 approximately 10 nations had been involved in the surface fishery for this species.

The skipjack fishery in the EPO is concentrated primarily in two areas. The most important fishery in the south is east of 100°W and near Central America, northern South America, Cocos Island-Brito Bank and the Galapagos Islands. The secondary, northern fishery is west of 100°W and is near Baja California, the Revillagigedo Islands and Clipperton Island. With rare exceptions, the cell of warm water off southern Mexico that divides the two fishing regions is devoid of commercial quantities of skipjack. In the northern area the largest seasonal catches are taken during June to October, with a peak in August. The fishery in the southern region lasts all year and peaks can occur late in the year, usually in the fourth quarter. Overall, the fishery for skipjack tends to take place closer to shore than for yellowfin, but sporadically in recent years, such as in 1981, 1983–84 and 1988–89, increasing amounts of catches have been made further offshore. Catches and relative abundance in the south are generally twice as large as in the north, and the quantity caught can vary considerably from year to year (Figure 10). During the late 1950s large catches of skipjack were made south of 5°S, and from the late 1950s to the early 1970s large amounts were taken close to shore off Ecuador and northern Peru. During the early 1970s, however, the centre of abundance of the southern group appeared to shift towards the waters farther off Colombia and Central America. Beginning in the early 1980s the centre seemed to move back to Colombia and Ecuador. Compared to the average annual catches during 1979–87 (Figure 7a), the most substantial catches in 1989 were made off Central America and northern South America. The size composition of skipjack samples collected in the EPO during 1984 through 1988 (Figure 11) demonstrate a relatively narrow range of intermediate-sized fish, and support the hypothesis that the fishery is sustained by immigration and temporary residence of fish from outside the EPO.

10.2 Western Pacific

The tuna fisheries of the WPO are extremely diverse and might be conveniently categorized into fisheries based in island countries, fisheries operated by foreign-based, distant-water fishing nations (DWFNs) and fisheries based in Southeast Asia, primarily the Philippines and eastern Indonesia. In many Pacific island countries, tuna are captured by artisanal fishermen for consumption and, where markets are accessible, for sale. Skipjack, and to a lesser extent juvenile yellowfin, are the main species taken, mostly from small boats by trolling and poling, using lures made from mother-of-pearl shell. These artisanal tuna fisheries are of most significance to the smaller island countries, where shortage of land limits both agricultural production and exploitation of inshore marine resources. Despite their social importance, artisanal tuna fisheries are relatively minor in terms of total catch. Although reliable statistics are not available, total tuna catches by artisanal fishermen from Pacific islands are estimated to be less than 10,000 mt annually (Doulman and Kearney, 1987).

Most of the domestic tuna fishery development in the Pacific Island countries has taken place with the assistance of foreign countries, most notably Japan, either in the form of joint venture arrangements or as aid packages. A domestic baitboat fishery in Papua New Guinea began in 1970 in joint venture with Japan, averaging nearly 30,000 mt annually between 1971 and 1981 (SPC, 1983) before unfavourable economic conditions resulted in its termination. The other major domestic tuna fishery in the region is based in the Solomon Islands. This fishery began in the early 1970s, again as a joint venture with Japanese interests, and currently consists of a fleet of small baitboats and several purse seiners. The total tuna catch from the fishery was approximately 30,000 mt in 1990, about 80 percent of which was skipjack (SPC, 1991). Smaller domestic tuna fisheries are currently based in Kiribati, Fiji, and French Polynesia (baitboat) and in Tonga and New Caledonia (longline).

Figure 10. Estimated catches of skipjack in the EPO north and south of 15°N. (After IIATTC, 1991).

Figure 11. Estimated length composition of the skipjack catch by 1-cm intervals in the EPO. The values in the upper left corners of the panels are average weights. (After IATTC, 1991).

Distant-water, commercial tuna fishing in the WPO dates back to the early 1920s, when Japanese baitboat fleets began to establish themselves in Micronesia (Doulman, 1987). The commercial fishery for skipjack in Japanese home waters operated well before this time. During the early 1950s, Japan re-established its distant-water fishing capability, which had been severely depleted during World War II, with baitboat fishing in Micronesia and longlining throughout the tropical WPO. In the late 1970s, baitboat fishing began to decline and was replaced by purse seining, initially by Japan and the USA, and more recently by the Republic of Korea, Taiwan, Philippines, and Indonesia. The distant-water purse-seine fleet is estimated to have caught approximately 700,000 mt of tuna in 1990, of which 520,000 mt was skipjack (SPC, 1991). This compares with 76,000 mt of skipjack taken by the Japanese distant-water baitboat fleet.

