REPORT OF THE ICCAT DATA PREPARATORY MEETING FOR SOUTH ATLANTIC SWORDFISH (Tamandare, Brazil - April 8 to 13, 1999)

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2 SCRS/99/19 REPORT OF THE ICCAT DATA PREPARATORY MEETING FOR SOUTH ATLANTIC SWORDFISH (Tamandare, Brazil - April 8 to 13, 1999) 1 Opening of the meeting The Data Preparatory Meeting for South Atlantic swordfish was held, at the invitation of Brazilian Government, at the Research and Training Center of IBAMA, in Tamandare, Pernambuco, Brazil, from April 8 to 13, The meeting time and venue was specified by the SCRS at its 1998 meeting. Mr. Celso Martins Pinto, the Director of the Research Division of IBAMA welcomed all the participants. While emphasizing the importance of the meeting, Mr. Martins wished the participants a successful meeting. The meeting was attended by the scientists from Brazil, Japan, EC-Portugal, EC-Spain, the United States and Chinese Taipei. The ICCAT Secretariat was represented by Dr. P. M. Miyake, Assistant Executive Secretary. The List of Participants is attached as Appendix 2. 2 Adoption of Agenda The Agenda, circulated prior to the meeting, was modified and adopted (Appendix 1). 3 Nomination of the Inter-sessional Chairman and nomination of Rapporteurs Dr. J. Powers, the SCRS Chairman presided over the entire session, in the absence of Dr. Julie Porter, the Convener for the Swordfish Species Group. He welcomed the participants and emphasized the importance of the session as the results are essential for assessments which will serve as the scientific base for south swordfish management. The following scientists were nominated as Rapporteurs: Drs. S.Turner (Item 5, K. Yokawa and J.A. Negreiros Aragau (Item 6) and J. Powers (Item 7); Dr. P. M. Miyake served as General Rapporteur. 4 Review of meeting objectives The Chairman reviewed the objectives of the meeting, which were as follows: (1) review of the catch and effort as well as size data available; (2) review of the adequacy of size data for analytical purposes and see what models can be applied; (3) create standardized CPUE and select those useful for further analysis; and (4) specify the items that need to be worked on prior to the 1999 swordfish stock assessment session in September. There were seven papers presented to the session (the list is attached as Appendix 3). These papers described the swordfish fisheries of Spain, Japan, Chinese Taipei, and various regions of Brazil. Initial efforts to create standardized CPUE were presented for data of Spain, Japan and Chinese Taipei in these

3 papers. All the papers were reviewed and will be discussed under various Agenda items where they are referred. 5 Size sampling The group reviewed the available size samples in the ICCAT data base for the purpose of deciding whether they would be sufficient for calculating the catch at size (CAS). It was noted that the a catch at size had been developed for South Atlantic swordfish by the Secretariat in 1996; however, to date, it has not been used by the group for age-structured or size-structured analyses. It was further noted that additional size samples became available since the CAS was first created, especially for the various Brazilian fisheries and Chinese Taipei. The group agreed to review: (1) the sampling fractions by nation and year, and (2) the year-to-year consistency of the size samples by nation and geographic area. It was decided that for these analyses, the same areas would be used as for the preliminary Spanish catch rate analyses (Figure 1) with the exception that the southern boundaries of areas 7 and 8 were extended to 6 o S (SCRS/99/32). As noted in section 7, those areas were proposed based on Spanish catch rate distributions, and the group recognized that further refinement might be necessary as more was learned about catch rates and size composition in the various fisheries. During this review many problems with the ICCAT data base were identified, including unreported data, out of date data which should have been updated, duplicate data entries, and erroneous size codes. It was noted that some of these problems were due to nations failing to report data or data corrections to the Secretariat and others are due to data management problems at the Secretariat. 5.1 Sampling fractions The group reviewed size sampling fractions by major fishing nation since 198 (Table 1, Figures 2 and 3). For Brazil, sampling fractions generally ranged from 1% to 3% during the period and 2-3% in the late 198's. (Note that the low sampling fractions for Brazil in 1983 and 1984 were reviewed; data were found to exist in Brazil, but not in the ICCAT data base addition. The sampling fractions for those years were thought to be similar to surrounding years and the full data sets for those years will be submitted to ICCAT (Note: to be checked and modified.). Spanish sampling fractions were relatively low (3-13%) during the initial years of its fishery and then increased to roughly 5-6% for Chinese Taipei had 1-3% sampling fractions in the early-mid 198's followed by 2-3% fractions during Japan had 4-8% sampling fractions until the early 199's which then decreased to about 2-3%. The group reviewed sampling fractions for Japan, Spain and Chinese Taipei for the western and eastern South Atlantic and noted similar fractions in both areas for each nation. The Group noted a higher sampling fraction for the Brazilian domestic fishery than for some of the leased vessel fisheries, particularly for the Brazil Chinese Taipei fishery for which no fish have been measured since 1994 (Table 2). Overall sampling fractions were generally in the range of 5-1% in the 198s and increased to 2-3% in the 199s (Figure 3) with 1,-2, fish measured annually in the 198s and increasing to more than 1, fish measured annually since However, a major part of the samples were taken from relatively a few fleets. The Group strongly recommended that nations with low sampling fractions review their sampling systems with the aim of increasing the sampling fraction perhaps through the use of species-specific sampling criteria especially for swordfish. Where possible, sex should also be recorded with size. The absolute sampling levels needed to adequately characterize the size composition will vary from fishery to

4 fishery. In the absence of research to define adequate sampling levels, the group recommended at least a large sampling fraction, perhaps as much as 2%. A suggestion was made that a manual for sex identification might be developed, if possible. 5.2 Consistency of size composition To review the consistency of size composition data, cumulative frequency distributions were examined for all nations, areas, and year strata in which at least 1 fish measure. The number of fish measured in length or weight (converted then length) by nation, area and year in the available ICCAT data base is shown in Table 3. However, the group decided to use the Spanish catch-at-size (CAS) data in the exploratory analyses because the original data are in 5x5 areas/month, the number of fish sampled by square is large, the number of substitutions required is very small, and the exploratory analyses are done in cumulative percentage. The group noted that the exploratory analyses were done using yearly data by tentative areas. So, the temporal variability (month or quarter) was not taken into consideration. Moreover, the group noted that only the information available was analyzed, and encouraged all nations to report their current and historical data and to maintain high sampling fractions. The cumulative frequency distributions (CFDs) were plotted by area and nation (Figure 4) and by nation, by area and year (Figure 5). In general, the group noted consistency in the cumulative frequency distributions (CFDs) by nation and area, but noted possible problems with some data sets. Spanish CFDs in areas 11 and 12 (the areas with large catches) showed roughly 1 cm and 2 cm ranges in sizes at 5% cumulative frequencies. However, this difference among years might be explained by changes in the seasonal distribution of fishing effort, the decrease in the mean weight of the catch (about 1 kg during the period available) reported from the Spanish data and other factors. The information for other areas also shows very consistent trends in CFDs. among years. Size frequency data for the domestic Brazilian fishery were reported by ICCAT area (western South Atlantic) without latitude and longitude information from 1971 through about 1984, and then with latitude and longitude thereafter when most the data occurred in area 9 (Table 3). There was general similarity in the CFDs within these periods, but there were differences between the periods. Additionally, in area 9, the 1998 CFD differed from those in earlier years ( ). The group recommended that these patterns be carefully reviewed to determine the reason for those differences. The Brazilian fishery conducted by leased Japanese vessels (Brazil- Japan, areas 7 and 13) generally showed consistent CFDs with a broader range in size composition in area 13 (western South Atlantic, 45 cm range at 5% CFD) than area 7 (about 3 cm range at 5% CFD). The greater variation in interannual size composition in area 13 might be interpreted as indicating shifts between inshore (such as area 7) fishing with small size composition and offshore fishing with larger size, as seen in many of the oriental, high seas, longline fisheries. Brazilian-leased vessel fishery for other than Japan developed in recent years. The limited number of comparisons available indicated relative consistency in CFDs.(Brasil Other; areas 7, 8, 9, 11 and 13). The group noted relative consistency in the CFDs between years for Japan in most of the areas with information, although with high variability among years. Review of the raw data for 198 in area 12 indicated possible erroneous information in the ICCAT data base and the group recommended a review

