SCTB16 Working Paper FTWG 5

Transcription

1 0 SCTB16 Working Paper FTWG 5 Fishery-related attributes associated with FAD and log fishing practices conducted by the U.S. purse seine fleet in the central-western Pacific Ocean, Al Coan Jr. and Paul Crone US National Marine Fisheries Service. La Jolla. USA. July 2003

2 Fishery-related attributes associated with FAD and log fishing practices conducted by the U.S. purse seine fleet in the central-western Pacific Ocean, Atilio L. Coan Jr. and Paul R. Crone National Marine Fisheries Service Southwest Fisheries Science Center La Jolla, CA USA July Report prepared for the 16 th Standing Committee on Tuna and Billfish, Fishing Technology Working Group, July 9-16, 2002, Mooloolaba, Austraila

3 Fishery-related attributes associated with FAD and log fishing practices conducted by the U.S. purse seine fleet in the central-western Pacific Ocean, Atilio L, Coan Jr. and Paul R. Crone National Marine Fisheries Service Southwest Fisheries Science Center La Jolla, CA 92038, USA INTRODUCTION U.S. Purse seiners have fished in the central-western Pacific (CWP) since 1988 under the South Pacific Regional Tuna Treaty. Landings, logbook, length frequency and species composition information are routinely collected from the fishery as part of the Treaty. Length frequency and species composition data are collected in American Samoa through a National Marine Fisheries Service (NMFS) port sampling program and on board vessels via an observer sampling program administered by the Forum Fisheries Agency (FFA). Logbook records show that during 1988 to 1995, the U.S. fleet fished mainly on free-swimming schools of skipjack (Katsuwonus pelamis) and yellowfin tunas (Thunnus albacares). During this period, these school-related operations accounted for 80 to 90% of the fleet s sets (Figure 1). However, in 1996, the fleet began changing its operations to sets associated with logs or Fish Aggregation Devices (FADs). These associated sets accounted for 40% of the fleet s sets in 1996 and quickly increased to 96% of the total sets in Associated sets typically catch smaller tunas and larger quantities of bigeye tunas (T. obesus) than unassociated sets (Sakagawa 2000). Further, associated sets are almost twice as effective in catching yellowfin, skipjack and bigeye tunas. As a result, small tunas flooded markets in 2000 and 2001 and the price for small tunas dropped from over $1,000 per short ton to $400 per short ton. In response, the percent of associated sets in 2001 and 2002 decreased to approximately 50%, as the fleet sought to increase its catches of larger tunas that are typically found in free-swimming school sets. While differences between free-swimming school and associated sets have been well documented for the U.S fleet fishing in the CWP, differences between log and FAD sets have not. This study addresses differences in fishing areas, fishing success, species composition and size composition between log and FAD sets for the U.S. fleet from 1997 to U.S. fleet logbook data are used to investigate annual differences in areas fished and average nominal catch per set estimates. Species composition data from port sampling are used to investigate annual differences in the average proportions of bigeye tuna in the yellowfin tuna landings. Species composition data from observer sampling were not used since samples for both log and FAD sets were only available for two years during the study period. Length frequency data from port sampling was used to 1

4 investigate annual differences in the average proportions of small (<7.5 lb) fish in the catch. Finally, it is important to note that the primary purpose of this study was to examine general statistical descriptors associated with major 'associated' set types, which should be considered the initial step in the overriding objective of providing fishery managers with meaningful advice regarding management of the tuna populations currently exploited by the purse seine fleets operating in the CWP. In this context, results presented here are intended to spur additional discussion within the forum of the Fishing Technology and Statistics Working Groups and thus, provide motivation for developing longer-term research schedules that will necessarily involve more detailed evaluations of the available sample data sets than relied upon here, e.g., deriving the most appropriate catch rate time series (e.g., using standardization techniques, such as general linear models) and employing the most rigorous statistical tests (e.g., using formal experimental design methods). METHODS Logbook data were summarized by set type for catches of yellowfin, skipjack, and bigeye tunas. Only sets identified as drifting FAD or floating log were used. Catches of bigeye tuna, reported in the logbooks, were combined with yellowfin tuna, as bigeye tuna are usually reported combined with yellowfin tuna. An annual average nominal catch (total yellowfin, skipjack and bigeye tuna) per set (not standardized via GLM analysis) by set type (log or FAD) was calculated along with the associated 95% confidence intervals (CI) around the estimates. Logbook data were also summarized by 1 square, year and set type and plotted to visually evaluate annual differences in occurrences of FAD and log sets in the CWP. Species composition data from port sampling were summarized by set type, log and FAD, and subsequently, the proportions of bigeye tuna in the yellowfin tuna landings was calculated. The annual average proportions of bigeye tuna in the yellowfin tuna landings was calculated along with the associated 95% CI around those estimates for log and FAD sets. Length-frequency data from port sampling were summarized by set type, log and FAD, and subsequently, the proportions of small fish in the catch was calculated. The annual average proportions of the total sampled yellowfin, skipjack and bigeye tuna less than 7.5 lb was calculated, as well as the corresponding 95% CI around each estimate. Statistical differences between the average nominal catch per set estimates, average proportions of bigeye tuna in the yellowfin tuna landings and average proportions of small fish in the catch by set type and year were determined by simply comparing the bounds of error associated with the mean statistics, i.e., means with bounds that did not overlap were considered statistically (P<0.05) different in this study. This ad hoc method is generally equivalent to paired t test analyses. 2