In the far western area, fisheries for small skipjack, yellowfin, and other scombrids have been developing in the Philippines and eastern Indonesia since the early 1970s. Tuna are captured by poling, trolling, handlining, ringnetting, purse seining, and other methods, mostly in association with FADs. In 1990, domestic fisheries in the Philippines and eastern Indonesia are estimated to have caught approximately 165,000 mt of skipjack (SPC, 1991). The total skipjack catch in 1990 in the WPO, including eastern Indonesia and the Philippines, is therefore estimated to have been in excess of 780,000 mt (SPC, 1991).

These fisheries are concentrated in tropical waters at 10°N–10°S (Figure 7b), although seasonal skipjack fisheries exist in waters adjacent to Japan, southeastern Australia, and the North Island of New Zealand. Comprehensive size composition data are generally not available, although it is generally held that size tends to increase from the far west, where large numbers of juvenile skipjack are caught in association with FADs, to the east.

11. TRENDS IN FISHING EFFORT, CATCH, AND CATCH-PER-UNIT OF FISHING EFFORT

11.1 Eastern Pacific

The trends in the catches of skipjack in the EPO (Figure 10 and Table 5) demonstrate that events in the two major fishing regions do not necessarily parallel one another. Substantial differences occurred in 1964–65, 1970–71, 1975, and in 1978–80, but since then the trends have been essentially similar. Part of the differences can be attributed to the major shifts in the location of the fisheries mentioned in Section 10.1. However, it is also thought that the increased catches in one region and the decline in the other may be due to the influx of recruits from the central Pacific favouring different regions (IATTC, 1978). During the 1970s the fishery also began to concentrate on small skipjack and yellowfin associated with logs. This was particularly evident in the fishery south of 15°N where the catches in 1975, 1976, and 1978 reached new levels of 107.6, 107.2, and 153.0 × 10³ mt, respectively. The decline in catches prior to and during the severe El Niño of 1982–83 was also more pronounced in the southern fishery as the vulnerability of fish diminished. Following the recovery of the fishery that began in 1984, the catches in both regions have increased again to the average levels attained during the 1960s and early 1970s.

Historically, the IATTC has progressively modified the standard against which the effort expended in the fishery is gauged. Originally, the standardization was made to Class-4 (182–272 mt capacity) baitoats when this type of gear dominated the fishery during the 1950s. Following the rapid conversion of the fleet to purse seiners by the early 1960s, Class-3 (92–181 mt) seiners became the new standard that persisted until 1967. By 1974, as a result of increased vessel construction that began in the mid-1960s, the fleet capacity of Class-3 seiners fell to 3 percent, while that of the most productive group, Class-6 (> 363 mt), rose to 81 percent. By 1983, Class-6 seiners represented at least 90 percent of the fleet and they were adopted as the new standard. Currently, raw Class-6 effort is used as the measure of effort in determining the catch per day's fishing (CPDF) without taking into account the minor contribution of smaller vessels. In addition, adjustments introduced in 1971, to account for improvements in fleet efficiency of vessels below Class-6, were discontinued when this class became the new standard.

The IATTC staff utilizes the sums of the logged catches (C) and effort (E) to derive the CPDF, and formerly the standardized CPSDF, as an index of the apparent abundance of skipjack. It is recognized that there are at least two problems associated with this approach. First, an index calculated as ΣC/ΣE, rather than as the average C/E weighted by area, i.e., C/E, is biased if the effort is not distributed equally over the fishing grounds (Griffith, 1960; Calkins, 1961). However, the early studies of Calkins (1961) and Joseph and Calkins (1969) indicated that, although the values of ΣC/ΣE over time tended to be both larger and more variable than C/E, the trends were quite similar. For this reason the CPSDF and CPDF are used directly as simple indices of abundance without weighting (Figure 12). Another application of the unweighted and weighted indices involved the ratio of (ΣC/ΣE)/(C/E) as a concentration index (CI). Values greater than one supposedly indicate that the fishermen were able to concentrate their effort in areas with above-average densities of fish. During most of the 1960s, the CI values for skipjack were close to two and approximately twice as large as those of yellowfin (IATTC, 1968), thus indicating that skipjack were vulnerable more consistently in recognized areas than yellowfin. Despite the seemingly useful properties of CIs, however, the IATTC staff discontinued their use in 1969 because of the statistical ambiguity of ratios of ratios, and the difficulty in interpreting the meaningful effects on the fishery of indices greater or less than one.

