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Influence of the Type of Longline on the Catch Rate and Size Composition of Swordfish, Xiphias Gladius

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Influence of the Type of Longline on the Catch Rate and Size Composition of Swordfish, Xiphias Gladius SCRS/2001/158 INFLUENCE OF THE TYPE OF LONGLINE ON THE CATCH RATE AND SIZE COMPOSITION OF SWORDFISH, XIPHIAS GLADIUS (LINNAEUS, 1758), IN THE SOUTHWESTERN EQUATORIAL ATLANTIC OCEAN Hazin, F.H.V.1.;Hazin, H.G and Travassos, P. SUMMARY In the present paper, the catch per unit of effort (CPUE) as well as the size composition of the swordfish caught by Brazilian boats operating from Natal, with a traditional, multifilament longline, from 1986 to 1997, were compared with the catches done by the same vessels, from 1998 to 2000, with a monofilament longline, using light sticks and squid as bait. Sharks, mainly night shark, Carcharhinus signatus, and blue shark, Prionace galuca, represented almost half of the catches done with the multifilament longline. In the monofilament longline, the swordfish was the most abundant species, accounting for about one third of the total catch. Total CPUE for the monofilament was about 40% higher than for the multifilament. The mean CPUE of most species were significantly different between the 2 gears. The mean CPUE of swordfish increased almost tenfold from the multifilament to the monofilament longline. The swordfish caught by the monofilament longline, were significantly larger than those caught by multifilament. RÉSUMÉ Le présent document compare la prise par unité d=effort (CPUE), ainsi que la composition de taille, de l=espadon capturé de 1986 à 1997 par les bateaux brésiliens basés à Natal avec des palangres traditionnelles en multi-filament et les prises réalisées par ces mêmes bateaux de 1998 à 2000 avec des palangres en mono-filament, avec des bâtons lumineux et du calmar comme appât. Des requins, surtout du requin de nuit, Carcharinhunus signatus, et du requin peau bleue, constituaient presque la moitié des captures effectuées avec les palangres en multi- filament. Pour ce qui est des palangres en mono-filament, l=espadon était l=espèce prédominante et représentait environ le tiers de la prise totale. La CPUE totale du mono-filament dépassait d=environ 40% celle du multi-filament. La CPUE moyenne de la plupart des espèces différait de façon significative entre les deux engins. La CPUE moyenne de l=espadon s=est accrue presque dix fois en passant du multi-filament au mono-filament. Les espadons capturés avec les palangres en mono-filament étaient sensiblement plus grands que ceux qui l=étaient par les palangres en multi-filament. RESUMEN En el presente documento se compara la captura por unidad de esfuerzo (CPUE) y la composición por tallas del pez espada capturado por los barcos brasileños que faenan desde Natal con palangre tradicional de multifilamento desde 1986 a 1997, con las capturas realizadas por los mismos barcos desde 1998 a 2000 con palangre de monofilamento, bastones de luz y utilizando calamar como cebo. Los tiburones, especialmente el tiburón nocturno, Carcharhinus ignatus, y el tiburón azul, Prionace glauca, representaron casi la mitad de las capturas realizadas con palangre de multifilamento. En el palangre de monofilamento la especie más abundante fue el pez espada, representando casi un tercio de la captura total. La CPUE total del monofilamento era casi un 40% mayor que la del multifilamento. La CPUE media de la mayoría de las especies era bastante diferente entre los 2 artes. La CPUE media del pez espada aumentó casi diez veces del palangre de multifilamento al de monofilamento. Los peces espada capturados por el palangre de monofilamento eran considerablemente mayores que los capturados con multifilamento. 1 Laboratório de Oceanografia Pesqueira – Departamento de Pesca/UFRPE, Av. Dom Manoel de Medeiros, s/n – Dois Irmãos – Recife-PE – Brasil / Fone/Fax. : 00 55 81 33021512/1511 / E-Mail: [email protected] INTRODUCTION The tuna longline fishery in Brazil was begun in 1956, by leased Japanese longliners, based at Recife. Although the operations of these vessels were highly successful, in 1964, due to political and commercial circumstances, they suspended their activities. In 1983, Brazilian tuna longline vessels begun to operate in the southwestern equatorial Atlantic, based at Natal, located in the northeast coast of Brazil. From that year on, up to 1996, they used a traditional multifilament longline. In middle 1996, an American longliner, operating with a monofilament longline, using light stick and squid as bait, and targeting swordfish, was leased by a local company. The good results obtained by that vessel prompted a quick assimilation and dissemination of this new technology among the Brazilian vessels, resulting in a significant increase of the swordfish catches. In the present paper, the catch per unit of effort (CPUE) as well as the size composition of the swordfish caught by Brazilian boats operating from Natal, with a traditional, multifilament longline, from 1983 to 1995, were