SCRS/2020/107 Collect. Vol. Sci. Pap. ICCAT, 77(7): 391-401 2020)

LENGTH COMPOSITION OF ALBACORE TUNA COLLECTED FROM THE CHINESE TAIPEI TUNA LONGLINE FISHERY IN THE NORTH ATLANTIC OCEAN

Nan-Jay Su1, Wei-Ren Lin1 and Kwang-Ming Liu2

SUMMARY

The albacore tuna caught by the Chinese Taipei tuna longline fleet were sampled and measured in the main fishing ground between 15°N to 40°N in the North Atlantic Ocean. In total 16 Chinese Taipei tuna longline vessels were selected for analysis because these vessels target albacore. During the 1980s and 1990s, the size of albacore caught in this fishery ranged from 80 to 120 cm FL with median values around 100 cm in fork length (FL). After 2000, size samples collected from these vessels centered around 90-110 cm FL. The sizes of albacore caught seemed to suddenly increase during 2000s for only two fishing vessels. However, for the other vessels that targets albacore, the sizes of albacore ca increased to around 110 cm FL from 2008 and remained relatively stable during the next 10 years to 2017 except for 2014.

RÉSUMÉ

Le germon capturé par la flottille de palangriers thoniers du Taipei chinois a été échantillonné et mesuré dans la principale zone de pêche située entre 15°N et 40°N dans l'océan Atlantique Nord. Au total, 16 palangriers thoniers du Taipei chinois ont été sélectionnés pour l'analyse car ces navires ciblent le germon. Au cours des années 1980 et 1990, la taille du germon capturé dans cette pêcherie était comprise entre 80 et 120 cm FL, avec des valeurs médianes d'environ 100 cm de longueur à la fourche (FL). Après 2000, les échantillons de taille collectés provenant de ces navires mesuraient principalement 90-110 cm FL. Les tailles de germon capturées semblent avoir augmenté soudainement au cours des années 2000 pour deux navires de pêche seulement. Cependant, pour les autres navires qui ciblent le germon, les tailles de germon ont augmenté pour atteindre environ 110 cm FL à partir de 2008 et sont restées relativement stables au cours des 10 années suivantes jusqu'en 2017, sauf en 2014.

RESUMEN

Se muestrearon y midieron ejemplares de atún blanco capturados por la flota palangrera atunera de Taipei Chino en el principal caladero entre 15°N y 40°N en el océano Atlántico norte. En total se seleccionaron 16 palangreros atuneros de Taipei Chino para el análisis porque estos buques tienen como especie objetivo el atún blanco. Durante los ochenta y los noventa, la talla del atún blanco capturado en esta pesquería osciló entre 80 y 120 cm FL con valores medios de aproximadamente 100 cm FL. Después del año 2000, las muestras de talla recogidas de estos buques presentaron tallas en torno a 90-110 cm FL. Las tallas de los ejemplares de atún blanco capturados experimentaron un repentino aumento durante la década de 2000 sólo para dos buques pesqueros. Sin embargo, en el caso de los demás buques que tiene al atún blanco como especie objetivo, la talla del atún blanco aumentó hasta llegar a unos 110 cm FL a partir de 2008 y se mantuvo relativamente estable durante los diez años siguientes hasta 2017, excepto en 2014.

KEYWORDS Catch-at-size, spatial distribution, mean length, albacore tuna, North Atlantic Ocean

1 Department of Environmental Biology and Fisheries Science, National Chinese Taipei Ocean University, Keelung, Chinese Taipei 2 Institute of Marine Affairs and Resource Management, National Chinese Taipei Ocean University, Keelung, Chinese Taipei * Email: [email protected] 391 1. Introduction

Albacore tuna are caught as a bycatch in equatorial waters by the tropical tuna purse seine fishery. There has been no evidence to date of cross-equatorial migratory movement from conventional tag-recapture experiments (Sagarminaga and Arrizabalaga 2010). In the North Atlantic Ocean, many countries including Spain, French, Japan, and Chinese Taipei fish for albacore (ICCAT 2014; ICCAT 2017). Adult albacore are caught mainly by longline vessels, while a large number of juveniles are taken using surface gears, including gillnets, baitboats, trolls, and trawls (Santiago and Arrizabalaga 2005; Yang and Yeh 2004; Su and Liu 2020).

