UPDATED WHITE MARLIN AND BLUE MARLIN CATCH RATES FROM THE TAIWANESE LONGLINE FISHERY IN THE ATLANTIC

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1 SCRS/2006/102 Col. Vol. Sci. Pap. ICCAT, 60(5): (2007) UPDATED WHITE MARLIN AND BLUE MARLIN CATCH RATES FROM THE TAIWANESE LONGLINE FISHERY IN THE ATLANTIC Yu-Min Yeh SUMMARY CPUEs of Atlantic white marlin (Tetrapturus albidus) and blue marlin (Makaira nigricans) caught by the Taiwanese longline fishery for - were standardized using a General Linear Model (GLM). The GLM model with year, fishing season, area, and target species as the main factors, and with fishing season and area interaction for the period from to- were carried out for the two hypotheses on the stock structure, one total Atlantic stock and separate North and South Atlantic stocks. Standardized CPUEs of white marlin show a decreasing tendency from to 1977, maintain a stable level from 1977 to, reach the lowest level in 1991, and then increase to the highest level in and decrease from onwards. Standardized CPUEs of blue marlin show a decreasing tendency from to 1987, which then increase to the level of the early s and then decrease again from 2001 onwards. RÉSUMÉ Les CPUE du makaire blanc (Tetrapturus albidus) et du makaire bleu (Makaira nigricans) de l Atlantique, capturés par la pêcherie palangrière du Taïpei chinois de à, ont été standardisées à l aide d un Modèle Linéaire Généralisé (GLM). Le modèle GLM a été appliqué avec l année, la saison de pêche, la zone et l espèce cible comme facteurs principaux, ainsi qu avec l interaction de saison de pêche et de zone pour la période - pour les deux hypothèses sur la structure du stock : un stock total pour l Atlantique et deux stocks distincts pour l Atlantique nord et sud. Les CPUE standardisées du makaire blanc présentent une tendance à la baisse de à 1977, se stabilisent de 1977 à, atteignent le niveau le plus faible en 1991, puis augmentent jusqu au niveau le plus élevé en et diminuent à partir de. Les CPUE standardisées du makaire bleu affichent une tendance à la baisse de à 1987, puis augmentent pour atteindre le niveau du début des années et diminuent de nouveau à partir de RESUMEN Se estandarizaron las CPUE de la aguja blanca del Atlántico (Tetrapturus albidus) y la aguja azul del Atlántico (Makaira nigricans) capturadas por la pesquería de palangre de Taipei Chino para - utilizando un modelo lineal generalizado (GLM). El GLM, con año, temporada de pesca, área y especie objetivo como factores principales, y con interacción de temporada de pesca y área para el periodo -, se llevó a cabo para dos hipótesis sobre la estructura del stock, un stock del total del atlántico y stocks separados en el Norte y en el Sur. Las CPUE estandarizadas de la aguja blanca muestran una tendencia descendente desde hasta 1977, mantienen un nivel estable desde 1977 hasta, llegan al nivel más bajo en 1991 y a partir de entonces aumentan hasta alcanzar el nivel más alto en y descienden de nuevo a partir de. Las CPUE de aguja azul muestran una tendencia descendente desde hasta 1987, posteriormente aumentan hasta el nivel de principios de los 70 y luego descienden de nuevo a partir de KEYWORDS Tuna longline fisheries, by-catch, catch and effort data, General Linear Model 1696

