Tsutomu Nishida
National Research Institute of Far Seas Fisheries
5-7-1, Orido
Shimizu, Shizuoka 424 Japan
ABSTRACT
The influence of the industrial purse seine fishery on the industrial longline fishery in the western Indian Ocean is examined. The two basic analyses carried out include a study of the temporal-spatial fluctuations of CPUE and size of yellowfin tuna caught by the longline fishery before and after the start of the purse seine fishery in 1982 and a study of the relationship of the longline CPUE and the purse seine catch. The results of these studies suggest that the influence of the purse seine fishery on the longline fishery is only moderate in areas of intensive purse seine fishing and that there appears to be no apparent influence of the purse seine fishery on the longline fishery outside the immediate area of intensive purse seine fishing.
1. INTRODUCTION
Large-scale commercial purse seine fishing in the Indian Ocean was initiated by French purse seine vessels which began operating in the western Indian Ocean in 1982. In 1983, Mauritius and Japan also began purse seining in the area. During 1982 and 1983, operations were on an exploratory basis and the total catch was about 4,500 mt and 24,000 mt for the two years, respectively. The purse seine catch made up 2% and 9% of the total tuna catch for FAO Area F51 (Figure 1). Due to the successful fishing trials in 1982-83, Spain, Ivory Coast, Panama and United Kingdom joined the purse seine fishery in 1984. Other countries (Cayman Islands, Malta, USSR and Seychelles) joined the fishery in more recent years. The total purse seine tuna catch increased significantly to 103,00 mt in 1984 representing about 32% of the total tuna catch in Area 51. The catch has continued to increase and reached 291,000 mt in 1992; the catch represented about 40% of the catch in Area 51. Among the purse seine fishing countries operating in the western Indian Ocean, France and Spain have the two dominant purse seine fleets in the area.
The species composition of the purse seine catch from 1984-92 was 52% skipjack, 43% yellowfin, 4% bigeye, and 1% other species. Among the tuna species, the yellowfin tuna is the most important commercial tuna species. The yellowfin is commonly exploited by purse seine and longline gear; the latter by the longline fleets of Korea, Japan, and Taiwan. The areas of fishing for yellowfin in the Indian Ocean are shown in Figure 2.
Figure 2. Yellowfin tuna longline and purse seine fishing grounds in the Indian Ocean. Shaded zone is the longline ground defined as 5º x 5º areas producing an annual average of more than 0.5 fish/1000 hooks during 1983-91. The purse seine ground is surrounded by thick borders, defined as the 5º x 5º areas where purse seine fishing was conducted by vessels from France or Spain at least once during 1983-91. The diagonally hatched area represents the major (heavy) purse seine fishing ground, i.e., producing more than 1.5 mt per year per 5º square.
With the introduction of purse seining in the western Indian Ocean, it was only a matter of time that the longline fishing nations would start to become concerned with the possibility of the purse seine operation impacting the longline fishery. It is also only a matter of course that artisanal fisheries of developing nations in the region (Maldives, Sri Lanka, Pakistan and other nations in the western Indian Ocean) would also start to worry about the possible impact of purse seining on their coastal tuna fisheries. Basically, there are six types of fisheries interactions to be considered, i.e., purse seine Û longline, purse seine Û artisanal fisheries, artisanal fisheries Û longline. Based on previous studies, it appears that the purse seine Û longline interaction is considered the most serious. In this paper, the descriptive analyses provided are based on fisheries information accumulated from the purse seine and longline fisheries.
In conducting fisheries interaction studies, the stock structure needs to be initially defined. This paper assumes the yellowfin population in the Indian Ocean is made up of two major stocks (west and east) and two possible minor stocks (far west and far east or Pacific stock); the assumed stock structure is shown in Figure 3. The stock structure is based upon (a) past studies of stock structure by Hirano and Tagawa (1956), Kurogane and Hiyama (1958), Mimura (1958), Kurogane (1960), Morita and Koto (1970), and Nishida (1991), and (b) results of tagging experiments in the Indian Ocean by Yano (1990), Cayre and Ramcharrun (1990), and Waheed and Anderson (1993). The present study concerns mainly the western yellowfin stock.