5 of that information. The range of variation for areas 1 and 12 is 4 cm at CFD of 5% with a mean size from 16 to 2 cm LJFL. Korean size composition is very limited but appeared to be relatively consistent in area 11 for the years available. The size composition is similar to the Japanese CFD in the same area. The CFDs for Chinese Taipei ( only; areas 13 and 14) indicated that larger fish occurred in the eastern South Atlantic than in the western south Atlantic. The similarity of the 1997 CFDs from the two areas and their average size greater than in earlier years was questioned. A review of the 1997 data held by scientists from Chinese Taipei indicated differences from the preliminary data received by ICCAT. The Group recommended that the two data bases be carefully compared and ICCAT be provided with the most accurate information. 5.3 Overview of size composition The review of the size composition by year and area with data from different fleets revealed a general pattern of smaller size composition for Brazil, Brazil Others and Spain compared with catches of deep longline of Korea, Chinese Taipei and especially Japan (Figure 5). These differences in size distribution among different types of gear (surface vs deep longline) could be linked to the fishing strategy of the different fleets, but it could also indicate a thermal stratification with a depth similar to that already known for latitude. If these differences are real, larger swordfish may have lower catchability with surface longline than with deeper longline. 5.4 Size composition general recommendations The group urged that all nations report raw size data to the Secretariat as had been agreed by the SCRS for many years. Reporting catch-at-size data does not waiver this responsibility. 5.5 Adequacy of size composition data for creation of catch at size The group concluded that if the problems identified with the size composition data bases are thoroughly investigated and, if necessary, corrected, then the data would be adequate for creating CAS, although prior to 199 the size data may be less adequate and this needs to be examined.. However, the group indicated that the overall sampling rate obtained after 199 is mostly due to the increase in catches and in sampling done by some fleets (i.e. Brazil and Spain which have high sampling coverage rates). The overall mean sampling level for the period was about 8% while the overall mean sampling rate for the period was 27%. The group recommended that reviews of size composition data be completed by April 3, that revised size composition be submitted to the Secretariat by May 31, and that catch at size be created by each nation for 198 to 1997 and submitted by July 31, If sampling coverage is sufficient for certain fisheries, the CAS should be created for an even earlier period. The Secretariat will create catch at size for monor fisheries such as Uruguay, Namibia, and Ghana. It was highly recommended that the procedures used for substitutions and raising to create catch at size be reported by the national scientists The group discussed strata for use in creating catch at size and recommended that the smallest possible time-area strata be used. The group recognized that stratification would depend on how the fishery and sampling are conducted, but it was suggested that a maximum area size of 1 o x1 o be considered.

6 6 Review of south Atlantic CPUE series The available catch-per-unit-effort (CPUE) data were examined and evaluated for their usefulness in developing indices of abundance for the south Atlantic swordfish stock (or portions thereof). As an initial step, the ICCAT Secretariat prepared a summary of the catch and effort data which were available in the data base. These data included information from Argentina, Brazil, Chinese Taipei, Cuba, Japan, Korea, South Africa, Spain, USA, Russia, Uruguay and Namibia. In many instances, the data series are not likely to be useful for indices of abundance since many covered only short periods. However, some sets appear to be useful. It should be noted that in the previous assessment only the data from Japan, Spain, and Chinese Taipei were used in the assessment analyses. After investigating the data base, the data from Brazil, Brazil-Chinese Taipei, Chinese Taipei, Spain, Japan and Portugal fleets were examined in more detail. Descriptions of these data sets are given below. Additionally, for Brazil, Spain, Chinese Taipei and Japan, the data were reexamined using the area divisions presented by Spain (SCRS/99/32) using a GLM to standardize the CPUE (Figure 1). This division differs from ICCAT areas, but it was adopted as a means to compare the CPUEs between fleets within areas. In particular, it noted if there was relative homogeneity between fleet CPUEs within an area. Trends of catch and effort and CPUE by fleet Chinese Taipei Swordfish is a by-catch of the Chinese Taipei tuna longline fleets (SCRS/99/34). An obvious transformation of fishing pattern occurred from the end of 198's (Hsu and Chen 1995, SCRS/95/97) which seemed to have resulted in the increasing catches of tropical tunas, swordfish and billfishes (Hsu 1996, SCRS/96/1). The proportion of swordfish in the total longline catch fluctuated from 1968 to the end of the 198s, increased abruptly in 1991 and 1992, decreased in 1993, and has maintained a relatively stable level. The Chinese Taipei series for Atlantic swordfish (i.e. Task II data in ICCAT base) covers a relatively long period since The data were by month and 5x5 degree areas, including catch in number and in weight (in kg) and effort in numbers of hooks. Length frequency data are also available both in actual size measured and catch at size starting from 1981 by the strata of quarters and ICCAT sampling areas (areas 96 and 97). Fishing effort (number of hooks) of this tuna longline fleet in the South Atlantic increased consistently from 1968 (19.8 million hooks) to 1997 (117 million hooks). The catch of swordfish in the South Atlantic increased from 488 MT in 1968 to 1281 MT in 197, fluctuated thereafter, decreased to the historical low of 199 MT in 1984, and increased abruptly from 896 MT in 199 to the peak 2876 MT in The lowest nominal CPUE occurred in 1976 (.35/1 hooks) and the highest in 1992 (1.49/1 hooks). All species of billfishes including swordfish are caught incidentally by Chinese-Taipei longline fleets in the South Atlantic (Hsu 1996). An obvious change in fishing pattern since the end of the 198s which seemed to result in the increasing catch ratio of swordfish might be another factor which affected Chinese-Taipei swordfish CPUE. Therefore, additional GLM analyses were run.