2 1. Introduction Taiwan is one of the major nations using longline gear fishing for tunas and tuna-like species in the Atlantic. Atlantic white marlin and blue marlin have been exploited as bycatch by the Taiwanese longline fishery since about However, the amounts harvested annually by the Taiwanese longline fishery are significant among the total Atlantic catches. Therefore, Taiwanese catch data was important for understanding the stock status of Atlantic white marlin and blue marlin. In this document, an updated standardized catch per unit effort series for white marlin and blue marlin based on the Taiwanese longline fishery operating in the Atlantic are provided using a general linear model (GLM). 2. Materials and methods 2.1 Basic data The landings for white marlin (Figure 1) and blue marlin (Figure 2) are compiled by similar procedures to those of Hsu (), from sources of Fisheries books of the Taiwan Are, Annual Catch Statistics of Taiwanese Tuna Longline fishery, and the tuna longline logbook which are submitted by captains of longline fleets and collected by the Department of Fisheries, Kaohsiung Municipal Government (Hsu, ). Catches of the species, white marlin and blue marlin, are verified and provided by Oversea Fisheries Development Council (OFDC) from to. The Task II date set was used to in the GLM analysis. 2.2 Standardized abundance indices The GLM model with the main factors, year, fishing season, area and target species were used with a fishing season*area interaction to adjust the nominal catch per unit effort data for white marlin and blue marlin. The GLM model used was: Log(CPUE+10% of grand mean) = u + effect + Fishing season effect + Area effect + Target species effect + Fishing season*area interaction + error term, where CPUE in the model is in number of billfish caught per 1000 hooks, u is the overall mean, and the error term follows a normal distribution with mean 0. The F-test was used on all effects and interactions to determine the significance of the contribution of each factor to the model. Fishing season was assigned as bimonthly period (Fishing season 1: January and February, Fishing season 2: March and April, and so on). Area classification (10 areas, as shown in Figure 4) was adopted from Hsu () which was done according to fishing effort distribution and nominal catch per unit effort of billfish (Hsu, ). White marlin and blue marlin catch rates were supposed to be affected by the shift of target species. From 1995, Taiwanese logbook data record the gear configuration information, that is, number of hooks per basket. The gear configuration information was used to identify the target effect in the catch data. The gear configuration was categorized into four levels: 4-6, 7-10, 11-14, and hooks per basket. The mean proportions of bigeye catch in the four species (bigeye, albacore, white marlin and blue marlin) for the four categories were calculated based on the logbook data from 1995 to. Three critical value of bigeye catch composition was determined to be 0.374, 0.634, and 0.8. Then the task II date set was categorized into four levels of target species effect. Catch data of bycatch species was subject to under-reporting in loneline fishery. To compensate for this problem in the CPUE analysis, the Task II data were raised to Task I catch. The detail explanation and the raising procedure were described in Chang (2003). 3. Results and discussions 3.1Catch, effort and nominal catch per unit effort White marlin and blue marlin were analyzed for the total Atlantic and also separately as north and south Atlantic stocks (separated at 5 N latitude). The historical total catch trend in the total, north, and south Atlantic from to for white marlin and blue marlin are given in Figure 1 and 2, respectively. The historical effort trend based on Task II data is given in Figure 3. The catches of white marlin indicate that there were two high-catch periods from 1969 to, and from 1987 to 1997, which then decline from the historical-high level (Figure 1). Those catches almost come from the south Atlantic (Figure 1) where conventional longline Taiwan vessels operated to albacore (in particular since 1987). 1697

3 The increasing catch may relate to the albacore targeted by Taiwanese longline fleets. A similar situation occurs for blue marlin (Figure 2). 3.2 Abundance indices The nominal CPUE for white marlin and blue marlin are shown in Figure 5. The standardized CPUE series for white marlin and blue marlin with estimated 95% confidence intervals are shown in Table 3, Table 4, Figure 6, and Figure 7. Adjusted catch per unit effort was standardized by the GLM method with year, fishing season, area and target species factors, and fishing season*area interaction as ICCAT species working groups used in previous assessments. The significant tests of standardized catch per unit effort for white marlin and blue marlin were given in Table 1 and 2, respectively. The models explained 26% and 38% of the variation in CPUE for white marlin and blue marlin, respectively for the total Atlantic. The adjusted catch per unit effort trend of white marlin for the Taiwanese longline fishery shows great fluctuation from to for the total Atlantic and north Atlantic series (Figure 6). In contrast, the south Atlantic series showed a decreasing trend from to 1977, was relatively flat through 1989, and then fluctuated through. The blue marlin index showed a decreasing trend from to, was relatively flat from the late s through the s, and then increase through for the total Atlantic series, and then decreasing after 1997 (Figure 7). Trends for the north and south series were similar to the total Atlantic results. The Taiwanese distant-water tuna longline fishery catches billfishes incidentally in the Atlantic. However, targeting is suspected for Taiwanese longline fleets operating joint venture in the waters off Brazil during the period from to. This may be one of the possible reasons for the increasing billfish catch during the s. There were between several and 20 or more Taiwanese conventional longline vessels operating in the southeastern and northeastern waters off Brazil, especially in the south-eastern waters during the period to (Hsu, ). Those vessels were joint ventures with Brazil, and targeted mainly albacore. Whether or not targeting on either swordfish or billfishes is suspected, the amounts of swordfish and billfish catches were highly significant. The 1967 Task II data were not included during standardization of catch per unit effort series because of the low coverage of 5x5 square blocks in Task II, which also pertained only to the last half of that year (from July to December). Therefore, the adjusted catch per unit effort 1967 may not represent the abundance index appropriately. Thus, only the data to were used for the standardization purpose. Finally, standardized CPUEs of white marlin show a decreasing tendency from to 1977, maintain in a stable level from 1977 to, reach at the lowest level in 1991, and then increase to the highest level in and decrease from onwards. Standardized CPUEs of blue marlin show a decreasing tendency from to 1987, which then increase to the level of the early s and then decreasing again from 2001 onwards. References CHANG, S.K Analysis of Taiwanese White Marlin Catch Data and Standardization of Catch Rates. Col. Vol. Sci. Pap., ICCAT, 55(2); HSU, C.C.. A Review of Catch Fishing Effort and Catch per Unit of Effort of Taiwanese Longline Fishery for Billfishes in the Atlantic. Col. Vol. Sci. Pap., ICCAT, 47; HSU, C.C.. Catch, Fishing Effort and Catch per Unit Effort of Taiwanese Longline Fishery for Blue Marlin and White Marlin in the Atlantic. Col. Vol. Sci.P ap. ICCAT, 53;

