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Interaction of the False Killer Whale (Pseudorca

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Interaction of the False Killer Whale (Pseudorca SCRS/2007/025 Collect. Vol. Sci. Pap. ICCAT, 62(6): 1721-1738 (2008) INTERACTION OF THE FALSE KILLER WHALE (PSEUDORCA CRASSIDENS) AND DEPREDATION ON THE SWORDFISH CATCHES OF THE SPANISH SURFACE LONGLINE FLEET IN THE ATLANTIC, INDIAN AND PACIFIC OCEANS A. Ramos-Cartelle and J. Mejuto1 SUMMARY On the basis of information provided by the fleet and by scientific observers during the 1992-2006 period, it was possible to identify the areas of interaction between the surface longline fishery and Pseudorca crassidens, the level of sporadic incidental by-catches of this cetacean and its depredation level carried out on the swordfish individuals caught by this fleet. In roughly 98% of the sets sampled by scientific observers, no depredation was detected on the swordfish longline catches and in only 2% there were signs evidencing this depredation. According to on-board scientific observations, the incidental catch rate of the false killer whale was estimated to be 1.464, 1.685 and 0.797 individuals per million hooks for the Atlantic, Indian and Pacific Oceans, respectively. The incidental mortality rate of the false killer whale was estimated 0.36 individuals per million hooks in the Atlantic and zero in the two other oceans. The intertropical band of the three oceans presented the greatest interaction with the swordfish fishery, reaching in some of the areas a mean impact affecting over 10% of the swordfish catch in number. On the basis of mean predation rates by region and quarter, the average number of swordfish estimated to have been depredated by the false killer whale in 2005 would range from 2999- 4804, 509-2706, 114-348 specimens, in the Atlantic, Indian and Pacific Oceans, respectively. These modest overall incidences compared with other fleets are probably due to fishing areas selected as well as the fleet’s effective practice of avoiding areas of interaction with the false killer whale. However, when attacks do occur, they can be devastating to the fishery interests of the vessel and may ruin their yields. Data from sets with HPUE>0 indicate that predation usually amounts to less than 5 swordfish per thousand hooks, although, it may sporadically reach or exceed 20 fish. Sets having HPR>0 indicate that, when attacks occur, depredation may affect a number of swordfish equivalent to 50% or more of the catch held on board and may even damage the catch in a proportion that is several times greater than the number of swordfish retained on board. For general purposes it was estimated that in 2005 Pseudorca crassidens carried out a mean overall depredation in the Spanish surface longline fleet of around 1.1-1.8%, 0.5-2.6%, 0.1-0.3% on the total number of swordfish caught in the Atlantic, Indian and Pacific Oceans, respectively. RÉSUMÉ D’après les informations soumises par la flottille et les observateurs scientifiques de 1992 à 2006, on a pu identifier les zones d’interaction entre la pêcherie palangrière de surface et la Pseudorca crassidens, le niveau de prises accidentelles sporadiques de ce cétacé ainsi que le niveau de déprédation causé par cette espèce sur les espadons capturés par cette flottille. Dans près de 98% des opérations échantillonnées par les observateurs scientifiques, aucune déprédation n’a été détectée sur les prises palangrières d’espadon et dans 2% des cas seulement cette déprédation était avérée. Selon les observations des scientifiques à bord, le taux de prise accidentel de cette fausse orque a été estimé à 1,464, 1,685 et 0,797 spécimens par million d’hameçons pour l’Océan Atlantique, l’Océan Indien et l’Océan Pacifique respectivement. Le taux de mortalité accidentel de la fausse orque a été estimé à 0,36 spécimens par million d’hameçons dans l’Atlantique et à zéro dans les deux autres océans. La bande intertropicale des trois océans présentait la plus grande interaction avec la pêcherie d’espadon, avec, dans certaines zones, un impact moyen affectant plus de 10% de la prise numérique d’espadon. Sur la base des taux moyens de prédation par région et trimestre, le nombre moyen d’espadon qui a été estimé comme ayant fait l’objet d’une déprédation par cette fausse orque en 2005 s’élèverait à 2.999-4.804, 509-2.706, 114-348 spécimens dans l’Océan Atlantique, l’Océan Indien et l’Océan Pacifique respectivement. Ces incidences globales modestes par rapport à celles d’autres flottilles sont probablement dues à certaines zones de pêche ainsi qu’à la pratique efficace de la flottille qui évite les zones d’interaction avec la fausse orque. Toutefois, lorsque des attaques se produisent, elles peuvent 1 Instituto Español de Oceanografía, P.O. Box. 130, 15080 A Coruña. Spain. 