7 Chinese Taipei CPUE of swordfish in the South Atlantic was standardized by using area designation shown in SWO/99/2. The original model used in the GLM analysis was: ln(cpue ijkl +const)=µ+yr+qt j +AR k +INTER+e ijkl INTER: combinations of two-way interaction of YR*QT) The trend of the CPUE in each area fluctuated and increased in the early 199s. One of main reasons for this fluctuation could be attributed to the change in fishing pattern at the end of the 198s (Liu and Wang, 1999, SCRS/99/34). Analysis including one additional factor, using catch ratio categories of swordfish in weight to the total weight of albacore, bigeye tuna, yellowfin tuna and swordfish of the ICCAT Task II data (aggregated by year, month and five degree square) were conducted to adjust for the effect caused by the change in target species. There were four categories of swordfish catch ratio used in the analysis: -25%, 25-5%, 5-75% and 75-1%. The results showed flat trends over the period analyzed for all areas (Figure 6). As the data used in this analysis were aggregated., the trend of CPUE (in weight) of the sum of albacore, bigeye tuna, yellowfin tuna and swordfish was also examined to see the effect of the CPUE trend of the sum of the aggregated species on that of swordfish. The CPUE trends of the sum of the aggregated species were also shown roughly to be flat. Thus, it was indicated that the factor of the swordfish catch ratio in the GLM analysis of Chinese Taipei CPUE has an effect on swordfish CPUE. However, the calculated CPUE of Chinese Taipei with the effect of the swordfish catch ratio might be affected by other factors. Therefore, further study on examining the factors that affect the CPUE trend, especially after 199, is necessary. A preliminary GLM analysis indicated that year, area, quarter, ratio and area-quarter interaction have significant effect on swordfish CPUE trend. This preliminary analysis trend indicated that CPUE decreased from 1968 to its lowest level in 1976, fluctuated during the , slightly increased in 1991, and decreased thereafter (Figure 6). Spain Data for the traditional Spanish fleet fishing on the North Atlantic swordfish stock are normally provided by records per trip obtained by the Spanish Institute of Oceanography (IEO) when fish are landed at the different base ports used by the Atlantic fleet. However, since the mid-198s vessels fishing the south stock (South of 5 o North latitude) are equipped with on-board freezers and whose trips last over 6 days at sea. In this case, the additional use of logbooks designed by the IEO specifically for this fleet has been introduced progressively and voluntarily. The information obtained per set, when available, is compiled in "sub-trips" (group of sets carried out consecutively in the same 5x5 degree area /month) and is treated as an observation or trip of the traditional fleet. Data from observers are also included in this analysis. Thus, the observations used in this analysis include data by trip as well as data by sub-trip, as was used in previous reports. A total 3,171 observations were finally used for the period. These observations include an average of 5,484 fishing days per year and around 15 million hooks per year. The overall coverage in terms of catch and effort included in these observations was about 87%. A minor number of observations is also available for 1986 and 1988, but these were not considered useful for CPUE analysis. Observations from 1989 are few and most of them are from only one area. However, they were used in the preliminary runs.

8 Following previous criteria, nominal effort was defined by number of hooks per trip or sub-trip (in thousands of hooks) calculated from the number of sets carried out and the mean number of hooks per set. The nominal CPUE, in kilograms round weight or in number of fish, was defined as catch per nominal effort (per thousand hooks). For the South Atlantic stock, the hypothetical boundary line was located at 5 o N latitude, as assumed by the ICCAT. Initially, six areas were defined for preliminary runs (see SCRS/99/32), but finally only four areas were used (because of limited observations and no directed fishing in Areas 8 and 1). The year sequence from 1989 to 1997 was analyzed in biomass and in number of fish per thousand hooks. The time breakdown used was quarters. The surface longline gear of the Spanish fleet has remained relatively constant over the years analyzed, in terms of structure and configuration. However, in the recent years small improvements and changes in fishing strategy were detected. An important change has been reported in the fishing strategy of the traditional fleets in the North stock, in some cases targeting swordfish jointly with other species. This new strategy could also affect, to some extent, the activity of the South Atlantic fleet in recent years due to market demand, although this effect is expected to be lower than in the North Atlantic stock. Although some trips were already clearly identified as targeting sharks and deleted, this classification is not an easy decision, especially for long trips, since the fishing strategy can even change several times during the same trip. The standardized analyses were done using the GLM procedure. The main effects considered were year, time and area. The following basic model was defined: Log(CPUEw)= overall mean + effect of year i + effect of quarter j + effect of area k + effect of area and quarter interactions + the normally distributed error term The variability explained by the model (CPUEw, i.e. CPUE in weight) was 31%. Standardized residual patterns by year obtained show a normally distributed shape when there are a suitable number of observations. Table 4 and Figure 7 provides information on CPUEw and upper and lower 95% confidence limits. Additionally, some standardized CPUEs by each area were obtained during the meeting (Figure 8) in order to see the trends in each area defined and to compare with the values obtained from other fleets using the same area definition (Figure 1). The standardized CPUE in areas 12 and 11 show a decreasing trend, while areas 9 and 7 show almost a flat trend. So, the general trend in the Spanish CPUE is mostly conditioned by the trend in area 12 where most of the observations were available at the beginning of the time series. Japan There are two kinds of data available from Japanese longliners: (1) from sample statistics aggregated by year, month, five-degree area, i.e., the TASK II data base submitted to ICCAT; and (2) that which has an additional item of number of branch lines per basket, which approximates the gear configurations of regular vs deep longline. The latter statistics have been collected since In these data bases the catch is given in number of fish. For , catch in weight were also available as estimated by

9 converting catch in number into weight (ton) using size composition by year, ICCAT swordfish area, and quarter. For purposes of CPUE standardization (SCRS/99/33), the five-degree areas which could obtain continuous and relatively high effort level (greater than 5, hooks per each year) were selected and used for the GLM analysis to secure the succession of data and reduce the area effect caused by the change of fishing ground. As a result, 7 and 14 five-degree areas which are the main fishing grounds for bigeye tuna and southern bluefin tuna, respectively, were selected on the eastern side of the South Atlantic. As only the most northern areas used in the analysis were overlapped with the fishing ground of Spanish longliners that target swordfish, the area used in the analysis was supposed to be the marginal distribution area of swordfish. The biomass CPUE was also standardized using the area stratification in SCRS/99/32 for The data series used in this analysis was the catch (MT) and effort (number of hooks) data aggregated by year, month and five-degree area. All the Japanese swordfish catch in the South Atlantic were done by longline, which started in 1956 (Figure 9). Since then, the effort level increased and peaked in 1965 with 54 million hooks and then decreased to 6 million hooks in From 1977, the effort level showed generally an increasing trend with some fluctuation and recorded a historical high in 1993 with 71 million hooks. Since 1994, effort started to decrease rapidly. The 1997 value was 48 million hooks. The catch trend (expressed in MT) had a relatively high correlation with the trend of effort. Before 1976, it peaked in 1965 and 197, with catches of 1,8 and 2,9 MT, respectively. Since 1977, it showed an increasing trend with some fluctuation and reached 6,7 MT in 199 and 5,3 MT in After that, catches decreased rapidly. The 1997 value was 1,4 MT. The ratio of the Japanese swordfish catch to that of the total South Atlantic catch was roughly above 4% for the period between (average: 44%) and (average: 41%) when the Japanese catch was relatively higher (Figure 1). For the period between , the ratio was roughly less than 2% (average:16%). The share of the Japanese catch has shown a steady decreasing trend since 1987 when the total swordfish catch in the South Atlantic showed a sudden increase and then decreased 8% in For the period before 198, Japanese longliners targeted albacore using less than 11 hooks between floats (NHF).From the beginning of the 198's deep longline (NHF=12-15) has been introduced to target bigeye tuna, and further, much deeper longline (NHF>15) was introduced since 1988 (Figure 11). The main fishing ground of Japanese longliners that target bigeye was in the tropical and subtropical areas in the southeastern Atlantic (Figure 12). The standardized CPUE of swordfish caught by Japanese longliners for the selected area in the southeast Atlantic were obtained for the period of in number and for in biomass (SWO/99/3). The models used for GLM analysis were: (number); ln(cpue ijkl +const)=(f)+(yri)+(qt j )+(AR k )+e ijkl (biomass); ln(cpue ijkl +const)=(f)+(yr i )+(QT j )+(AR k )+(GE l )+(INTER)+e ijkl (INTER: combinations of two-way interaction of YR*QT, AR*QT, YR*AR and YR*GE.)