Figure 4 shows the trend of the annual catch of yellowfin landed by the purse seine, longline, and artisanal fisheries from 1972 to 1992; the data are based upon statistics for FAO Area 51 which were summarized by Nishida (1993). The yellowfin catch by the purse seine fishery is about 2-5 times that of the longline fishery. The yellowfin catch by the longline fishery was more or less constant ranging from 17,000-38,000 mt for the period covering before and after the start of the purse seine fishery.
Under the above mentioned situation, this paper attempts to investigate the possible impact of the purse seine fishery on the longline fishery for the western Indian Ocean yellowfin stock. Past research on fisheries interactions on yellowfin are reviewed, followed by the present study which involves descriptive (spatial and temporal) analyses of fisheries statistics (catch, CPUE, and size of fish).
2. REVIEW
Table 1 summarizes previous studies relating to fisheries interactions of yellowfin tuna in the Indian Ocean; these studies are classified into qualitative and quantitative analyses. Regarding the influence of the purse seine fishery on the longline fishery, all of the studies predicted existence of a moderate impact of the purse seine fishery on the longline fishery. Marsac and Hallier (1987) and Bertignac (1993) showed the impact by conducting Ricker-type simulation analyses. In these yield per recruit (Y/R) simulation studies, the impacts of one fishery on the other were done by changing fishing mortality (F). Both papers concluded that the purse seine fishery moderately impacted the longline fishery. Review papers by Marcille (1986) and Suzuki (1987) also predicted effects of purse seine over longline by referring to similar cases in the Atlantic and Pacific Oceans, respectively. Nishida (1988) found local impacts of purse seine on longline in areas of overlapping fisheries. Nishida (1988) also found a positive impact (increases in purse seine catch accompanied by increase in longline CPUE) in the surrounding waters of the operational areas of purse seine and longline. It should be noted here that Nishida (1988) only had four years of data after the start of the purse seine fishery; thus, the long term effect needs to be re-investigated. Tsuji (1994) made similar comments about local impacts of purse seine on longline for the yellowfin fisheries in the western Pacific Ocean. Bertignac (1993) recommended that spatially concerned models should be developed since his interaction study was not spatially aggregated and thus may have masked the true situation. The present descriptive analyses of spatial and temporal changes in CPUE and size frequency data were carried out with consideration of past studies. It is the intent of this study to determine why the predicted impacts of the purse seine fishery on the longline fishery are not apparent in the collected fisheries statistics.
Figure 3. Assumed stock structure (upper panel) and results of tagging experiments (lower panel). IPTP=Indo-Pacific Tuna Programme, JAMARC=Japan Marine Fishery Resources Research Center, IOC=Indian Ocean Commission.
Figure 4. Annual yellowfin tuna catch trend by longline (LL), purse seine (PS) and artisanal (AF) fisheries in the western Indian Ocean (FAO statistical area F51).
Table 1. Papers and reports of fisheries interactions for yellowfin tuna in the Indian Ocean.
|
Type of Analysis |
Method |
Author |
Year |
Conclusion |
|
Quantitative
|
Ricker-type simulation model for Y/R
|
Marsac and Hallier |
1987 |
Increasing purse seine catches generate a moderate negative effect on
longline and no effect on artisanal fisheries |
|
Bertignac |
1993 |
Reduction of purse seine effort could result in 20-30% increase of longline
catch, but such impacts were not observed. If effort by purse seine (log)
is reduced, purse seine (free), longline and artisanal catches could increase
15-20%. Oman-type fisheries impact purse seine and longline. |
||
|
Qualitative
|
Review
|
Marcille |
1986 |
Purse seine could reduce longline catch by 30-50% over next few years. |
|
Suzuki |
1987 |
Total yellowfin catch should increase by developing purse seine fishery
and reduced longline catch. |
||
|
Analysis of CPUE/size |
Nishida |
1988 |
Purse seine impacts longline within same fishing grounds. No conclusion
about interaction between purse seine and artisanal fisheries. |
The available historical fisheries data (catch, effort, and fish size) from the Indo-Pacific Tuna Development and Management Programme (IPTP) and the National Research Institute of Far Seas Fisheries (NRIFSF) were used in this study of spatial and temporal changes in CPUE and size frequency distribution of yellowfin tuna caught on longline gear before and after the start of the purse seine fishery in the western Indian Ocean. In addition, the relationship between CPUE of longline-caught yellowfin and the purse seine catch of yellowfin were investigated for possible evidence of interaction between fisheries.