1721 être dévastatrices pour les intérêts halieutiques du navire et détruire ses productions. Les données d’opérations avec HPUE>0 indiquent que la prédation totalise généralement moins de 5 espadons par mille hameçons, même si elle peut parfois atteindre ou dépasser 20 poissons. Les opérations avec HPR>0 indiquent que lorsque des attaques se produisent, la déprédation pourrait affecter 50% ou plus de la prise d’espadon conservée à bord et même détruire la prise dans une proportion plusieurs fois supérieure au nombre d’espadons retenus à bord. On a généralement estimé que la Pseudorca crassidens a causé en 2005 une déprédation générale moyenne dans la flottille palangrière de surface espagnole de 1,1-1,8%, 0,5-2,6%, 0,1-0,3% environ sur le nombre total d’espadons capturés dans l’Océan Atlantique, l’Océan Indien et l’Océan Pacifique respectivement. RESUMEN Sobre la base de la información proporcionada por la flota y por los observadores científicos durante el periodo 1992-2006, ha sido posible identificar las zonas de interacción entre la pesquería palangrera de superficie y Pseudorca crassidens, el nivel de capturas fortuitas incidentales esporádicas de este cetáceo y su nivel de depredación en ejemplares de pez espada capturados por esta flota. En aproximadamente el 98% de las caladas de palangre muestreadas por los observadores científicos no se detectó depredación en las capturas de pez espada con palangre, y sólo en un 2% se observaron signos que evidenciaban esta depredación. Según las observaciones de los científicos embarcados, se estimó que la tasa de captura incidental de orca falsa (Pseudorca crassidens) ascendía a 1,464, 1,685 y 0,797 ejemplares por millón de anzuelos para los océanos Atlántico, Índico y Pacífico, respectivamente. La tasa de mortalidad incidental de orca falsa se estimó en 0,36 ejemplares por millón de anzuelos en el océano Atlántico y en cero en otros océanos. La franja intertropical de los tres océanos presentaba la mayor interacción con la pesquería de pez espada, alcanzando en algunas zonas un impacto medio que afectaba a más del 10% de la captura de pez espada en número. Sobre la base de las tasas medias de depredación por región y trimestre, el promedio de peces espada que se estima que han sido depredados por la falsa orca en 2005 oscilaría entre 2.999 y 4.804, 509 y 2.706, 114 y348 ejemplares para los océanos Atlántico, Índico y Pacífico, respectivamente. Estas incidencias globales, modestas en comparación con otras flotas, se deben probablemente a las zonas de pesca seleccionadas así como a la práctica efectiva de la flota de evitar las zonas de interacción con orcas falsas. Sin embargo, cuando se producen los ataques, éstos pueden ser devastadores para los intereses pesqueros de los buques y pueden arruinar su rendimiento. Los datos de las caladas con HPUE>0 indican que la depredación se sitúa en menos de 5 ejemplares de pez espalda por 1.000 anzuelos, aunque esporádicamente puede alcanzar o superar la cifra de 20 ejemplares. Los caladas de palangre con HPR>0 indican que, cuando se producen ataques, la depredación puede afectar a un número de peces espada equivalente al 50% o más de la captura retenida a bordo y podrían incluso dañar la captura en una proporción varias veces superior al número de peces espada retenidos a bordo. A efectos generales, se estimó que en 2005 la Pseudorca crassidens llevó a cabo una depredación global media en la flota palangrera de superficie española de aproximadamente el 1,1-1,8%; 0,5-2,6% y 0,1-0,3% del número total de peces espada capturados en los océanos Atlántico, Índico y Pacífico, respectivamente. KEYWORDS Pseudorca crassidens, false killer whale, swordfish, depredation, surface longline 1. Introduction The false killer whale (Pseudorca crassidens), the only species of its genus, is one of the largest cetacean members of the Delphinidae family. While it might sometimes be confused with other cetacean having a similar morphology, such as the pygmy killer and melon-headed whales, and less commonly, pilot whales, fishermen seem to have no trouble identifying it and often see the damage it inflicts on their catches and bait. The shape of the head, dorsal fin, and flippers will be the best characters to use in distinguishing them2 (Baird, 2005). The taxonomic details are (Carwardine, 1995): Kingdom: Animalia. Phylum: Chordata. Class: Mammalia. Order: Cetacea. Family: Delphinidae. Genus: Pseudorca, Reinhardt, 1862. Species: P. crassidens. 2 http://www.marinethemes.com/falsekiller.html 1722 The habitat of this species ranges from the tropical to temperate zones of both hemispheres. It appears to have a widespread distribution in temperate and tropical oceanic waters. However, unlike the killer whale (Orcinus orca), its distribution is preferably tropical, although it can often appear in subtropical and in some cases in temperate regions, particularly during certain seasons of the year. This species was reported in fairly shallow waters of the Atlantic, Indian and Pacific Oceans as well as in the Mediterranean Sea and Red Sea.
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