The second problem concerning the index of abundance is that while the catches refer to skipjack alone, the effort represents that directed towards both yellowfin and skipjack. This situation arises because of the difficulty in obtaining a rational separation of effort in a fishery that essentially involves two species, yellowfin and skipjack, but is primarily directed towards yellowfin. To determine the relative apparent abundance in different years in such a situation, Joseph and Calkins (1969) first identified core regions of skipjack effort in the northern and southern fisheries in which the catches in 1° areas were at least 50 percent skipjack in successive years. These baseline regions were augmented with additional 1° areas of high catch specific to different years (i) before determining relative values of (C/E)_i Similarly, and more recently, Forsbergh (1989) identified 22, 5° areas in which most of the skipjack were caught in the 1961–76 period before calculating the relative CPDFs. Some of the important conclusions that emerge from these studies and that of Calkins (1961) are that skipjack catches in successive years are highly variable (Table 5), sometimes differing by a factor of three, but the changes in catch do not appear to be related to effort in preceding years. In fact, and in contrast to yellowfin, a plot of CPDF versus days fishing (Figure 13) indicates that the CPDF is independent of effort. In this situation, considering that skipjack recruitment is derived from outside the EPO, the CPDF is probably indicative of apparent abundance in only the EPO, and is not useful for predicting future catch rates. For this reason the CPDF for skipjack may be more usefully interpreted as an economic indicator of fishing success. Caution is also urged in interpreting the CPDF values in Table 5 and Figure 12 because of two biases: the restricted, coastal operating range of baitboats during the 1960s, and the tendency of the fleet to concentrate on yellowfin in recent years.

Figure 12. Catch per days fishing for yellowfin and skipjack in the EPO in Class-6 pure-seine units. The 1988 data are preliminary. (After IATTC, 1989).

Figure 13. Plot of catch per days fishing (CPDF) and effort for skipjack in the EPO using data for all 5° areas. (After IATTC, 1985).

TABLE 5. Historic catches of skipjack in the EPO by the surface fleet, total effort, catch per days fishing (CPDF), and carrying capacity and number (No.) of vessels participating in the fishery. (Adapted from IATTC, 1991).

* Effort for all surface vessels in class -6 (>363 mt) purse-seine units
@ For Class-6 purse seiners
# All data for 1989 are preliminary

11.2 Western Pacific

The overall catch trend for skipjack in the WPO is shown in Figure 14. The catch has been increasing at a rapid and fairly constant rate since the early 1970s (SPC, 1991). A number of events have fueled this expansion. In the 1970s, the development of baitboat fisheries in Papua New Guinea and the Solomon Islands and the expansion of the Japanese distant-water, baitboat fishery led to the first large increases. In the 1980s, the development of large-scale purse seining in the WPO and the subsequent influx of vessels from several DWFNs resulted in further increases. In the 1990s, the policy of many canneries not to buy tuna caught in association with dolphins has resulted in the relocation of many purse seiners to the WPO, where problems of dolphin-associated tuna schools do not exist. This, along with tuna fishery development in Southeast Asia, has brought about further increases in the skipjack catch in the WPO. This trend is likely to continue for the next few years at least.

Figure 14. Catch trend for skipjack in the WPO. (After SPC, 1991).

The SPC routinely monitors total CPDF for both the baitboat and purse-seine fisheries for which it receives data. To date, abundance indices such as those described in the previous section for the EPO have not been routinely derived, although a preliminary study was reported in the SPC (1990). Here, indices were constructed using (C/E)_j for Japanese purse seines, where the strata, j, were defined as areas of 2° of latitude by 5° of longitude within the overall area of 10°N–6°S, 130° – 165°E, and time periods of either one or three months. These indices, along with raw CPDF, (ΣC/ΣE), suggest similar, steadily-increasing trends in apparent abundance (Figure 15). This is also true of CPDF in the Japanese distant-water, baitboat fishery. Whether these trends are real, i.e. they reflect skipjack abundance, or result from increasing fleet efficacy or other factors, has not yet been determined. In either case, it appears that the abundance of skipjack in the WPO is substantially higher than in the EPO, based on the observation of CPDF levels in the WPO 3–4 times those in the EPO (Figures 12 and 15) (although the effects of yellowfin targeting may bias such comparisons to some extent).