4 Table 1. ANOVA results for the GLM procedure for adjusted catch per unit effort for white marlin in the Atlantic, -. ( 1 )Total Atlantic Sources DF Sum of Mean F value Pr>F Model < <.0001 Bi-Month Area <.0001 Target <.0001 Bi-Month*Area <.0001 Error Corrected Total ( 2 ) North Atlantic Sources DF Sum of Mean F value Pr>F Model < <.0001 Bi-Month <.0001 Area <.0001 Target Bi-Month*Area Error Corrected Total ( 3 ) South Atlantic Sources DF Sum of Mean F value Pr>F Model < <.0001 Bi-Month <.0001 Area <.0001 Target <.0001 Bi-Month*Area <.0001 Error Corrected Total

5 Table 2. ANOVA results for the GLM procedure for adjusted catch per unit effort for blue marlin in the Atlantic, -. ( 1 )Total Atlantic Sources DF Sum of Mean F value Pr>F Model < <.0001 Bi-Month <.0001 Area <.0001 Target <.0001 Bi-Month*Area <.0001 Error Corrected Total ( 2 ) North Atlantic Sources DF Sum of Mean F value Pr>F Model < <.0001 Bi-Month <.0001 Area <.0001 Target <.0001 Bi-Month*Area <.0001 Error Corrected Total ( 3 ) South Atlantic Sources DF Sum of Mean F value Pr>F Model < <.0001 Bi-Month <.0001 Area <.0001 Target <.0001 Bi-Month*Area <.0001 Error Corrected Total

6 Table 3. White marlin standardized catch rate (No./1000 hooks), and limits of the 95% confidence interval (CI) of the standardized index. Lower Upper North Lower Upper South Lower Upper Atlantic 95% CI 95% CI Atlantic 95% CI 95% CI Atlantic 95% CI 95% CI

7 Table 4. Blue marlin standardized catch rate (No./1000 hooks), and limits of the 95% confidence interval (CI) of the standardized index. Lower Upper North Lower Upper South Lower Upper Atlantic 95% CI 95% CI Atlantic 95% CI 95% CI Atlantic 95% CI 95% CI

8 At. Whm White marlin N. At. Whm S. At. Whm 1200 Catch(mt) Figure 1. Total catches for white marlin in the total, north, and south Atlantic by the Taiwanese longline fishery. At. Bum Blue marlin N. At. Bum 1750 S. At. Bum Catch(mt) Figure 2. Total catches for blue marlin in the total, north, and south Atlantic by Taiwanese longline fishery. Effort (million hooks) Atlantic N. Atlantic S. Atlantic 20 0 Figure 3. Effort trend based on Task II data. 1703

9 Figure 4. Area division used for general linear model analysis on the standardization of catch per unit effort of white marlin and blue marlin by the Taiwanese longline fishery in the Atlantic. Nominal CPUE (No./1000 hooks) Atlantic N. Atlantic S. Atlantic White marlin 0 Atlantic Blue marlin Nominal CPUE (No./1000 hooks) N. Atlantic S. Atlantic 0 Figure 5. Nominal CPUE of white marlin and blue marlin in the total, north, and south Atlantic by the Taiwanese longline fishery. 1704

10 Entire Atlantic White Marlin North Atlantic White Marlin South Atlantic White Marlin Figure 6. Standardized CPUE of white marlin with estimated 95% confidence intervals in the entire, north, and south Atlantic for the Taiwanese longline fishery. 1705

11 Entire Atlantic Blue Marlin North Atlantic Blue Marlin South Atlantic Blue Marlin Figure 7. Standardized CPUE of blue marlin with estimated 95% confidence intervals in the entire, north, and south Atlantic for the Taiwanese longline fishery. 1706