In these analyses some caution needs to be exercised in interpreting the results, i.e., a decrease in CPUE of the longline-caught yellowfin or a change in size frequency distribution do not necessarily imply impacts of purse seine on longline. Other factors such as environmental conditions, presence of strong cohorts, availability of fish, etc. may be responsible for the changes. However, if consistent patterns of changes occur with different analyses, the possibility of a fishery interaction is strengthened.
3.1 CPUE (long-term influence)
3.1.1 Spatial Analyses
In order to examine the wide-spread effects of the purse seine fishery on the longline fishery spatial analyses were carried out using 5º x 5º areal units. These analyses were performed by comparing distributional maps of annual average CPUE of longline-caught yellowfin for the periods before and after the start of the purse seine fishery in 1983 and by examining distribution maps for differences in CPUE between these two periods. Figure 5 (revised from data provided by Nishida, 1988) shows the short term situation (four years before and after the start of purse seining), while Figure 6 shows the long term case (eight years before and after the start of purse seining). Both the short and long term situations show decreases in CPUE for a majority of the 5 degree squares in and around the major purse seine fishing grounds. On the other hand, increases in CPUE are observed in a majority of the 5 degree squares in the waters surrounding the intensive purse seine fishing area.
3.1.2 Trend of Annual CPUE (entire purse seine fishing ground)
The historical annual CPUE data of yellowfin tuna caught by the longline fleets of Korea, Japan, and Taiwan in the purse seine fishing grounds have been compiled. Trends of CPUE from 1954 to 1991 are shown in Figure 7 (top), while those for 1975-91 are shown enlarged in Figure 7 (bottom). The purse seine fishing ground is defined as the 5º x 5º area where at least one purse seine operation was conducted during 1983-91 (Figure 2).
Based on Figure 7, the average CPUE of longline-caught yellowfin decreased drastically from about 90 fish/1,000 hooks at its peak in the mid-1950s to less than 10 fish/1,000 hooks after 1973. The trend in CPUE for the period before and after the start of the purse seine fishery (Figure 7, bottom panel) show relatively low level catch rates which ranged from 3 to 9 fish/1,000 hooks; an exception occurred in 1977-78 when the catch rate exceeded 13 fish/1,000 hooks. Patterns of the CPUE trends for Korea, Japan, and Taiwan are almost identical; however, the CPUE values of the Taiwan fleet are lower than those of Korea and Japan. This difference is probably because Taiwan targets mainly albacore; the fishing efficiency for yellowfin in this case is lower than the other two fleets.
3.1.3 Annual CPUE Trend (intensive purse seine fishing ground)
As observed in the spatial analysis, the CPUE of the longline fishery decreased in a majority of the 5 degree squares in the major purse seine fishing grounds after the start of the purse seine fishery. The major purse seine fishing ground is defined as the 5º x 5º square areas which produced more than 4,500 mt of yellowfin per year (combined French and Spanish purse seine catch). Figure 8 shows the annual average CPUE of the combined Korea, Japan, and Taiwan longline catch of yellowfin for the period 1955-91 (Figure 8, top panel); the period from 1975-91 is shown enlarged (Figure 8, bottom panel). The time series covering recent years (Figure 8, bottom panel) indicates a moderate decreasing trend after the start of purse seining in 1983. The overall average CPUE before the start of purse seining (1975-82) was 4.7 fish/1,000 hooks, while the average CPUE after the start of purse seining was 2.9 fish/1,000 hooks; a decline of 36%.
3.2 Size Frequency Distribution (long term influence)
The CPUE analyses showed that there were almost no changes in CPUE of longline-caught yellowfin from the wider regions of the purse seine fishing grounds; however, there appeared to be moderate decreases in CPUE in the major purse seine fishing area. Similar analyses were carried out to assess the changes in size of yellowfin.
The data on size of fish from the Japanese and Taiwanese fleets were combined and standardized into 2-cm class intervals and by 5º x 5º area; the data were then converted into percent frequency distributions. In the analyses, eight years of data before and after the start of purse seining (1975-82 and 1983-90) were utilized.
During processing of the Japanese size data, some of the data were suspected to have been measured originally by 5 cm or 10 cm class intervals instead of the 1 cm or 2 cm class intervals requested. This possibility was suggested by the sharp pulses noted in the size frequency distributions. It is possible that some Japanese longline fishermen measure size of fish roughly by 5 cm or 10 cm intervals because it is easier to estimate fish into these wider class intervals, especially during rough weather conditions and under busy working conditions (Shiohama, NRIFSF, pers. comm.). This problem was corrected by smoothing frequencies by taking a moving average of five class intervals.