10 The results of those analyses were as follows: Figure 13, standardized CPUE of swordfish in number; Figure 14, standardized CPUE of swordfish in biomass (the broken line shows the values of standardized biomass CPUE for a wide area). The CPUE trend was generally stable for the period between and 1975 and The CPUE showed a rapidly decreasing trend until 1997 after the sudden increase observed in 1984 and In order to compare with other fleets CPUE, a standardized CPUE of Japanese longliners was also obtained during the meeting, using the area stratification of SCRS/99/2 (Figure 1) for the years of (Figure 15). The model used for the GLM analysis was; ln(cpue ijkl +const)=(f)+(yr i )+(QT j )+(AR k )+(GE l )+(INTER)+e ijkl (INTER: combinations of two way interaction of YR*QT,) The results (Figure 15)show the value in area 7 might not reflect the actual trend of the swordfish stock in that area as the number of data used in the analysis was very few. The observed high variation of CPUE in this area came mainly from the yearly change of concentration of fishing effort within the area. This indicates that further area stratification is necessary to standardize the CPUE in each area. Brazil The available data on swordfish for the Santos based fleet are catch (in weight) and effort by month (1971 to 1997); catch (in weight) and effort by 5x5 area (1971 to 1989); in weight and number (1992 to 1997) by 5x5 area. Effort has shown a increasing trend since 1971 and the total catch fluctuated, with peaks in 198 and The targeted species were tunas in the 197s, and swordfish during part of 198 and However, since 1979, effort has gradually been directed at sharks (mainly blue shark) until mid In 1994, some boats began to change their gear in order to catch swordfish again. As the number of hook lines per set may define the target species, the correlation between the number of lines per set and the catch rates of swordfish was investigated for the data of Brazil Chinese Taipei, according to categorizations used by Japan. The results of the analysis showed high swordfish catch rates for longline sets with 4, 6 and 7 branch lines. Values for set with four branch lines were about six times higher, suggesting the possibility of using gear configuration as an indication of target species effect in further GLM analysis. Initial GLMs were conducted at the meeting for the Brazilian national fleet and for the Brazil-Chinese Taipei fleet. For the Brazilian national fleet, a total of 2,579 observations were used for the period Data used comprised records per trip, aggregated by month, 5x5 block and number of branch lines between buoys (per set). The results of GLM analysis showed that 47.7% of the variability in CPUE was explained by the model. For the Brazil-Chinese Taipei fleet, data from logbooks, obtained per set were aggregated by month, 5x5 block and by number of branch lines between buoys (per set). A total of 1,233 observations were used for the period. The results of GLM analysis showed that 27.7% of the variance in CPUE was explained by the model. The results are shown in Appendices 4 and 5, Tables 5 and 6, and Figures 16 and 17. The standardized CPUE (kg/1 hooks) for Brazilian fleets shows an increasing trend from 1986 to 1994, then a decrease from 1994 to 1996, and a minor recovery in CPUE trends for the Brazil-

11 Chinese Taipei fleet show a sharp decrease from 1991 to 1992, then a gradual decrease until 1996 and then showed a recovery in It was recommended that IBAMA implement an observer program for leased and national tuna fishing fleets for the northeast and southeast regions of Brazil. They can obtain biological sampling (e.g. cleytrum-keel measurement and lower jaw fork length) and also many data on by-catch and fishing activities. Portugal The information presently available was extracted from the logbooks of Portuguese longliners, operating in the South Atlantic during years 1997 and The raw data matrix consists of swordfish catches and non-targeted species catches, by daily fishing operation (set) and vessel. During this period, data on a total of 846 longline fishing sets (456 in 1997 and 39 in 1998) were collected from five Portuguese longliners. Geographical area are recorded according to the FAO statistical areas (Divisions 34 and 41, and their respective sub-divisions). Concerning FAO area 34, only the sets located from 5ºN and 5ºS were selected as belonging to the South Atlantic swordfish stock, as assumed by ICCAT. As the specific catch sets are reported in dressed weight, they were converted to live weight using the equations recommended by ICCAT. Nominal effort units are in number of hooks per set. An exploratory analysis of the daily sets was made, with the main purpose of understanding some elementary descriptive data characteristics, such as centrality and dispersion over time among the geographic areas considered. Basic statistic characteristics, namely, centrality measures (modes, medians and averages) and dispersion measures (ranges, standard deviations, variances, minimums, maximums, ranges, etc.) were estimated. Histograms of the observations by year and region were also obtained. In order to understand the inter-annual variation and trends of the nominal CPUE by month for the main areas, several procedures were made. Box plots were made of the observed daily set swordfish catches by month and main areas, as well as plots of average catches and standard error trends by month for the main areas. The CPUE estimates by month and area were determined using arithmetic means of catches by sets to investigate overall CPUE trends. This data treatment and analysis was also made at the sub-areas level. However, given the low number of observations by sub-area, only the nominal CPUE series were considered. In general, the limited number of years in question (two) did not permit a more in-depth studies than that described above. Nevertheless, monthly plots of nominal CPUE show, in some way, Portuguese longline fishing in those waters (Figure 18). Some attempts of standardizing fishing effort based on univariate GLM methods were made. The chosen categorical variables were: Year, Month, Quarter, Area and three Ratio categories (I- ratios ; II ratios ; and III- ratios 7-1). Several models were tested using combinations of the different factors. In general, some results were considered satisfactory. However, the source data need to be revised in order to confirm some detected errors. Implications of CPUE analysis results The examination of the CPUE in the papers prepared for this meeting and in the analyses conducted at this meeting were very useful in determining both the extent and usefulness of the data for subsequent assessment work. When the various data sets were stratified by area, the similarities and differences in