5 RESULTS The number of log and FAD sets summed from logbook reports is shown in Figure 2 for 1997 to Number of sets ranged between 113 and 1,535 for log sets and 1,980 and 3,046 for FAD sets. The number of log and FAD sets was almost equal in 1997 and The number of log sets quickly dropped in 1999 to approximately 200 sets in 1999 to 2001, and has remained relatively low in FAD sets, however, peaked in 1999 at over 3,000 and gradually declined to approximately 1,500 in Figures 3-5 show the 1 distribution of occurrences of FAD and log sets for 1997 to Figure 3 shows that logs and FADs occurred in roughly 50% of the same 1 squares in 1997 and In 1999 to 2002 (Figures 4 and 5), the occurrences of log sets decreased and the percent of 1 squares with both FAD and log sets also decreased to only 2% in 2001 and 8% in It seems that logs were less prevalent in than in 1997 and In 1997, when both FAD and log sets occurred in the same 1 square, 49% had more FAD than log sets, 36% more log sets than FAD sets and 15% had equal numbers of both set types. Also, FAD and log sets occurred in the same 1 squares most frequently between 174 E longitude and 171 W longitude with log sets dominate in 1 squares west of 180 and FAD sets in 1 squares east of 180. In 1998, this trend reversed; when both FAD and log sets occurred in the same 1 square, 48% had more log sets than FAD sets, 33% had more FAD sets than log sets and 19% had equal numbers of both set types. Also, FAD and log sets occurred in the same 1 squares most frequently between 176 E longitude and 175 W longitude and log sets seemed to be equally dominate in both eastern and western areas. Average nominal catch per set, for FADs was 5 to 6 t higher than average nominal catch per set for logs in 1998 and 1999 (Figure 6). In 1997, and , average nominal catch per set for both log and FAD sets were approximately the same. The 95% confidence intervals for log and FAD average annual nominal catch per set overlapped for all years except Therefore, no statistical difference (P>0.05) could generally be found between log and FAD average nominal catch per set, except in The number of species composition samples available to determine the average proportions of bigeye tuna in the yellowfin tuna landings by set type and year is shown in Figure 7. The number of log and FAD species composition samples available was essentially the same in 1997 and However, the number of log set samples available in 1999 to 2002 was less than 50, while the number of FAD set samples was always over 150, and as high as 450 (1999). The annual average proportions of bigeye tuna in the yellowfin tuna landings, estimated from these species composition samples, was higher for FAD sets than log sets in all years except 1999 (Figure 8). However, the estimates were statistically (P<0.05) different in only 1997 and The number of length frequency samples available to determine the proportions of small fish in the catch by set type and year is shown in Figure 9. The number of 3

