SCRS/2019/106 Collect. Vol. Sci. Pap. ICCAT, 76(4): 255-264 (2020)

SURVIVAL AND SEX RATIO OF WHITE MARLIN (KAJIKIA ALBIDA) CAUGHT IN THE CHINESE TAIPEI LONGLINE FISHERY IN THE ATLANTIC OCEAN

N.-J. Su1,2 and Y. S. Lu1

SUMMARY

Sex ratios and the condition (alive or dead) of Atlantic white marlin (Kajikia albida) reported by onboard observers were summarized for the Chinese Taipei distant-water longline fishery targeting tunas in this study. The sex ratio ranged between 0.381 and 0.538 from 2007 to 2017, with an overall sex ratio estimated at 0.414. Survival ratios of fish alive when hooking were estimated at 0.713 and 0.615 for two periods of 2007-2009 and 2014-2017, respectively. Lower values (0.182 and 0.286) were derived for particular years due to small sample sizes. The survival ratios were almost identical between sexes (0.655 for females and 0.671 for males). High survival ratios of Atlantic white marlin around 0.650 for commercial tuna longline fishery in this study suggest that alive discards could be an effective measurement to reduce bycatch mortality of the species because high probability of releasing the fish alive could be expected.

RÉSUMÉ

La présente étude offre un résumé des ratios des sexes et de la condition (vivant ou mort) du makaire blanc de l'Atlantique (Kajikia albida) déclarés par les observateurs à bord de palangriers hauturiers du Taipei chinois ciblant les thonidés. Le ratio des sexes variait entre 0,381 et 0,538 de 2007 à 2017, avec un ratio des sexes total estimé à 0,414. Les taux de survie des poissons vivants lors de la capture étaient estimés à 0,713 pour 2007-2009 et à 0,615 pour 2014-2017. Des valeurs inférieures (0,182 et 0,286) ont été calculées pour des années spécifiques en raison de la petite taille des échantillons. Les taux de survie étaient presque identiques entre les sexes (0,655 pour les femelles et 0,671 pour les mâles). Les taux de survie élevés du makaire blanc de l'Atlantique (0,650 environ) pour la pêcherie palangrière thonière commerciale présentés dans cette étude suggèrent que les rejets vivants pourraient constituer une mesure efficace pour réduire la mortalité par capture accessoire de l'espèce, car une forte probabilité de remise à l'eau à l’état vivant peut être escomptée.

RESUMEN

En este estudio se resumieron las proporciones de sexos y la condición (viva o muerta) de la aguja blanca del Atlántico (Kajikia albida) comunicada por los observadores a bordo para la pesquería de palangre de aguas distantes de Taipei Chino dirigida a los túnidos. La ratio de sexos osciló entre 0,381 y 0,538 entre 2007 y 2017, con una ratio general de sexos estimada en 0,414. Las ratios de supervivencia de peces vivos cuando se enganchan en el anzuelo se estimaron en 0,713 y 0,615 para dos períodos de 2007-2009 y 2014- 2017, respectivamente. Se obtuvieron valores más bajos (0,182 y 0,286) para determinados años debido a los pequeños tamaños de la muestra. Las ratios de supervivencia fueron casi idénticas entre los sexos (0,655 para las hembras y 0,671 para los machos). Las altas tasas de supervivencia de la aguja blanca del Atlántico en torno a 0,650 para la pesquería palangrera comercial de túnidos en este estudio sugieren que los descartes vivos podrían ser una medida eficaz para reducir la mortalidad de la especie debida a la captura fortuita, ya que podría esperarse una alta probabilidad de liberar el pez vivo.

KEYWORDS

Survival condition, alive and dead discard, sex ration, bycatch mortality

1 Department of Environmental Biology and Fisheries Science, National Chinese Taipei Ocean University, Keelung 20224, Chinese Taipei 2 Documents are not cited without permission from authors

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1. Introduction

White marlin, Kajikia albida (formerly Tetrapturus Albidus; Collette et al., 2006), is a highly migratory species widely distributed in the Atlantic Ocean (Goodyear, 2003). Several studies have shown that white marlin could migrate a long distance more than 1000 km, consisting of both trans-Atlantic and trans-equatorial movements (Orbesen et al., 2008; Wells et al., 2010). The assessment of white marlin in the Atlantic Ocean was made based on the assumption of a single unit stock (Restrepo et al., 2003). Previous stock assessment conducted by the ICCAT (International Commission for the Conservation of Atlantic Tunas) indicated that the current status of this population was likely to be overfished, with estimated exploitable biomass well below the level needed to maintain maximum sustainable yield, and suffering overfishing (ICCAT, 2012).

As a bycatch species in commercial longline fisheries targeting tunas, white marlin also supported economically important recreational fisheries throughout the Atlantic Ocean (Prince and Brown, 1991). The Chinese Taipei tuna distant-water longline fishery has operated throughout the Atlantic Ocean since the early 1960s, and become one of the major longline fleets thereafter. The fishery targeted solely on albacore tuna (Thunnus alalunga) in the beginning of early stage throughout 1980s, but part of fishing effort has shifted to target on bigeye (Thunnus obesus) since the late 1980s, with the development of the deep longlining (Chang, 2003). However, the survival and condition of white marlin when hooking has never been reported for this fishery (Su and Lu, 2019).