12 trends became more clear (Figure 19). Trends between Japanese, Brazilian and Spanish indices were similar for some areas and different for others, whereas, the Chinese Taipei indices appeared to be different in some areas (Figure 6). Also, it is noted that the Japanese indices, where showing similar area-specific trends as the Spanish, were typically more variable from year to year, especially in the early years of the indices. The issue of how to handle target species in the GLM analyses was common over several different data sets including Japanese, Chinese-Taipei and Brazilian data. For both the Japanese and Chinese Taipei fisheries swordfish are reported not to be the targeted species. Thus, CPUE may be affected by the availability of other species. The definition of targeting was also suggested to be a problem with the Brazilian CPUE, because the fleet targeted sharks in the early period. One possible solution could lie in the use of pre-defined ratios as categorical variables to mimic targeting strategies. Boat type and captain characteristics might be useful for specify targeting categories as well. Gear types are an obvious method of categorization, and has been used in several data sets. However, because the specificity in the size composition of the catch between deep vs surface longlines, the CPUEs so obtained may track different portion of the stock The targeting differences and area-specific differences in CPUE trends are important aspects that should be investigated. Development of combined CPUE indices will necessitate standardization of factors to include in the model. Substantial progress was made at the meeting in that area specific catch rate patterns, which generally were observed to be similar between fleets. The observation that indices developed from the Japanese data had lower variances when analyses were restricted to small geographic areas than when large geographic areas were used, suggests that further research on use of substrata is needed before the assessment meeting in September. Additional progress was made in identifying factors in fleet-specific data sets which appear to be useful in identifying targeting or no-targeting. However the differences between fleets in the information for identifying targeting or non-targeting [e.g. hooks between floats, type of longline (surface or conventional) or catch ratio] and the absence of gear information related to targeting for some fleets could present a problem for development of combined indices. It is therefore also recommended that each nation conduct analyses on the utility of catch based indices of targeting for possible use at the assessment Fleet Combined CPUE Initial efforts to get a combined CPUE across fleets were conducted at this meeting. Noting the discussions on the differences in CPUE between the fleets, an attempt was made to develop the fleetcombined CPUE tentatively, even though any problems discussed in each CPUE standardization are still involved in these tentative runs. There are two kinds of fisheries in the south Atlantic, swordfish targeted and non-targeted fisheries. As a first step, an attempt was made to develop country-combined CPUE by the target and non-target fisheries. The examination of the CPUE data suggest that there are some consistencies between CPUE data sets when they are appropriately stratified by area (Figure 2), but that several indices occur only in limited areas. Thus, there could be some problem in using those data as indices of the stock-wide biomass. Therefore, there is a need to obtain combined biomass indices across fleets. It was suggested that this should be done in two ways: (1) using a GLM, combine the data of fleets targeting swordfish (Spain and Brazil) into an index and then combine the by-catch fleets data (Japan and Chinese Taipei) into an index; and (2) using a GLM, combine all the fleets data sets into a single biomass index.

13 As the non-target fishery CPUE, the Japanese and Chinese Taipei CPUE were to be combined. At this stage, the fleet-combined CPUE was developed by the areas which were used for the standardization of Spanish CPUE (Figure 1). The tentative model for the standardization is ln(cpue+.1)=year + Quarter + Country + %_of_swordfish. The % of swordfish, which is categorized as -25%, 26-5%, 51-75%, and 76-1%, means the percentage of swordfish catches in the total catch of bigeye, yellowfin, albacore, and swordfish (SWO/99/4) in each observation (a 5x5 block in a month). In this stage, the %_of_swordfish is also applied to the Japanese data, because there is no direct information on gear configuration, such as number of branch lines, in the Chinese Taipei database. The resultant CPUEs are shown in Figure 19. Generally, the country-combined CPUE shows an intermediate trend between Japanese and Chinese Taipei CPUEs. However, the trend in the recent years may be more affected by the Japanese CPUE than by that of Chinese Taipei. In this analysis, the trend should be affected to some degree by the annual change in fishing effort distribution, due to the absence of sub-areas in each area, and some other problems discussed in each CPUE section are still involved in this analysis. Furthermore, year*fleet, year*%_of_swordfish, and other interactions are significant for most areas as shown in Table 7, for example, although Figure 2 shows the results obtained by the model only with main effects. The discussion had to be centered on how to treat the year*fleet interaction as well as some other interactions with year. It was agreed to investigate further the problems on CPUE series in each country, that have been pointed out at this meeting before developing the fleet-combined CPUE. 7 Assessment options for South Atlantic swordfish An assessment of South Atlantic swordfish is scheduled for September, The above examination of the size and CPUE data was done in order to facilitate the assessment analyses. Given the results of the current Data Preparatory Meeting, there are several assessment options to be considered. Production modeling An application of production models is the first option to be considered. Preliminary production modeling was done in the previous assessment. The data needed for such an application are the time series of catch (in weight) and an index (or indices) of the biomass trend. Thus, it is possible to include the multiple indices directly in the production model analyses. However, as noted in the previous section, the examination of the CPUE data suggests that there are consistencies between some CPUE data sets when they are stratified by area, but that several indices occur only in limited areas. Thus, it may be problematical to use those data as indices of the stock-wide biomass. Therefore, combined biomass indices across fleets may be useful. It is suggested that this should be done in two ways: (1) using a GLM, combine the data of the fleet targeted swordfish (Spain and Brazil) into an index and then combine the by-catch fleets data (Japan and Chinese Taipei) into an index; and (2) combine (using a GLM) all the fleets data sets into a single biomass index. Initial efforts to do (1) were conducted at this meeting. Item (2) needs to be done prior to the assessment meeting. Since production models are used to determine long-term potential productivity (i.e. MSY), it is important to utilize data throughout the history of the fishery so that contrasts in the data may be shown (if they exist). Note that the time series of catch does not have to be of the same duration as the CPUE series. Therefore, the production models should include catches that are available back into the 196s. Long time series of CPUE are also important for determining productivity. In the case of South Atlantic swordfish, the long CPUE series are provided by Japan and Chinese Taipei. It is important to include these historical data back to at least the early 197s. However, to do this the quality of the index data in early years should be re-examined.

14 Age-structured modeling As noted in section 5, an estimation of catch at size is scheduled for South Atlantic swordfish. These data could be utilized in an age-structured model using the biomass index (or indices) developed above. The most obvious choice of models in this regard is an age-structured production model (ASPM) in which a population dynamics model is used to predict biomass and catch at size and these predictions are matched with the observed indices of abundance and with the estimated catch at size using maximum likelihood estimation of initial stock biomass, catchability constants, and stock-recruitment parameters, as appropriate. In order to implement this type of model, the underlying growth parameters and maturity parameters must be specified. However, the SCRS has not had major discussions about what these parameters might be specifically for South Atlantic swordfish. Given that these parameters can be specified, then initial ASPM analyses can be examined in September. Other age-structured approaches such as ADAPT require an estimation of catch at size and age in addition to indices of abundance. It also would be helpful to have indices of abundance in number for separate age groups. At this stage, it is unlikely that such analyses will be available for the September assessment. 8 Recommendations for future work Recommendations for future work are categorized into short- term (i.e. efforts needed prior to the September assessment meeting), and long-term (those research efforts that are needed for overall improvement). -- Short term Review and correct size sample data by the Secretariat as well as by national scientists; Recreate catch at size; Investigate targeting criteria for GLM analyses of CPUE s, especially for by-catch fisheries; Careful attention to treatment of 's in non-target fisheries; Combine over fleets to get indices of biomass; Evaluate available growth and maturity parameters for South Atlantic swordfish. -- Long term Growth and maturity studies; Estimation of catch at age by sex; Age-and-sex-specific indices of abundance in numbers; Carry out scientific observer program for all swordfish fishery fleets. 9 Adoption of the report The draft report was reviewed and modified. The report was adopted. 1 Closure of the meeting The meeting was adjourned. At the closure, the hospitality and technical support by the host, IBAMA has been very much appreciated. Also the hard work by all the participants were commended and the very efficient Chairmanship was also appreciated by the participants.