6 samples available for log and FAD sets in 1997 and 1998 was close to 400. However, the number of log length frequency samples available in 1999 to 2002 decreased to 91 in 2001 and 34 in The number of FAD length frequency samples available was over 400 in all years, with a high of 1,157 in The annual average proportions of small fish, determined from these length frequency samples, varied between 0.3 and 0.6 for log sets and 0.3 and 0.5 for FAD sets (Figure 10). Proportions of small fish in FAD sets were higher than log sets in all years except 1998 and The proportions were statistically (P<0.05) different in DISCUSSION Logbook data for 1997 and 1998 show that if log sets are present in large quantities they are distributed in approximately the same areas as FAD sets. However, when log and FAD sets occurred in the same areas, FAD sets were more prevalent in 1997 and log sets in More log sets are found independent of FAD sets west of 160 E longitude and further, if FAD sets are found independent of log sets, they usually occur in areas east of 160 E. Logbook data, especially for seem to indicate that there were very few logs in the areas fished. Possible causes may be an absence of severe weather in the area during these years and more severe weather in 1997 and 1998 that caused more logs to be washed out to sea or a misreporting of log sets as FAD sets. A difference between log and FAD set nominal catch per set estimates, average proportions of bigeye tuna in yellowfin tuna landings, or proportions of small fish could not be statistically (P>0.05) documented across each year of the entire time series ( ). Statistical (P<0.05) differences were found only in the following comparisons: 1) log and FAD average nominal catch per set estimates were only statistically different in 1998; 2) the average proportions of bigeye tuna in the yellowfin tuna landings in 1997 and 1998; and 3) the average proportions of small fish in the catch in Only in 1998 was there a difference between log and FAD set nominal catch per set, average proportions of bigeye tuna in the yellowfin tuna landings and average proportions of small fish. If this one year is considered alone, then one conclusion would be that FAD sets have higher nominal catch per set estimates and average proportions of bigeye tuna in the yellowfin tuna landings than log sets and log sets have a higher average proportions of small fish than FAD sets. However, as noted above, this conclusion is not consistent from year to year. For the most part, statistical (P<0.05) differences between log and FAD set types were not documented here. It is important to note that statistical power associated with the formal analyses was likely quite low (e.g., low sample sizes for log sets in ). Further, it is also possible that potential biases were associated with sample data analyzed in this study. For example, it is unclear whether FAD sets are always reported or whether at times they are misreported as log sets. FADs are sometimes tied to logs or natural logs are tied together (with transponders) or attached to corks and FAD materials, 4

7 blurring the definition of what is a log or a FAD. Additionally, the first time a log is encountered it maybe recorded as a log set, but it is unclear whether, after the log is modified, it is recorded consistently as a FAD or log set. Observer data may be useful in clarifying these situations. LITERATURE CITED Sakagawa, G. T The impact of FAD innovation on the performance of U.S. tuna purse seine operations in the Pacific Ocean. pp In: Jean-Yves Le Gall, Patrice Cayré, and Marc Taquet (eds). Pêche thonière et dispostifs de concentration de poissons, October 15-19, 1999, Trois-Îlets, Martinique. Actes de colloques n Institut français de recherche pour l'exploitation de la mer (Ifremer), Plouzané, France. 5

8 100 FAD SETS PERCENT OF SETS FREE-SWIMMING SCHOOL SETS LOG SETS Figure 1. Set type (in percent) time series for U.S. purse seiners fishing in the centralwestern Pacific ( ). YEAR FAD SETS LOG SETS SETS YEAR Figure 2. FAD and log set type (in number) time series for U.S. purse seiners fishing in the central-western Pacific ( ). 6

9 Figure 3. Distribution of the occurrences of FAD and log sets for U.S. purse seiners fishing in the central-western Pacific in 1997 and

10 Figure 4. Distribution of the occurrences of FAD and log sets for U.S. purse seiners fishing in the central-western Pacific in 1999 and

11 Figure 5. Distribution of the occurrences of FAD and log sets for U.S. purse seiners fishing in the central-western Pacific in 2001 and

12 FAD SETS LOG SETS NOMINAL CATCH PER SET YEAR Figure 6. Log and FAD average nominal catch per set and associated 95% confidence interval for U.S. purse seiners fishing in the central-western Pacific ( ) NUMBER OF SAMPLES FAD SETS LOG SETS Figure 7. Species composition samples (in number) used to determine the average proportions of bigeye tuna in yellowfin tuna landings for U.S. purse seiners fishing in the central-western Pacific ( ). YEAR 10

13 0.6 PROPORTION OF BIGEYE TUNA IN YELLOWFIN LANDINGS FAD SETS LOG SETS YEAR Figure 8. Log and FAD average proportions of bigeye tuna in yellowfin tuna landings and associated 95% confidence interval for U.S. purse seiners fishing in the central-western Pacific ( ) FAD SETS LOG SETS 1000 NUMBER OF SAMPLES YEAR Figure 9. Length frequency samples (in number) used to determine the average proportions of small fish (<7.5 lb) in the catch for U.S. purse seiners fishing in the central-western Pacific ( ). 11

14 FAD SETS LOG SETS 0.6 PROPORTION SMALL FISH YEAR Figure 10. Log and FAD average proportions of small fish (<7.5 lb) in the catch and associated 95% confidence interval for U.S. purse seiners fishing in the central-western Pacific ( ). 12