Catch information is important and essential to stock assessment and fisheries management (Hilborn and Walters, 1992). Underestimate of catch or unreported catches could lead to substantial bias to the assessment results and thus the management plan to recover the population. The objective of this study was therefore to provide catch information for important bycatch species, Atlantic white marlin, in commercial tuna longline fishery as reported by observers, including the location where they were caught, condition when hooking, i.e., alive or dead, and the sex ratios. Such information is useful to reduce bycatch mortality rates of billfish species especially for white marlin in large-scale longline fisheries targeting tunas.

2. Materials and methods

Catch information of white marlin for the Chinese Taipei distant-water tuna longline fishery in the Atlantic Ocean was obtained from the Overseas Fisheries Development Council of the Republic of China (OFDC, Taipei). Data were collected by onboard observers in large-scale longline vessels from 2002 to 2017. During each observation (8 hours a day), observers recorded every fish they saw in each operation, including tunas, sharks, and billfishes. This data set from observers contains information on time (year and month), fishing location (5º latitude and longitude), species, size (lower jaw fork length), condition when catching (alive or dead), and gender if possible. The samples of white marlin were plotted in geographic maps to show the location of capture, and summarized by sex and condition using histograms and ratios for each year. The sex ratios and survival ratios (condition) of white marlin were calculated as follows:

Sex ratio = Females / All male and female individuals recorded.

Survival ratio = Alive individuals when hooking / All alive and dead individuals recorded.

3. Results and discussion

There were in total 925 white marlin recorded by onboard observers for the Chinese Taipei tuna longline fishery in the Atlantic Ocean during 2007-2018. However, 82 samples were removed from the analysis due to no length, sex, or condition information available for the same specimen. The other 842 individuals were used to calculate sex and survival ratios by year, of which 696 and 698 fish have sex or condition information available, respectively; most of them were recorded during 2007-2009 (n = 329) and 2015-2017 (n = 350). There were 130 white marlin recorded by onboard observers during 2010-2012, but no condition or survival information (alive or dead) could be obtained for the fish observed (Table 1).

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Size samples of white marlin collected by onboard observers were summarized in histogram by sex and by year, with the sample size varying among years. However, most of the fish caught were within the sizes ranging from 150 to 175 cm in lower jaw fork length (Figure 1). The locations where the white marlin catch was reported by onboard observers were shown by condition (alive or dead), which seems that no obvious pattern of survival ratio was found within the geographic range in the Atlantic Ocean (Figures 2-3). Most of the white marlin records occurred in pelagic waters of the South Atlantic Ocean, as a result of most observers assigned to the large-scale longline vessel operating in tropical waters of the equatorial Atlantic Ocean targeting bigeye tuna.

The sex ratios ranged between 0.381 and 538 among years from 2007 to 2017 with an overall sex ratio at 0.414, except for 2018 where only 6 samples were available, suggesting that male white marlin were more than females in number (Table 1). The survival ratios were 0.713 and 0.615 for 2007-2009 and 2014-2017, respectively. Lower values were derived for 2013 and 2018 (0.182 and 0.286) due to small sample sizes (n =11 and 7). In general, a high survival ratio of Atlantic white marlin around 0.65 was found for the Chinese Taipei distant-water tuna longline fishery (Table 1), which suggests that alive discards of white marlin could be an effective measurement to reduce bycatch mortality because high probability of releasing the fish alive could be expected.

The survival ratios were almost identical between sexes (0.655 and 0.671 for females and males, respectively). The results were very similar to those for coastal fisheries off western United States, ranging from 0.54 to 0.80 for white marlin (Pacific Fishery Management Council, 2003). Relatively high post-release survival rates were also estimated at 0.63 based on short-duration pop-up satellite archival tags (PSATs) by Kerstetter and Graves (2006), and an estimate of 0.69 was shown in White Marlin Biological Review Team (2007). Both of the results were fairly consistent with those from current study. However, a lower percentage of 0.44 was estimated by Jackson and Farber (1998) for white marlin alive at the time of longline gear retrieval.

As a bycatch species in commercial longline fisheries targeting tunas and swordfish, white marlin also supported economically important recreational fisheries throughout the Atlantic Ocean (Prince and Brown, 1991). Results from this study indicated that white marlin could survive the trauma resulting from the interaction with pelagic longline gear, which further indicates that the management measures requiring the release of live white marlin at the time of longline gear retrieval could substantially reduce the fishing mortality on the Atlantic-wide stock.

In addition to recording the number of live fish releases and the number of dead discards, studies with electronic tagging or auxiliary information should be conducted to derive estimates of live discard mortality and evaluate the degree of injury at release and the condition of the fish at release, in order to accommodate the ICCAT management measure that requires the release from longline vessels of blue and white marlin that are alive at haul back (ICCAT, 2012).