15 SCRS/99/19 RAPPORT DE LA RÉUNION PRÉPARATOIRE SUR LES DONNÉES DE L ESPADON DE L ATLANTIQUE SUD (Tamandaré, Brésil, 8-13 avril 1999) 1. Ouverture de la réunion La réunion préparatoire sur les données de l espadon de l Atlantique Sud s est tenue du 8 au 13 avril 1999, suite à l invitation du gouvernement brésilien, au Centre de Recherche et de Formation d IBAMA à Tamandaré, Pernambuco (Brésil). La date et le lieu de cette réunion avaient été fixés lors de la réunion du SCRS de M. Celso Martins Pinto, le Directeur de la Division de Recherche d IBAMA, a souhaité la bienvenue aux participants. Il a insisté sur l importance de la réunion et a formulé le voeu qu elle soit couronnée de succès. Les scientifiques du Brésil, du Japon, de la CE-Portugal, de la CE-Espagne, des Etats-Unis et du Taïpei chinois ont participé à la réunion. Le Secrétariat de l ICCAT a été représenté par le Dr P.M. Miyake, Secrétaire Exécutif Adjoint. La liste des participants est jointe en Appendice Adoption de l ordre du jour L ordre du jour qui avait été distribué avant la réunion a été modifié et adopté (Appendice 1). 3. Désignation du Président de la session et des Rapporteurs Le Dr Powers, Président du SCRS, a présidé toute la session en l absence de la Dr Julie Porter, Présidente du Groupe de l Espèce Espadon. Il a souhaité la bienvenue aux participants et a souligné l importance de la session en indiquant que les résultats seront essentiels pour les évaluations qui serviront de base scientifique à la gestion de l espadon du sud. Les scientifiques suivants ont été désignés comme Rapporteurs: Dr S. Turner (Point 5), K. Yokawa et J.A. Negreiros Aragau (Point 6) et J. Powers (Point 7); Le Dr P.M. Miyake a assumé la fonction de Rapporteur Général. 4. Examen des objectifs de la réunion Le Président a passé en revue les objectifs de la réunion qui sont les suivants: (1) examen de la capture et de l effort ainsi que des données de taille disponibles; (2) examen de l utilisation des données de taille à des fins analytiques et détermination des modèles applicables; (3) création des CPUE standardisées et sélection des CPUE utiles pour les analyses ultérieures et (4) spécification des points qui doivent être traités avant la session d évaluation du stock de l espadon de Sept articles ont été présentés à la réunion (la liste est jointe en Appendice 3). Ces articles ont décrit les pêcheries d espadon de l Espagne, du Japon, du Taïpei chinois et de différentes régions du Brésil. Les efforts initiaux pour créer des CPUE standardisées ont été présentés dans ces articles pour les données de l Espagne, du Japon et du Taïpei chinois. Tous ces articles ont été revus et seront examinés dans les différents points de l ordre du jour où ils sont mentionnés.

16 5. Echantillonnage des tailles Le Groupe a examiné les échantillons de taille disponibles dans la base de données ICCAT afin de déterminer s ils étaient suffisants pour calculer la capture par taille (CAS). Il a été noté qu une capture par taille pour l espadon de l Atlantique Sud avait déjà été élaborée en 1996 par le Secrétariat, mais qu elle n a pas jamais été utilisée par le Groupe dans les analyses structurées par âge ou par taille. On a également précisé que des échantillons complémentaires par taille étaient devenus disponibles après la création de la CAS, en particulier pour différentes pêcheries du Brésil et du Taïpei chinois. Le Groupe a décidé d examiner: (1) les fractions d échantillonnage par nation et par année, et (2) la cohérence d une année à l autre des échantillons de taille par nation et par zone géographique. Il a été décidé d utiliser pour ces analyses les mêmes zones que pour les analyses préliminaires des taux de capture espagnole (Figure 1), si ce n est que les limites méridionales des zones 7 et 8 ont été repoussées jusqu à 6ºS (Document 32). Comme on le verra au point 7, le Groupe a proposé de fonder ces zones sur les distributions des taux de capture espagnole et a reconnu qu il pourrait être nécessaire d affiner ces zones étant donné les progrès réalisés dans la connaissance des taux de capture et dans la composition des tailles dans les différentes pêcheries. Cet examen a permis d identifier une série de problèmes ayant trait à la base de données ICCAT, tels que les données non déclarées, les données dépassées qui auraient dû être actualisées, les doubles entrées de données et les codes incorrects de taille. Certains de ces problèmes sont imputables au fait que des nations n ont pas transmis leurs données ou les corrections de données au Secrétariat, tandis que d autres sont dus à des problèmes de gestion au Secrétariat. 5.1 Fractions de l échantillonnage Le Groupe a examiné les fractions de l échantillonnage par taille des principales nations de pêche depuis 198 (Table 1, Figures 2 et 3). Dans le cas du Brésil, les fractions de l échantillonnage s étendaient de 1% à 3% pour la période et de 2% à 3% à la fin des années 198. (Signalons que les faibles fractions d échantillonnage enregistrées pour le Brésil en 1983 et 1984 ont été revues; les données existaient au Brésil, mais pas dans la base de données ICCAT. On a estimé que les fractions pour ces années étaient semblables à celles des années environnantes et les jeux complets de données de ces années seront présentés à l ICCAT). Les fractions d échantillonnage espagnoles étaient relativement basses (3-13%) pendant les premières années de sa pêcherie et ont augmenté jusqu à environ 5-6% pendant la période Le Taïpei chinois avait des fractions d échantillonnage de 1-3% du début au milieu des années 198, suivies de fractions de 2-3% pendant les années Le Japon avait des fractions d échantillonnage de 4-8% jusqu au début des années 199, puis de 2-3%. Le Groupe a revu les fractions d échantillonnage du Japon, de l Espagne et du Taïpei chinois pour l Atlantique Sud oriental et occidental et a observé des fractions similaires dans ces deux zones pour chaque nation. Le Groupe a constaté l existence d une fraction d échantillonnage plus importante pour la pêcherie nationale brésilienne que pour celle de certaines des pêcheries de bateaux loués, en particulier pour la pêcherie Brésil-Taïpei chinois pour laquelle aucun poisson n a été mesuré depuis 1994 (Table 2). Dans l ensemble, les fractions de l échantillonnage étaient généralement comprises entre 5% et 1% dans les années 198, puis sont passées à 2-3% dans les années 199 (Figure 3) avec poissons mesurés chaque année en 198 et plus de 1. poissons mesurés chaque année depuis Une grande partie des échantillons a cependant été prélevée d un nombre relativement faible de flottilles. Le Groupe a vivement recommandé que les nations ayant de faibles fractions d échantillonnage revoient leurs systèmes d échantillonnage dans le but d augmenter la fraction, si nécessaire en utilisant