Total catches of albacore in the North Atlantic Ocean have decreased gradually from more than 60,000 tons in the 1960s to about 30,000 tons, owing to many factors such as changes in stock abundance, shifts in targeting, and other causes. The catch of albacore tuna from the Chinese Taipei distant-water longline fishery increased to a maximum of about 20,000 tons during the middle of the 1980s, but dropped substantially since the late 1980s due to the targeting shift to bigeye tuna (Thunnus obesus)(Hsieh et al. 2010; Chang and Yeh 2017).

This paper aims to provide available fishery information in size data for albacore tuna caught in the Chinese Taipei tuna longline fishery in the North Atlantic Ocean from 1981 to 2017. Spatial distribution for catch-at-size samples and mean lengths based on samples recorded by captains in logbooks from the Chinese Taipei tuna longline fleet were analyzed and presented by year and for each vessel that target on this species in the North Atlantic Ocean during the period.

2. Materials and methods

Catch-at-size data of albacore tuna caught in the Chinese Taipei distant-water tuna longline fishery in the North Atlantic Ocean were collected from the logbooks, which were obtained from the Oversea Fisheries Development Council (OFDC) of Chinese Taipei. Size data of albacore tuna were collected through an on-board sampling that the measurement is carried out by fishermen for initial 30 fishes (including tunas, billfishes, sharks and any other species) for each operation, which is an operating protocol for the Chinese Taipei distant-water tuna longline fishery.

Size data in fork length (FL, in cm) for the studied species were collected for the Chinese Taipei longline fleet in the North Atlantic Ocean, and summarized with smallest and largest observations and length frequencies using boxplots. The mean lengths by year for each vessel that targets on albacore in the fishing ground were estimated to examine the temporal variation, given the targeting shift between albacore and bigeye tunas in this area.

3. Results and discussion

The albacore tuna sampled and measured by the Chinese Taipei tuna longline fleet occurred mainly in the fishing ground between 15°N to 40°N in the North Atlantic Ocean during 1981-1987, while the size samples became less and less for 1989-1990 (Figure 1) due to the targeting shift of this fishery to bigeye tuna in tropical waters of the Atlantic Ocean. Very few size samples of albacore were collected from this fishery during the 1990s (Figure 2) and 2000s except for 2002-2005 (Figure 3). After 2011, the albacore tuna were measured in the northwestern Atlantic Ocean between 15°N to 40°N for the Chinese Taipei tuna longline fleet (Figure 4).

There were in total 16 tuna longline vessels that target albacore in the western North Atlantic Ocean, which were selected for analysis in this study (Figures 5-8). In the early period (1980s), the size of albacore caught ranged from 80 to 120 cm FL, with median values around 100 cm FL (Figure 5). Very few size samples were collected during the 1990s for the albacore targeting vessels (Figures 6-7) because these fishing vessels changed to catch bigeye tuna due to the development of super frozen equipment from the late 1980s (Su and Liu 2020).

During the 2000s, nine tuna longline vessels were chosen for analysis because of their targeting on albacore. In general, size samples collected from these vessels centered around 90-110 cm FL. The sizes of albacore caught seemed to increase during 2000s for two albacore-targeting vessels (Figure 8, top two panels). However, for the case of other albacore-targeting fishing vessels, the sizes of albacore catch jumped to around 110 cm FL from 2008 and remained relatively stable during the next 10 years except for 2014 (see the lower panel in Figure 8).

392 References

Chang, F.C. and Yeh, S.Y. 2017. CPUE standardization on northern Atlantic albacore, dating from 1967 to 2016, based on catch statistics of Chinese Taipei longliners. Col. Vol. Sci. Pap. ICCAT, 74: 663-686.