As in the EPO, similar difficulties exist in determining effective effort on skipjack in a purse-seine fishery that also targets yellowfin. In contrast to the EPO, however, calculated concentration indices suggest that the distribution of effort of at least Japanese purse seiners is influenced more by the apparent abundance of skipjack than that of yellowfin (SPC, 1990).

12. POPULATION DYNAMICS

If the skipjack fishery in the EPO is open to both immigration and emigration, as is currently believed, then it is doubtful whether stock production modelling or cohort analysis can produce useful results. Forsbergh (1987) drew attention to this point when values of F, and consequently recruitment estimates, failed to converge after applying Pope's (1972) cohort analysis to quarterly catches and various starting values and attrition rates. On the other hand, by incorporating information on growth rates, a length-weight relationship and approximate attrition rates into yield-per-recruit (Y/R) analysis, some insight can be gained into the dynamics of the fishery. For example, the yield-per-recruit isopleths in Figure 16 were constructed using a linear growth rate of 24 cm/yr (Forsbergh, 1989) and Hennemuth's (1959) length-weight formula from Table 2. The instantaneous attrition rates M ′ = 1.5 (upper panel) and M ′ = 2.0 (lower panel), which include both natural mortality and emigration, correspond to current estimates of the annual attrition rates of 78 and 86 percent, respectively. The effort levels represent multiples of the 1986–87 average which is set equal to one. The importance of the attrition rate with respect to the Y/R is apparent in that an increase of M ′ from 1.5 to 2.0 reduces the Y/R by nearly half. At the lower value of M ′ and recent effort levels there is also a marginal improvement in the Y/R by delaying the size at first capture until 45 cm, but at the higher attrition rate the Y/R is nearly constant in the size range from 30–40 cm. In general, fishing effort considerably greater than or sizes of entry considerably less than has been the case so far would be required to overfish the stock in the EPO in the Y/R sense (IATTC, 1991). The reason is that losses to the total weight of a cohort due to M ′ exceed the gains to it by growth, even when the fish are less than 50 cm and growing rapidly. In summary, the model does not indicate the need for management of the skipjack fishery in the EPO at present levels of effort. However, it is apparent that at the higher attrition rate in Figure 16 increased effort eventually leads to a decline in the marginal return of Y/R. Conceivably, high levels of effort could reduce recruitment in subsequent years, but this seems unlikely from what is known of the spawning behaviour and population structure.

Figure 15. Various indicies of skipjack apparent abundance based on CPDF in the Japanese purse-seine fishery. “CPUE” refers to unadjusted CPDF, “Month” refers to average CPDF stratified by month and 2° latitude by 5° longitude rectranglar areas, and “Quarter” refers to average CPDF stratified by quarter and 2° latitude by 5° longitude recatangular areas. (After SPC, 1990).

Figure 16. Yields (lbs) per recruit for skipjack with multiple levels of fishing effort in 1986–1987 (=1), length at entry into the fishery, and the sum of natural mortality and emigration (M ′ ). (Adapted from IATTC, 1991).

In the WPO, comparable modelling of skipjack dynamics has not been carried out, due primarily to the lack of representative size composition and total catch and effort data, although this situation is gradually being rectified. Most studies relating to population dynamics and stock assessment have been based on the SSAP tagging data. The SPC produced a series of reports during the early 1980s that described the dynamics of skipjack stocks in the Exclusive Economic Zones (EEZs) of all its South Pacific member countries. In all, twenty such reports were produced along with a more general report dealing with the aggregate data set (Kleiber et al., 1983; 1987). In the latter, total standing stock of skipjack in the WPO was estimated at 3 × 10⁶ mt, with a total throughput of 6.2 × 10⁶ mt per year. The overall harvest ratio (proportion of total attrition due to fishing mortality) was estimated at 0.04, while harvest ratios for seven subareas for which catch and effort data were available ranged from 0.02 to 0.46; only one exceeded 0.17. Kleiber et al. (1987) concluded that “…low harvest ratios over most of the study area during the period tags were at large imply a potential for increased skipjack catches in many subareas and in the whole study area.” At that time the total skipjack catch in the WPO was approximately 230,000 mt per yr.