17 des critères d échantillonnage spécifiques aux espèces, surtout dans le cas de l espadon. Le sexe devrait également être enregistré avec la taille chaque fois que cela était possible. Les niveaux absolus d échantillonnage requis pour caractériser de façon adéquate la composition des tailles varieront d une pêcherie à l autre. A défaut de recherche permettant de définir les niveaux d échantillonnage appropriés, le Groupe recommande de réaliser au moins une fraction d échantillonnage étendue, qui pourrait être de 2%. Il a été suggéré d élaborer, dans la mesure du possible, un manuel d identification du sexe. 5.2 Cohérence de la composition des tailles Pour réviser la cohérence des données de composition des tailles, on a examiné des distributions cumulées de fréquences pour toutes les nations, zones et strates annuelles dans lesquelles au moins 1 poissons ont été mesurés. Le nombre de poissons mesurés en taille ou en poids (converti ensuite en taille) par pays, zone et année dans la base de données disponible de l ICCAT est illustré à la Table 3. Le Groupe a néanmoins décidé d utiliser la prise par taille (CAS) espagnole dans les analyses exploratoires étant donné que les données originales sont établies par zone de 5ºx5º/mois, que le nombre de poissons échantillonnés par carré est important, que le nombre de substitutions requises est très faible et que les analyses exploratoires sont établies en pourcentage cumulatif. Le Groupe a fait remarquer que les analyses exploratoires ont été réalisées en utilisant des données annuelles par zone expérimentale. La variabilité temporelle (mois ou trimestre) n a donc pas été prise en considération. Par ailleurs, le Groupe a indiqué que seules les informations disponibles ont été analysées; il a encouragé tous les pays à déclarer leurs données actuelles et historiques et à maintenir des fractions d échantillonnage élevées. Les distributions cumulées des fréquences (CFD) ont été projetées par zone et pays (Figure 4), ainsi que par pays, zone et année (Figure 5). En général, le Groupe a constaté une cohérence entre les distributions cumulées des fréquences (CFD) par nation et par zone, mais a observé des problèmes potentiels avec certains jeux de données. Les CFD espagnoles dans les zones 11 et 12 (zones de grandes captures) ont montré des gammes de taille d environ 1 et 2 cm pour des fréquences cumulées de 5%. Cette différence entre les années pourrait néanmoins s expliquer par des modifications de la distribution saisonnière de l effort de pêche, par la baisse du poids moyen de la prise (environ 1 kg pendant la période disponible) déclaré à partir des données espagnoles ainsi que par d autres facteurs. Les informations concernant d autres zones montrent également des tendances très cohérentes pour les CFD entre les différentes années. Des données de fréquence de taille pour la pêcherie nationale brésilienne ont été déclarées pour la zone ICCAT (Atlantique sud-occidental) sans information de latitude et longitude de 1971 à environ 1984, et, par la suite, avec des informations de latitude et longitude lorsque la plupart des données correspondaient à la zone 9 (Table 3). On a constaté, dans l ensemble, que les CFD étaient semblables au sein de ces périodes, tout en présentant des différences entre les périodes. En outre, les CFD de 1998 correspondant à la zone 9 différaient de celles des années précédentes ( ). Le Groupe a recommandé de vérifier attentivement ces modèles afin de déterminer la cause de ces différences. La pêcherie brésilienne pratiquée par des bateaux japonais loués (Brésil-Japon, zones 7 et 13) a montré de façon générale des CFD cohérentes avec un spectre plus large pour la composition des tailles dans la zone 13 (Atlantique sud-occidental, gamme de 45 cm pour des CFD de 5%) que dans la zone

18 7 (gamme d environ 3 cm pour des CFD de 5%). La plus grande variation que connaît la composition des tailles inter-annuelles dans la zone 13 pourrait être interprétée comme un indice de déplacements entre la pêche côtière (telle que la zone 7) avec une composition de petites tailles et la pêche hauturière avec des tailles plus grandes, comme cela a été le cas dans un nombre élevé de pêcheries palangrières orientales de haute mer. La pêcherie des bateaux brésiliens en régime de location, destinés à d autres nations que le Japon, s est développée ces dernières années. Le nombre limité de comparaisons disponibles a indiqué une certaine cohérence dans les CFD. (Brésil Autres, zones 7, 8, 9, 11 et 13). Le Groupe a observé une certaine cohérence dans les CFD au fil des années dans la plupart des zones ayant donné lieu à des informations, même s il existait une grande diversité entre ces années. La révision des données brutes pour 198 dans la zone 12 a laissé entrevoir l existence d informations erronées dans la base de données ICCAT, ce qui a incité le Groupe à recommander la révision de ces informations. L échelle de variation des zones 1 et 12 est de 4 cm pour des CFD de 5% avec une taille moyenne de 16 à 2 cm LJFL. La composition coréenne des tailles est certes très limitée, mais semble être relativement cohérente dans la zone 11 pour les années disponibles. La composition des tailles est semblable à celle des CFD japonaises de la même zone. Les CFD pour le Taïpei chinois (uniquement ; zones 13 et 14) ont indiqué la présence de poissons plus grands dans l Atlantique sud-oriental que dans l Atlantique sud-occidental. La similitude des CFD de 1997 issues de ces deux zones et leur taille moyenne supérieure à celles des années précédentes ont été mises en cause. Une révision des données de 1997 réalisée par des scientifiques du Taïpei chinois a montré des différences par rapport aux données préliminaires reçues par l ICCAT. Les participants ont recommandé de comparer attentivement ces deux bases de données et d envoyer les informations les plus précises à l ICCAT. 5.3 Aperçu de la composition des tailles L examen de la composition des tailles par année et par zone à l aide de données provenant de différentes flottilles a révélé un modèle général de composition de tailles plus petites pour le Brésil, Brésil Autres et l Espagne comparé aux captures des palangres profondes de la Corée, du Taïpei chinois et surtout du Japon (Figure 5). Ces différences dans la distribution des tailles entre différents types d engin (surface contre palangre) peuvent être associées à la stratégie de pêche des différentes flottilles, mais pourraient également indiquer une stratification thermique avec une profondeur semblable à celle que l on connaît déjà pour la latitude. Si ces différences sont réelles, il se peut que les grands espadons soient plus difficiles à capturer à la palangre de surface qu à la palangre profonde. 5.4 Recommandations générales sur la composition des tailles Le Groupe insiste pour que tous les pays déclarent leurs données brutes de capture au Secrétariat comme l a demandé le SCRS il y a plusieurs années déjà. Le fait de déclarer leurs données de capture par taille ne les dispense pas de cette responsabilité. 5.5 Utilisation des données de composition par taille pour créer la capture par taille Le Groupe a conclu que si les problèmes identifiés au sujet des bases de données de la composition par taille sont examinés à fond et, le cas échéant, résolus, ces données pourront être utilisées pour créer