compared with the catches done by the same vessels, from 1998 to 2000, with a monofilament longline, using light sticks and squid as bait. MATERIAL AND METHODS The catch data were obtained from the logsheets of Brazilian vessels, operating in the southwestern equatorial Atlantic, from 0o to 10oS latitude and from 25o to 40oW longitude (Fig. 1). From 1986 to 1997, all vessels used a traditional multifilament longline, with 6 to 7 secondary lines between buoys. In 1996, due to the good results obtained by a leased vessel, some of the boats begun to operate with a monofilament longline, using light sticks and squid as bait and targeting swordfish. From 1998 on, all vessels were already operating with this new model of longline. In the present paper, data from 1986 to 1997 were compared to those from 1998 to 2000. Catch per unit of effort (CPUE) for each set was calculated as the number of fish caught per hundred hooks. The mean CPUE for each species for the monofilament longline and the multifilament longline was calculated and compared through ANOVA, at a significance level of 1%. The lower jaw to fork length (LJFL) frequency distribution of the swordfish caught by each type of longline were calculated and compared through a Kolmogorov-Smirnov test, also using a significance level of 1%. The mean LJFL of the swordfish caught by both gears was also calculated and compared by using a Student’s t test. RESULTS AND DISCUSSION A total of 107,379 fishes were caught during the studied period, 56,375 by the traditional longline and 51,004 by the monofilament gear (Table 1). Sharks represented almost half of the catches done with the multifilament longline, 90% of them being gray sharks (66%), mainly night shark, Carcharhinus signatus, and blue shark (25%), Prionace galuca. The yellowfin tuna, Thunnus albacares, was the second most important species, accounting for about 30% of the total catches. The catch composition of the monofilament gear was very much different from the multifilament. The swordfish was the most abundant species, accounting for about one third of the total catch, whilst yellowfin tuna ranked second, representing about 15%. The bigeye tuna was the third most caught species, amounting to 12% of total catches. Total CPUE for the monofilament was about 40% higher than the multifilament. The mean CPUE of all species were significantly different between the 2 gears, except for the white marlin, other fish (mainly dolphin fish), and tresher shark, Alopias superciliosus. The mean CPUE of swordfish increased almost tenfold from the multifilament to the monofilament longline, jumping from 0.12 to 1.03. The CPUE of bigeye tuna showed a similar trend, increasing from 0.05 to 0.38. The mean CPUE of albacore and blue marlin also rose significantly, more than doubling. The mean CPUE of the sharks of the genus Carcharhinus with the monofilament longline, on the contrary, was cut by one third. The yellowfin tuna, the white marlin and the blue shark also showed lower CPUE with the monofilament longline than with the multifilament one. The much higher CPUE of swordfish and bigeye tuna obtained by the monofilament longline than by the multifilament one might be related to a number of factors. The use of light stick and squid as bait certainly is one of the main reasons. The behavior of the fishing gear in the water, as well as the depths attained by the hooks, are also important. The fishing efficiency of the longline seems to be very sensitive to even small changes in gear configuration. Broadhurst and Hazin (2000), for instance, have shown that the position of the bait in the hook may significantly change its efficiency. They showed that the use of mackerel in a horizontal position, instead of vertical, increased fish attraction, while, at the same time, reduced bait theft. Higher CPUE for monofilament longlines have been reported for other species too, such as cod (Bjordal and Lokkeborg (1996). The LJFL length frequency distribution of the swordfish caught by both gears were significantly different (Figure 2). The fish caught by the monofilament longline, with a mean LJFL of 193.7, were significantly larger than those caught by multifilament (mean LJFL = 145.0) (P< 0.01). These results indicate that although the use of monofilament longline significantly increases the catch of swordfish, the higher size of the fish caught by this gear might, at least partly, help to alleviate the impact on the stocks resulting from the increase of catchability for the species. REFERENCE BIBLIOGRAPHIC Bjorda, and Lokkeborg S. Longlining. Fishing News Books, Oxford, England, 156p. 1996 Broadhurst, K. M and Hazin F.H.V. Influences of type and orientacion of bait on catches of swordfish (Xiphias gladius) and other species in an artisanal sub-surface longline fishery off northeastern Brazil. Fish. Res. 1159 (2000) 1-11. Table 1- Number, percent and CPUE of the species caught by Brazilian vessels, operating off northeast Brazil, whit monofilament and multifilament longline. Longline multifilament Longline monofilament Species nº.
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