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References

Chang, S.K. 2003. Analysis of Chinese Taipei white marlin catch data and standardization of catch rates. Col. Vol. Sci. Pap. ICCAT, 55: 453-466.

Collette, B.B., McDowell, J.R., Graves J.E. 2006. Phylogeny of recent billfishes (Xiphioidei). Bull. Mar. Sci., 79: 455-468.

Goodyear, C.P. 2003. Spatio-temporal distribution of longline catch per unit effort, sea surface temperature and Atlantic marlin. Mar. Freshw. Res., 54: 409-417.

Hilborn, R., Walters, C.J. 1992. Quantitative fisheries stock assessment: Choice, dynamics and uncertainty. Chapman and Hall, London.

ICCAT. 2012. Report of the white marlin stock assessment session and white marlin data preparatory meeting. Col. Vol. Sci. Pap. ICCAT, 68: 1273-1386.

Jackson, T.L., Farber, M.I. 1998. Summary of at-sea sampling of the western Atlantic Ocean, 1987-1995, by industrial longline vessels fishing out of the port of Cumana, Venezeula. ICCAT Enhanced Research Program for Billfish 1987-1995. ICCAT, SCRS/1998.

Kerstetter, D.A., Graves, J.E. 2006. Survival of white marlin (Tetrapturus albidus) released from commercial pelagic longline gear in the western North Atlantic. Fish. Bull., 104: 434-444.

Orbesen, E.S., Hoolihan, J.P., Serafy, J.E., Snodgrass, D., Peel, E.M., Prince, E.D. 2008. Transboundary movement of Atlantic istiophorid billfishes among international and U.S. domestic management areas inferred from mark-recapture studies. Mar. Fish. Rev., 70: 14-23.

Pacific Fishery Management Council. 2003. U.S. west coast fisheries for highly migratory species, fishery management plan: Environmental impact statement. National Oceanic and Atmospheric Administration, United States.

Prince, E.D., Brown, B.E. 1991. Coordination of the ICCAT Enhanced Research Program for Billfish. In: Guthrie, D., Hoenig, J.M., Holliday, M., Jones, C.M., Mills, M.J., Moberly, S.A., Pollock, K.H., Talhelm, D.R. (Eds.), Fisheries Management, American Fisheries Society Symposium 12, pp13-18.

Restrepo, V., Prince, E.D., Scott, G.P., Uozumi Y. 2003. ICCAT stock assessments of Atlantic billfish. Mar. Freshw. Res., 54: 361-367.

Su, N.J., Lu, Y.S. 2019. CPUE standardization for white marlin (Kajikia albida) caught in the Chinese Taipei distant-water longline fishery in the Atlantic. ICCAT SCRS/2019/038.

Wells, R.J.D., Rooker, J.R., Prince, E.D. 2010. Regional variation in the otolith chemistry of blue marlin (Makaira nigricans) and white marlin (Tetrapturus albidus) from the western North Atlantic Ocean. Fish. Res., 106: 430-435.

White Marlin Biological Review Team. 2007. Atlantic white marlin status review. Report to National Marine Fisheries Service, Southeast Regional Office. United States.

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Table 1. Catches of white marlin caught in the Chinese Taipei distant-water tuna longline fishery in the Atlantic Ocean reported by onboard observers. Results were shown by sex (males and females) and condition (alive or dead), and the overall sex and survival ratios to all samples. Information on sex or alive/dead was lack for some samples during 2010 and 2012.

Sex Survival Ratio = Ratio = Year Female Male F/(F+M) Alive Dead A/(A+D) Total 2007 57 104 0.354 105 67 0.610 172 2008 27 48 0.360 69 7 0.908 76 2009 32 48 0.400 50 16 0.758 81 2010 4 4 0.500 63 2011 56 2012 11 2013 5 6 0.455 2 9 0.182 11 2014 4 6 0.400 10 6 0.625 16 2015 48 78 0.381 76 51 0.598 127 2016 85 73 0.538 100 60 0.625 160 2017 25 36 0.410 39 24 0.619 63 2018 1 5 0.167 2 5 0.286 7 288 408 0.414 453 245 0.649 843

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Figure 1. Length frequency histograms by sex for white marlin caught in the Chinese Taipei distant-water tuna longline fishery in the Atlantic Ocean. Red bars are for females and blue for males. Sex unknown individuals are indicated by white bars. Sample sizes for that year were shown.

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Figure 2. Maps showing the location where female white marlin was caught in the Chinese Taipei distant-water tuna longline fishery in the Atlantic Ocean. The symbol “A” indicates the fish caught alive; “D” the fish caught as dead. Note that information on sex or condition (alive/dead) was lack or insufficient from 2010 to 2014. Sample sizes for the year were shown.

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Figure 2. Continued.

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Figure 3. Same as Figure 2, but for males.

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Figure 3. Continued.

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