The purse seine size data were also processed in the same way as the longline size data in order to allow for proper comparisons. In the purse seine fishery large yellowfin (80-160 cm) are caught mainly from free-swimming schools, while smaller size yellowfin (20-80 cm) are caught mainly from log-associated schools. In the longline fishery the catch mainly consists of large yellowfin (80-160 cm); thus, 80-150 cm yellowfin are commonly exploited by purse seiners fishing free-swimming schools and longliners, while purse seiners fishing log-associated schools catch small yellowfin.
Figure 9 shows size frequency distributions of yellowfin for the longline fishery (before and after the start of purse seine fishing) and the purse seine fishery for the entire purse seine fishing ground (Figure 9, upper panel) and the most intensively fished purse seine area. For the broad purse seine fishing area, the size frequency distributions for the longline fishery before and after the start of purse seining appear to be similar. The average size of yellowfin landed was 116 cm for the period prior to the start of purse seining and 120 cm for the period after the start of purse seining. On the other hand, for the area of intense purse seine fishing, the modal group for the longline fishery after the start of purse seining is larger than for the period prior to the start of purse seining. The average size of yellowfin was 122 cm and 110 cm for the two periods, respectively. For the purse seine fishery, the size of yellowfin for the extended fishing grounds and the intensively fished areas were similar. There appear to be two distinct modal groups with a large mode of small-size fish around 50 cm for log-associated schools and a smaller modal group of larger fish at 125 cm for free-swimming schools.
3.3 Relationship between CPUE of the Longline Fishery and Purse Seine Catch (immediate influence)
For the final analysis, a relationship between CPUE of the longline fishery and the purse seine catch of yellowfin was investigated. The analysis was conducted by plotting monthly CPUE (fish/1,000 hooks) of the longline fishery and purse seine catch of yellowfin by 5º x 5º degree squares. For the longline data the monthly average CPUE for the fleets of Korea, Japan, and Taiwan were combined, while the purse seine data consisted of the combined French and Spanish purse seine fleet for the entire area of operation. Figure 10 (upper panel) shows the basic scatter diagram for the period 1982-91. It clearly shows that all plots are located within the diagonal border made by the two outer points. This implies that the relationship between CPUE of the longline fishery and the catch of the purse seine fishery are inversely related.
The scatter plots of the purse seine catch are classified into four clusters (Figure 10, bottom panel). Cluster A includes 90% of the total points, while the remaining 10% is distributed among the other three clusters. Hence, the relationship between CPUE of the longline fishery and the catch of the purse seine fishery is dominated by Cluster A. This dominance implies there is no obvious relationship between CPUE of the longline fishery and the catch of the purse seine fishery; at least for the ranges encountered.
4. DISCUSSION
Basic descriptive analyses of spatial and temporal fluctuations of CPUE of the longline fishery (all ages pooled) and size frequency distributions in response to the catch of the purse seine fishery were carried out to examine possible impacts of one fishery on the other. In this type of analysis, careful interpretations of changes of CPUE and size frequencies must be examined. This is because factors other than the impact of one fishery on the other may be responsible for the changes. The factors include changes in environmental conditions, presence of strong cohorts, and availability of fish. If consistent changes are observed based on different types of analyses, it is possible to provide evidence of the impact of the purse seine fishery on the longline fishery.
The results of this study showed that the CPUE of the longline fishery remained relatively stable over the broad area of the purse seine fishery for the period prior to and after the start of the purse seine fishery. When data for the heavily fished purse seine area were examined the longline CPUE showed a modest decrease. Table 2 summarises the analyses of CPUE trends. It clearly demonstrates the differences between the extended purse seine fishing area and the area of intensive fishing.
The question of why there is only a moderate decrease in CPUE of the longline fishery despite the large purse seine catch will be discussed in the following sections for the (a) long-term situation, and (b) the immediate situation.