13. INTERACTIONS

During the 1950s and early 1960s, baitboats and purse seiners frequently competed for tuna resources in nearshore waters in the EPO. However, purse seiners make up over 97 percent of the current fleet capacity (Table 5), and gear competition for skipjack in particular is virtually nonexistent in the EPO. Similarly, purse seiners now account for most of the catching capacity in the WPO; in 1990, purse seiners caught 66 percent of the estimated total skipjack catch of 785,000 mt (SPC, 1991). However, the possibility of interaction between purse-seine and baitboat fisheries is of concern in countries like the Solomon Islands and Fiji, where domestic fisheries have existed for some time. In the case of the Solomon Islands, purse-seine fishing has increased in recent years, and the SPC has been collaborating with the Solomon Islands Government in a tagging experiment designed to estimate the magnitude of the purse-seine/baitboat interaction. The results of the study suggest that the effect of purse-seine catches on baitboat CPUE at recent levels of catch (baitboat ≈ 25,000 mt per year, purse seine ≈ 6,000 mt per year) is slight (SPC, 1992).

Although gear interactions are largely minor, interactions among geographical areas, particularly EEZs, are of concern in some areas. In the WPO, the main area of operation of the purse-seine and baitboat fisheries is composed primarily of the largely contiguous EEZs of Philippines, Indonesia, Palau, Federated States of Micronesia, Papua New Guinea, the Solomon Islands, Nauru, Kiribati, and the Marshall Islands. The degree of interaction among areas such as these will be determined by the size of the areas, the distances between them, skipjack movement rates, the natural mortality rate, and the intensity of the fisheries (SPC, 1988). As noted in Section 6.4, there has been some controversy regarding movement rates of skipjack and their possible effects on spatially-separated fisheries. However, some specific analyses have been carried out. Kleiber et al. (1984), using SSAP tagging data, calculated a series of interaction coefficients based on the proportions of total throughput in receiver EEZs derived from immigration from donor EEZs. These results are summarized in Table 6. Most of the coefficients are low, indicating that under conditions prevailing when the SSAP data were gathered, there was generally little potential for fishery interaction. Not surprisingly, most cases of significant exchange occurred between adjacent EEZs. In particular, the results suggested that 37 percent of throughput in the Marshall Islands EEZ at the time of tagging resulted from immigration from Federated States of Micronesia. Relatively high interaction coefficients were also observed for Northern Mariana Islands ⇄ Federated States of Micronesia and to a lesser extent Palau → Federated States of Micronesia and Papua New Guinea ⇄ Solomon Islands, indicating some potential for fishery interaction between those countries. The only case of a relatively high interaction coefficient for widely separated areas was New Zealand → Fiji; however, this may have been an artifact of the timing of tag releases into the highly seasonal New Zealand Fishery (Argue and Kearney, 1983).

TABLE 6. Coefficients of interaction between fisheries operating in various countries and territories in the WPO. Receiver countries are listed at the top of the table and doner countries down the left margin. (After Kleiber et al., 1984).

This relatively simple representation of interaction does not explicitly specify the controlling factors noted above. Sibert (1984) derived a more rigorous method to estimate interaction between two countries and applied the method to Papua New Guinea and the Solomon Islands, both of which had substantial baitboat fisheries for skipjack at the time of the tagging project. Exchange rates between the two EEZs, losses from natural mortality and movement to other areas, the proportions that remained resident and lived and the proportions that were caught locally on a monthly basis were estimated. The Solomon Islands' stock was found to be relatively stable with a low rate of natural mortality and emigration (resulting in high survival) and low rate of movement to Papau New Guinea. The Papua New Guinea stock was found to be more dynamic with a higher rate of natural mortality and emigration (lower survival), but with a low rate of movement to the Solomon Islands. Sibert (1984) estimated from these results that an increase in the catch in either EEZ of 1,000 mt would result in a decrease in the steady-state catch of the other of only 1–3 mt.

Incomplete catch and effort data availability has, until now, hindered a more thorough analysis of skipjack movement dynamics, and its interpretation with respect to interaction, in the WPO. This situation is now being rectified through a new, collaborative project under the auspices of the FAO Expert Consultation on Interactions of Pacific Ocean Tuna Fisheries.

14. ACKNOWLEDGEMENTS

We would particularly like to thank Bill Bayliff, Kurt Schaefer, and Pat Tomlinson for generously sharing their insights on many interesting and controversial points in the manuscript. Tony Lewis and Albert Caton also provided valuable comments.

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