19 les CAS, même s il se peut que les données de taille datant d avant 199 soient moins appropriées et qu il soit nécessaire de remédier à cette situation. Ceci dit, le Groupe a indiqué que le taux global d échantillonnage obtenu après 199 est essentiellement dû à la hausse des captures et des échantillonnages réalisés par certaines flottilles (notamment le Brésil et l Espagne qui ont des taux élevés de couverture d échantillonnage). Le niveau moyen global de l échantillonnage pour la période était d environ 8%, tandis que le taux moyen global de la période était de 27%. Le Groupe a recommandé que les examens des données de composition par taille soient terminés le 3 avril, que les révisions des compositions par taille soient présentées au Secrétariat le 31 mai et que les captures par taille soient créées par chaque nation pour la période et soient présentées le 31 juillet Si la couverture d échantillonnage est suffisante pour certaines pêcheries, la CAS devra être créée pour une période remontant encore plus loin dans le temps. Le Secrétariat créera des captures par taille pour les petites pêcheries telles que l Uruguay, la Namibie et le Ghana. Il a été vivement recommandé que les scientifiques nationaux transmettent des informations sur les procédures utilisées pour les substitutions et les extrapolations pour créer la capture par taille. Les participants ont discuté des strates à utiliser pour créer les captures par taille et ont recommandé d appliquer les strates spatio-temporelles les plus petites possibles. Le Groupe a reconnu que la stratification dépendra de la façon dont la pêcherie et l échantillonnage seront réalisés, tout en suggérant d envisager une dimension maximale de zone de 1ºx1º. 6. Examen de la série de CPUE de l Atlantique Sud Les données disponibles de capture par unité d effort (CPUE) ont été examinées et évaluées pour déterminer leur utilité dans la mise au point d indices d abondance pour le stock de l espadon de l Atlantique Sud (ou de parties de ce dernier). Le Secrétariat ICCAT a préparé, comme première mesure, un résumé des données de capture et d effort qui étaient disponibles dans la base de données. Ces données contenaient des informations de l Argentine, du Brésil, du Taïpei chinois, de Cuba, de la Corée, de l Afrique du Sud, de l Espagne, des Etats-Unis, de la Russie, de l Uruguay et de la Namibie. Dans de nombreux cas, les séries de données ne pourront pas être utilisées dans les indices d abondance étant donné que bon nombre d entre elles ne couvrent que de courtes périodes. En revanche, d autres séries semblent être utiles à cette fin. Il faut encore signaler que seules les données du Japon, de l Espagne et du Taïpei chinois ont été utilisées dans les analyses d évaluation antérieures. Après avoir examiné la base de données, on a étudié en détail les données des flottilles du Brésil, du Brésil-Taïpei chinois, de l Espagne, du Japon et du Portugal. Les descriptions de ces séries de données figurent ci-dessous. Par ailleurs, les données du Brésil, de l Espagne, du Taïpei chinois et du Japon ont été réexaminées en utilisant les divisions géographiques présentées par l Espagne (SCRS/99/32) à l aide d un GLM pour standardiser la CPUE (Figure 1). Cette division diffère des zones ICCAT, mais a été utilisée comme un instrument qui sert à comparer les CPUE entre les flottilles au sein des zones. Elle a notamment mis en évidence une homogénéité relative entre les CPUE des flottilles au sein d une même zone. Tendances des captures, efforts et CPUE par flottille Taïpei chinois L espadon est une prise accessoire des flottilles palangrières du Taïpei chinois (SCRS/99/34). Le modèle de pêche a subi une transformation évidente à partir de la fin des années 198 (Hsu et Chen 1995, SCRS/95/97) qui semble avoir abouti à l augmentation des prises de thonidés tropicaux, d espadons et des

20 poissons porte-épée (Hsu 1996, SCRS/96/1). La proportion d espadons dans le total des prises palangrières a fluctué de 1968 à la fin des années 198, puis a connu une hausse brutale en 1991 et 1992 et a diminué en 1993 avant de se maintenir à un niveau relativement stable. La série du Taïpei chinois pour l espadon de l Atlantique (données Tâche II dans la base de données ICCAT) couvre une période relativement étendue depuis Ces données sont présentées par mois et par zone de 5ºx5º et contiennent les prises en nombre et en poids (en kg) et l effort en nombre d hameçons. Les données de fréquence de taille sont également disponibles, en tailles mesurées et en capture par taille, à partir de 1981 par trimestre et par zone d échantillonnage ICCAT (zones 96 et 97). L effort de pêche (nombre d hameçons) de cette flottille palangrière visant les thonidés dans l Atlantique Sud a considérablement augmenté entre 1968 (19,8 millions d hameçons) et 1997 (117 millions d hameçons). La prise d espadon dans l Atlantique Sud est passée de 488 TM en 1968 à TM en 197. Elle a fluctué dans les années suivantes, est retombée au seuil historique de 199 TM en 1984 et s est redressée brutalement en passant de 896 TM en 199 au plafond de TM en La CPUE nominale a connu son niveau le plus faible en 1976 (,35/1. hameçons) et le plus élevé en 1992 (1,49/1. hameçons). Toutes les espèces de poissons porte-épée, y compris l espadon, sont prises de façon fortuite par les flottilles palangrières dans l Atlantique Sud (Hsu, 1996). La modification évidente du modèle de pêche observée depuis la fin des années 198, qui semble avoir provoqué la hausse du taux de capture d espadons, peut également être un facteur supplémentaire qui aurait affecté la CPUE de l espadon du Taïpei chinois. C est pour cette raison qu on a réalisé des analyses GLM complémentaires. La CPUE de l espadon du Taïpei chinois dans l Atlantique Sud a été standardisée en utilisant la désignation géographique montrée dans SWO/99/2. Le modèle original utilisé dans l analyse GLM était le suivant: In(CPUE ijkl +const)=µ+yr+qt j +AR k +INTER+e ijkl INTER: combinaisons d une interaction à deux voies de YR*QT La tendance de la CPUE dans chaque zone a fluctué et s est accrue au début des années 199. Une des principales raisons de cette fluctuation pourrait être le changement du modèle de pêche observé à la fin des années 198 (Liu et Wang, 1999, SCRS/99/34). On a effectué des analyses comprenant un facteur supplémentaire, en utilisant les catégories de ratio de captures d espadon en poids par rapport au poids total de germon, du thon obèse, de l albacore et de l espadon des données Tâches II (regroupées par année, mois et carré de cinq degrés) dans le but de tenir compte de l effet causé par le changement de l espèce visée. Quatre catégories de ratio de capture d espadon ont été utilisées dans l analyse: -25%, 25-5%, 5-75% et 75%-1%. Les résultats montrent des tendances planes pendant la période analysée pour toutes les zones (Figure 6). Comme les données utilisées dans cette analyse ont été regroupées, la tendance de la CPUE (en poids) de la somme de germon, thon obèse, albacore et espadon a également été examinée afin d observer l effet de la tendance de la CPUE de la somme des espèces regroupées sur celle de l espadon. Les tendances de la CPUE de la somme des espèces regroupées semblaient également planes. On a indiqué, par conséquent, que le facteur du ratio de capture de l espadon dans l analyse GLM de la CPUE du Taïpei chinois affecte la CPUE de l espadon. Par contre, la CPUE calculée du Taïpei chinois avec l effet du ratio de capture de l espadon pourrait être influencée par d autres facteurs. C est pour cette raison qu il est nécessaire d examiner les facteurs affectant la tendance de la CPUE, en particulier après 199.