Gary T. Sakagawa
Southwest Fisheries Science Center
US National Marine Fisheries Service
P.O. Box 271
La Jolla, California 92038-0271 USA
ABSTRACT
The Western Pacific Yellowfin Tuna Research Group has been coordinating research on the population biology of yellowfin tuna of the central-western Pacific Ocean since 1989. One of the Group's objective is to study the interaction among fisheries in the region. In August 1994, the Group reviewed results of longline-purse seine fisheries interaction studies conducted by members. It concluded that the sizes of yellowfin tuna caught by the fishing gears overlap, at least some yellowfin caught and tagged in surface waters are later recaptured by longline gear, and the longline-purse seine fisheries interaction appears weak within the central-western Pacific as a whole. This weak interaction is largely because the exploitation rate is currently modest, averaging 0.20. The Group also concluded that some localities in the region are experiencing substantially higher exploitation rates than this average and may be experiencing significant fisheries interactions at the local level.
1. INTRODUCTION
The Western Pacific Yellowfin Tuna Research Group (WPYRG) is an informal organization of scientists and fisheries officers studying the population biology of yellowfin tuna, Thunnus albacares, of the central-western Pacific Ocean (CWP). The Group's purpose is to exchange information and data, plan and cooperate in collaborative research projects, foster a common understanding of the condition of the yellowfin tuna stock, and offer scientific advice on fishery management issues. One of the Group's objectives is to evaluate interactions among the various fisheries that catch yellowfin tuna in the central-western Pacific Ocean.
In 1990, the Group reduced this objective into research questions. The first question that the Group chose to address was, what is the level of interaction between longline and purse seine fisheries on a region-wide basis? The Group felt that once this interaction was better understood, research could then focus on other more local fisheries interactions using the experience gained from this first study. Reported in this document is the progress made by the WPYRG with the longline-purse seine fisheries interaction study. More detailed information on the Group's findings is found in WPYRG meeting reports (WPYRG, 1993; 1994a; 1994b).
2. SCALE OF STUDY
The study area adopted by the WPYRG is a region bound on the north by 40°N, on the west by Australia and 120°E, on the east by 150°W, and on the south by 40°S (Figure 1). Within this area, there are various types and sizes of fisheries that catch yellowfin tuna as a target and as a bycatch species. In 1993, the fisheries caught approximately 397,000 mt of yellowfin tuna. Most were taken with purse seine (62%) and longline (16%) gears; consequently, these gears are often suspected of having major effects on the yellowfin tuna population and on other fisheries in the area. Hampton (1994), for example, showed that out of the 35 cases of tuna fisheries interactions issues identified in the CWP, only three do not involve purse seine or longline gears (Table 1).
This suspicion is reinforced by data on sizes of yellowfin tuna in the catches. Although different sizes of fish are caught by the different gears, the range of sizes taken by purse seine gear overlaps sizes taken by other gears (Figure 2). The USA purse seiners, for example, typically catch yellowfin tuna from approximately 35 cm fork length (FL) to 150 cm FL. Japanese longliners catch primarily large yellowfin tuna at sizes that overlap those of the purse seiners in the range of 60 cm FL to 150 cm FL. Within this overlapping size range, the purse seiners catch 5 to 8 times as many yellowfin tuna as longliners. Philippine ring net gear, on the other hand, catches primarily small yellowfin tuna in the range of 15 cm FL to 50 cm FL. Fish caught by this gear overlap somewhat sizes taken by purse seiners, but not sizes taken by longliners.
3. LONGLINE - PURSE SEINE FISHERIES INTERACTION
The WPYRG investigated the interaction between the large-scale purse seine and longline fisheries to determine the extent and level of the fisheries interaction. Studies were undertaken using three methods: (1) correlation analysis using catch-effort statistics, (2) movement and tag-returns analysis using tag-recapture experiments, and (3) qualitative analysis using biological information.