Long-term situation
In the case of long term influence, the reason for the moderate decrease in CPUE of longline-caught yellowfin in the presence of large catches of yellowfin by the purse seine fishery may be due to the existence of a cryptic biomass. The longline gear is deployed from 50-300 m water depth depending upon the species targeted. Images of tuna have been reported at depths of 500-2,000 m water depth (Yamada, 1991; Kawamura, 1994). Since tunas range vertically beyond the depth range of the longline gear and horizontally beyond the range of the tuna fishery, a portion of the yellowfin resource is assumed to be out of reach of the longline fishery. This portion of the resource is defined as the cryptic biomass. Fonteneau (1994) provides a detailed discussion of cryptic biomass and its relationship to the resource.
In the western Indian Ocean, the CPUE of adult yellowfin caught by the longline fishery is currently 20-25% of the level achieved in the 1950s when the longline fishery first started in the Indian Ocean. Hence, at present, the adult population of yellowfin tuna is apparently at a low level as indicated by the low CPUE values. If one accepts the concept of cryptic biomass, the true population size of yellowfin is probably much larger than predicted based on the level of CPUE. This is again the major reason why the CPUE of the longline fishery is not influenced by the large catch of yellowfin by the purse seine fishery. This may describe the situation as predicted by the simulation studies undertaken by Marsac and Hallier (1987) and Bertignac (1993).
Table 2. Summary of longline CPUE analyses.
|
Area Analysed |
Longline |
Time Scale |
B: Average Longline CPUE Before Purse Seine, 1975-82
fish/1000 hooks) |
A: Average Longline CPUE After Purse Seine, 1983-91
(fish/1000 hooks) |
Rate of Change (%) from B to A = (A-B)*100/B |
|
Whole PS fishing ground
|
Korea |
Annual |
8.2 |
7.5 |
-9% |
|
Japan |
Annual |
8.2 |
7.6 |
-7 |
|
|
Taiwan |
Annual |
2.8 |
3.0 |
-7 |
|
|
Heavy PS fishing ground
|
Average |
Annual |
4.7 |
2.9 |
-38 |
|
|
Quarterly |
11.7 |
6.6 |
-44 |
|
|
|
Quarterly |
12.2 |
10.6 |
-13 |
|
|
|
Quarterly |
2.7 |
4.7 |
+74 |
|
|
|
Quarterly |
11.0 |
4.5 |
-59 |
In case of the immediate situation, Figure 10 provides some interesting insights for the reasons why the moderate increases in CPUE of the longline fishery in the area of intense purse seining is not reflected outside this area. As shown in Figure 10 (bottom panel), each cluster has its own upper limit of CPUE of the longline fishery. Cluster A includes about 90% of the total data points and within its range it is not clear that the purse seine catch affects the longline catch of yellowfin; this may be due to the wide range of the CPUE of the longline fishery. On the other hand, in other clusters (B-D) there are clear upper limits of CPUE. Thus, it is obvious that the catch of the purse seine fishery affects the longline fishery when the purse seine catch exceeds 1 ton/month/5º x 5º area. However, since there are only 5% of the data points in these three clusters, it can be generally concluded that there are no serious immediate influences of the purse seine fishery impacting the longline fishery; at least at current catch levels. This immediate impact is mainly caused by the purse seine catch of the larger size yellowfin (80-160 cm) since this size group is also exploited by the longline fishery. However, this influence may be masked by the existence of the cryptic biomass discussed earlier.
Further studies
There is a need to investigate the cryptic biomass of the yellowfin population in the western Indian Ocean by conducting tagging experiments using archival and/or sonic tags. Data obtained from these experiments will help elucidate the time and depth distribution of adult yellowfin tuna and also elucidate the proportion the cryptic biomass that make up the total yellowfin population in the region. Additionally, it will be necessary to estimate the unbiased CPUE of the longline fishery by correcting for the cryptic biomass. It may be desirable to cross-check the accuracy of the corrected CPUEs by carrying out simulation studies. The General Linear Model (GLM) analyses may be effective in assessing the influence of the purse seine fishery on the longline fishery quantitatively. This examination can be conducted by the following model:
Longline (CPUE)=(mean)+year+season+area+purse seine catch+environmental factors
where the area term includes the heavily purse seine fished area as well as the extended fishing area.
5. ACKNOWLEDGMENTS
The author thanks Mr. D. Ardill and Mr. S. Amarasekara of IPTP for providing the large data base used in this paper. The author further extends his appreciation to Mrs. A. Shimuzu and Mrs. H. Nukaya of the NRIFSF for their assistance in the graphic works; their patient work is much appreciated.
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