Hsieh, C.H., Chang, F.C. and Yeh, S.Y. 2010. Standardized northern Atlantic albacore (Thunnus alalunga) CPUE, from 1967 to 2008, based on Chinese Taipei longline catch and effort statistics. Col. Vol. Sci. Pap. ICCAT, 65: 1390-1400.

ICCAT 2014. Report of the 2013 ICCAT North and South Atlantic albacore stock assessment meeting. Col. Vol. Sci. Pap. ICCAT, 70: 930-995.

ICCAT 2017. Report of the 2016 ICCAT North and South Atlantic albacore stock assessment meeting. Col. Vol. Sci. Pap. ICCAT, 73: 1147-1295.

Sagarminaga, Y. and Arrizabalaga, H. 2010. Spatio-temporal distribution of albacore (Thunnus alalunga) catches in the northeastern Atlantic: relationship with the thermal environment. Fish. Oceanogr., 19: 121-134.

Santiago, J. and Arrizabalaga, H. 2005. An integrated growth study for North Atlantic albacore (Thunnus alalunga Bonn. 1788). ICES J. Mar. Sci., 62: 740-749.

Yang, S.H. and Yeh, S.Y. 2004. Standardized CPUE trend of Chinese Taipei longline fishery for northern Atlantic albacore from 1968 to 2001. Col. Vol. Sci. Pap. ICCAT, 56: 1412-1416.

Su, N.J. and Liu, K.M. 2020. CPUE standardization of albacore tuna (Thunnus alalunga) for the Chinese Taipei longline fishery in the North Atlantic Ocean. SCRS/2020/102. ICCAT Atlantic Albacore Stock Assessment Meeting, June 29-July 8, 2020. Online Meeting.

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Figure 1. Spatial distribution for catch-at-size samples of albacore tuna from the Chinese Taipei distant-water tuna longline fishery in the North Atlantic Ocean for 1981-1990. The squares in various colors represent the sample size for each spatial grid.

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Figure 2. Spatial distribution for catch-at-size samples of albacore tuna from the Chinese Taipei distant-water tuna longline fishery in the North Atlantic Ocean for 1991-2000. The squares in various colors represent the sample size for each spatial grid.

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Figure 3. Spatial distribution for catch-at-size samples of albacore tuna from the Chinese Taipei distant-water tuna longline fishery in the North Atlantic Ocean for 2001-2010. The squares in various colors represent the sample size for each spatial grid.

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Figure 4. Spatial distribution for catch-at-size samples of albacore tuna from the Chinese Taipei distant-water tuna longline fishery in the North Atlantic Ocean for 2011-2017. The squares in various colors represent the sample size for each spatial grid.

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Figure 5. Boxplots of albacore size samples by year for the Chinese Taipei distant-water tuna longline vessels that target albacore in the North Atlantic Ocean (left panels) and the catch-at-size distribution of albacore sampled from those vessels (right panels). The squares in various colors represent the sample size of length sample for each spatial grid.

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Figure 6. Boxplots of albacore size samples by year for the Chinese Taipei distant-water tuna longline vessels that target albacore in the North Atlantic Ocean (left panels) and the catch-at-size distribution of albacore sampled from those vessels (right panels). The squares in various colors represent the sample size of length sample for each spatial grid.

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Figure 7. Boxplots of albacore size samples by year for the Chinese Taipei distant-water tuna longline vessels that target albacore in the North Atlantic Ocean (left panels) and the catch-at-size distribution of albacore sampled from those vessels (right panels). The squares in various colors represent the sample size of length sample for each spatial grid.

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Figure 8. Boxplots of albacore size samples by year for the Chinese Taipei distant-water tuna longline vessels that target albacore in the North Atlantic Ocean (left panels) and the catch-at-size distribution of albacore sampled from those vessels (right panels). The squares in various colors represent the sample size of length sample for each spatial grid.

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