The first method was applied to time-series of fisheries statistics, building on results from earlier studies (Suzuki, 1994). Using Japanese longline data, catch-per-unit of effort (CPUE) of yellowfin tuna were computed and time-series trends analysed. The CPUE for the region as a whole indicated a sharp declining trend between 1959 and 1973 followed by a significant upward trend until 1979 before declining again to a low level (Figure 3). The sharp decline in the 1960s occurred well before the build-up of the purse seine fishery and associated surface catches (Figure 3). The more recent decline, on the other hand, occurred during the period of expansion of the surface fishery as well as during the period when the Japanese longline fleet switched to targeting bigeye tuna (Suzuki et al. 1977). Recent analysis that corrects for this switch in target species produces a stable CPUE trend since 1989 in the Japanese longline CPUE (Miyabe, 1994). Thus, analysis of CPUE data has not demonstrated a region-wide interaction between the two gears for yellowfin tuna.
Figure 1. Western Pacific Yellowfin Tuna Research Group statistical areas.
Table 1. Catalogue of tuna fishery interactions issues in the western and central Pacific Ocean (from Hampton, 1994.)
|
Description
|
Species
|
Afecting fishery |
Affected fishery |
Class
|
||
|
Area |
Gear |
Area |
Gear |
|||
|
Large-scale interaction between PS and PL fisheries |
SJ |
WCPO |
PS |
WCPO |
PL |
A or C |
|
Interaction between DWFN surface fisheries based commercial surface fisheries
|
SJ.YF |
WCPO |
PS.PL |
ID |
PS.PL |
C |
|
SJ.YF |
WCPO |
PS.PL |
PH |
Various |
C |
|
|
SJ |
WCPO/PU |
PS.PL |
PU |
PL |
A,C |
|
|
SJ.YF |
WCPO/SB |
PS.PL |
SB |
PS.PL |
A,C |
|
|
SJ |
WCPO/KI |
PS.PL |
KI |
PL |
A,C |
|
|
SJ |
WCPO |
PS.PL |
FJ |
PL |
C |
|
|
SJ |
WCPO |
PS.PL |
PF |
PL |
C |
|
|
Interaction between industrial fisheries and artisanal/subsistence fisheries |
SJ,YF.BE |
WCPO |
PS, PL LL |
All Pac. Is. & SE Asian countries |
Various |
A,C |
|
SJ.YF |
SB |
PS.PL |
SB |
TR.HL |
A,C |
|
|
SJ |
KI |
PL |
KI |
PL TR. HL |
A |
|
|
SJ.YF |
ID |
PL.PS |
ID |
TR.HL |
A,C |
|
|
SJ.YF |
PH |
PS |
PH |
Various |
A,C |
|
|
Interaction between DWFN fisheries in different EEZs
|
SJ.YF. |
EEZs in |
PS. PL.LL |
EEZs in |
PS.PL.LL |
C |
|
BE.AL |
WCPO |
|
WCPO |
|
|
|
|
Interaction between different fleets in Solomon Islands |
SJ.YF |
SB (non- archipelagic) |
PS(Co.#l) |
SB (non- archipelagic) |
PS (Co. #2) |
A |
|
SJ |
SB (archi- pelagic) |
PL(Co.#l) |
SB (archi- pelagic) |
PL (Co. #2) |
A |
|
|
SJ |
SB (non- archi pelagic) |
PS |
SB (archi- pelagic) |
PL |
A or C |
|
|
Interaction between DWFN PS fishery and locally based "fresh- sashimi
LL fisheries
|
YF,BE |
WCPO/PU |
PS |
PU |
LL |
B,C |
|
|
WCPO/FM |
PS |
FM |
LL |
B,C |
|
|
|
WCPO/MI |
PS |
MI |
LL |
B,C |
|
|
|
WCPO |
PS |
GU |
LL |
B,C |
|
|
|
WCPO/NC |
PS |
NC |
LL |
B,C |
|
|
|
WCPO |
PS |
FJ |
LL |
B,C |
|
|
|
WCPO/AU |
PS |
AU |
LL |
B,C |
|
|
Interaction between DWFN PS fishery and "frozm-sashimi" LL fishery |
YF.BE |
WCPO |
PS |
WCPO |
LL |
B,C |
|
Interaction between DWFN' frozen-sashimi' LL and locally- based "fresh-sashimi"
fisheries
|
YF.BE |
WCPO/PU |
LL |
PU |
LL |
A,C |
|
|
WCPO/FM |
LL |
FM |
LL |
A,C |
|
|
|
WCPO/MI |
LL |
Ml |
LL |
A,C |
|
|
|
WCPO |
LL |
GU |
LL |
C |
|
|
|
WCPO |
LL |
NC |
LL |
C |
|
|
|
WCPO/FJ |
LL |
FJ |
LL |
A,C |
|
|
|
WCPO/AU |
LL |
AU |
LL |
A,C |
|
|
Interaction between albacore difrent and troll fisheries |
AL |
STCZ Tasman Sea |
DN |
STCZ,NZ |
TR |
A,C |
|
Interaction between surface and longline fisheries for albacore |
AL |
STCZ Tasman Sea |
DN,TR |
South Pacific |
LL |
B,C |
|
Gear codes: |
PS: purse seine. PL: pole-and-line. LL: longlire, DN:
diferent. TR: troll. HL: handline |
|
|
|
|
Country/area codes: |
AU: Australia. FM: Federated States of Micronesia. FJ: Fiji.
