Spatial patterns of the epipelagic species caught incidentally in the tuna fishery on floating objects, in the eastern Pacific Ocean

Abstract

Recibido en diciembre de 2000; aceptado en junio de 2001 RESUMENSe analizaron los patrones espaciales de la fauna asociada con objetos flotantes en el Océano Pacífico oriental (OPO). Para ello, se calcularon diferentes índices de presencia para los grupos de especies capturados incidentalmente en lances de pesca de atún sobre objetos flotantes, utilizando la base de datos de los observadores de la Comisión Interamericana del Atún Tropical (1993 a 1997). Los índices permitieron una primera agrupación de especies con respecto al porcentaje de presencia en los lances de pesca sobre objetos flotantes y al tipo de cardúmenes formados (especies solitarias o en cardúmenes). Posteriormente, de acuerdo con los grupos de especies, zona, periodo anual y tipo de objetos flotantes, se realizó una clasificación por medio del uso de diversos modelos de funciones discriminantes cuadráticas, basados en diferentes estructuras de varianzas y covarianzas. De ellos, se escogieron tres modelos para la clasificación de la fauna de acompañamiento, de acuerdo con las pruebas de razón de verosimilitud y con la reducción de la tasa de error observada. Los resultados muestran la existencia de dos zonas en el OPO, con diferente composición de grupos de especies y origen de los objetos flotantes. Las zonas tienen como frontera común la Contracorriente Ecuatorial localizada, aproximadamente, sobre los 4ºN. La zona norte presenta una fauna asociada con objetos flotantes dominada por adultos y reclutas del grupo de dos especies de tiburón, el sedoso (Carcharhinus falciformis) y el punta negra (C. limbatus); por reclutas de atún aleta amarilla (Thunnus albacares); y, en menor proporción, por el jurel (Seriola lalandi). En la zona sur, por su parte, dominan los reclutas de patudo (T. obesus). Las dos zonas comparten fauna que sólo se encuentra lejos de la costa, como es el caso del tiburón punta blanca (C. longimanus).Palabras clave: objetos flotantes, fauna asociada con objetos flotantes, Océano Pacífico oriental, pesca de atún, función discriminante cuadrática.

Incidental catch of dolphinfish (Coryphaena spp.) reported by the Mexican tuna purse seiners in the eastern Pacific Ocean

“…Conversely, Solana-Sansores (2001) mentions that smaller fishes are found near the shore and larger ones in more oceanic waters. We found no size-related distribution pattern for the incidentally caught dolphinfish, all three sizes occurring in both oceanic and coastal areas (among years as well as quarters analyzed).…”

“…Spatially, Arenas et al (1999) and Solana-Sansores (2001) mention that the incidental catch of dolphinfish is very frequent in sets on floating objects in the EPO, and that they are most frequently caught in its central part, between 10 • N and 10 • S, though they can also be found in coastal areas. The dolphinfish habit to associate with floating objects is not entirely evident.…”

“…Solana-Sansores (2001) mentions that almost half of the fishes recorded by IATTC observers are mediumsized and that the other half are distributed in the other two size categories. Comparing our results with those of Solana-Sansores (2001), there seems to be a decrease in the size of the dolphinfish caught incidentally by the purse-seine tuna fishing fleet; however, it is important to bear in mind that this study was based on the mean size of the fish caught (in the case of two or more specimens caught in the same set) and that this could result in a significant bias, since PNAAPD observers register the minimum and maximum sizes of the fish caught but do not specify any possible dominance of fishes of a given size in the catch of a particular set. Furthermore, the numbers of sets made on floating objects by the international fleet are far greater than the numbers of sets on floating objects by the fraction of the Mexican fleet analyzed in this work.…”

“…Most sets on floating objects were recorded in the equatorial fringe between 0 • N and 10 • N. Solana-Sansores (2001) indicated that floating objects are more likely to be found drifting at these latitudes because of wide river mouths draining into the ocean from areas of high coastal vegetation density (De la Rosa, 1995). This is noteworthy because it has been reported that tuna fisheries that set their nets on floating objects develop mainly in regions close to tropical forests with important freshwater drainage into the ocean (IATTC, 1993;Hallier, 1995;Caddy and Majkowski, 1996;Ariz et al, 1999).…”

“…Fishes were then grouped into three size categories based on a prior classification reported by Solana-Sansores (2001): small (<90 cm TL), medium (>90 cm TL and <150 cm TL), and large (>150 cm TL). To determine significant differences among the three size categories per year, quarter, and set type, Kruskal-Wallis analyses of variance were performed.…”