PF: French |
|
|
Polynesia, GU: Guam. ID: Indonesia. KI: Kinbau. MI: Marshall
Islands. NC: |
|
|
New Caledonia. NZ.: New Zealand, PU: Palau. PH:
Philippines. SB: Solomon Islands. WCPO: western Pacific Ocean. STCZ;
sub-tropical convergence zone |
|
|
|
|
Species codes: |
AL albacore. BE: bigeye. SJ: skipjack. YF:
yellow-fun |
Returns from 181 large (>100 cm FL) yellowfin tuna during 1991-93 were used to test the null hypothesis that the relative probabilities of capture of the tagged yellowfin tuna by purse seine and longline gears are determined only by the relative catches by these gears. The null hypothesis would be accepted if yellowfin tuna were equally available to both gears, and rejected if unequally available to the gears. Overall, approximately 160 purse seine returns and 21 longline returns would be expected under the null hypothesis. Statistical tests showed that the observed number of longline returns (4 out of 181) was significantly less than that expected under the null hypothesis. Low reporting rates of longline-caught tagged fish, low returns, lack of thorough mixing of tagged fish, and reduced availability of tagged yellowfin tuna to longline gear are potential reasons for the lower than expected longline returns. Also, the expected longline returns may be artificially overestimated because they are based on ratio of catches, which are probably incomplete, especially for Korean and Taiwan purse seine catches. The tagging results, therefore, demonstrated surface to deep movement of yellowfin tuna, but so far not at a high rate to demonstrate a significant longline-purse seine fisheries interaction.
A unique tagging study has been proposed to determine whether this lower return rate from longlines is related to the source of the tagged fish; that is, tagged recoveries are principally surface-caught and tagged fish. The assumption is that there is vertical homogeneity and surface-caught fish move freely into the longline-vulnerable group. The proposed study is to tag a large number of longline-caught yellowfin tuna and evaluate the return rates of these releases by surface-gear and longline recaptures.
Size-frequency information was used in one study that investigated the extent of interactions between longline and purse seine fisheries. The study examined the size-frequency distribution of yellowfin tuna caught by the longline fishery in two different areas of the western Pacific in which there were relatively high and low purse seine fishing intensities. In areas with a history of high purse seine fishing intensity, there was a shift in the distribution with time towards a higher proportion of small yellowfin tuna in the longline catch with increase in purse seining intensity. In areas of low purse seine fishing intensity, no change in the size distribution was found with time. These results suggest a longline-purse seine fisheries interaction, but an alternative explanation could be that the dynamics of the yellowfin tuna population is more complex than assumed. For example, mixing of yellowfin tuna spatially throughout the central-western Pacific might occur slowly and not completely, the population might be significantly larger and more productive than generally assumed from experience with yellowfin tuna stocks in other regions, or the two fisheries might be exploiting separate stocks even in common fishing areas (Suzuki, 1994; WPYRG, 1994b).
Figure 2. Length-frequency distribution of yellowfin tuna caught by different gears in the central-western Pacific Ocean.
Another study now underway using biological information may shed some light on the longline-purse seine interaction. It is a large-scale study on the reproductive biology of yellowfin tuna. The study is designed to use sex ratio, reproductive condition, spawning time and location, and other ecological information to determine the relationship of yellowfin tuna caught by purse seine and longline gears in similar areas and periods. Progress with this study so far has been made in collection of samples and in standardizing the technique for histological analysis of gonads. Final results are anticipated within two years.
The findings of the WPYRG fisheries interaction studies have been mixed, with evidence tending towards a weak or little interaction between the longline and purse seine fisheries in the CWP as a whole. This conclusion is further supported by evidence on exploitation rate of the yellowfin tuna population, which is currently low, averaging 0.20. On the other hand, some localities in the region are experiencing substantially higher exploitation rates than this average, resulting in local depletion and possibly significant fisheries interactions at the local level (WPYRG, 1994b).
4. LOCAL DEPLETION AND FISHERIES INTERACTION
The WPYRG has gained experience and a better understanding of processes involved in yellowfin tuna fisheries interactions in the WCP region. It has substantially completed its task of addressing the region-wide issue of longline-purse seine fisheries interaction. In 1994, it identified local depletion and local fisheries interaction as the next priority for concentrated, cooperative investigation. This new challenge for the Group will be considerably more difficult to address than the region-wide issue, largely because local issues are more complex (e.g., Hampton et al., 1996) and the number of cases in the region is growing rapidly with developments and expansion of the fisheries. Furthermore, non-resource mediated factors, e.g., culture and economics, may be as important as the resource mediated factors in understanding fisheries interactions at the local level. The Group is preparing to address local interaction issues with its expertise in tuna resource and fisheries science. It has proposed several new research initiatives, such as tagging of longline-caught yellowfin tuna, development of an analytical model that includes spatial and size-structure information, as well as movement information and morphological studies of air bladder development related to deep swimming or surface swimming preference (WPYRG, 1994b). It has also called for parallel studies on non-resource mediated factors to be conducted, which should contribute to informed decisions for resolving local tuna fisheries interactions issues.
5. REFERENCES CITED
Hampton, J. 1994. A review of tuna fishery-interaction issues in the western and central 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): 138-157.
Hampton, J., T. Lawson, P. Williams and J. Sibert. 1996. Interaction between small-scale fisheries in Kiribati with the industrial purse seine fishery in the western and central Pacific Ocean. In: Shomura, R.S., J. Majkowski and R.F. Harman (eds.). Scientific Papers from the Second FAO Expert Consultation on Interactions of Pacific Tuna Fisheries, 23-31 January 1995, Shimizu, Japan. [This volume]
Miyabe, N. 1994. Japanese yellowfin tuna fisheries in the western and central Pacific and updated CPUE from those fisheries. Paper presented at the 4th Meeting of the Western Pacific Yellowfin Tuna Research Group, Koror, Palau, 9-11 August 1994. S. Pac. Comm. 12 p.
Suzuki, Z. 1994. A review of interaction between purse seine and longline on yellowfin (Thunnus albacares) in the western and central 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): 158-181.
Suzuki, Z., Y. Warashina and M. Kishida. 1977. The comparison of catches by regular and deep tuna longline gears in the western and central equatorial Pacific. Bull. Far Seas Fish. Res. Lab. 15: 51-89.
WPYRG (Western Pacific Yellowfin Tuna Research Group). 1993. Report of the second meeting of the western Pacific yellowfin tuna research group, 17-24 June 1992, Honolulu, Hawaii. S. Pac. Comm. 79 p.
WPYRG (Western Pacific Yellowfin Tuna Research Group). 1994a. Report of the third meeting of the western Pacific yellowfin tuna research group, 21-23 June 1993, Pohnpei, Federated States of Micronesia. S. Pac. Comm. 59 p.
WPYRG (Western Pacific Yellowfin Tuna Research Group). 1994b. Report of the fourth meeting of the western Pacific yellowfin tuna research group, 9-11 August 1994, Koror, Republic of Palau. S. Pac. Comm. 66 p.
