versión On-line ISSN 0718-1957
Rev. biol. mar. oceanogr. v.44 n.2 Valparaíso ago. 2009
Revista de Biología Marina y Oceanografía 44(2): 523-526, agosto de 2009
NOTAS CIENTÍFICASSouthernmost record of Isurus paucus Gitart Manday, 1966 (Elasmobranchii: Lamnidae) in the southeast Pacific Ocean
Registro más austral de Isurus paucus Gitart Manday, 1966 (Elasmobranchii: Lamnidae) en el Océano Pacífico sudeste
Carlos Bustamante1, Francisco Concha2, Fernando Balbontín2 and Julio Lamilla1
1Laboratorio de Elasmobranquios (ELASMOLAB), Instituto de Zoología, Universidad Austral de Chile. Casilla 567, Valdivia, Chile
2Facultad de Ciencias del Mar y de Recursos Naturales, Universidad de Valparaíso, Valparaíso, Casilla 5080 Reñaca, Viña del Mar, Chile
Se informa sobre el registro más austral del marrajo de aleta larga Isurus paucus Gitart Manday, 1966 en el Pacífico suroriental. El ejemplar correspondió a una hembra de 310 cm de longitud total, capturada por un pesquero artesanal frente a la costa de Valparaíso, Chile, por un pesquero artesanal cuya especie objetivo era el marrajo de aleta corta, I. oxyrinchus. Los principales caracteres taxonómicos utilizados en la identificación del marrajo de aleta larga que permitieron distinguirlo de la otra especie del género Isurus (marrajo de aleta corta), fueron la forma de la cabeza y hocico, la longitud de las aletas pectorales, la posición relativa de la primera aleta dorsal con respecto al margen posterior de las aletas pectorales y la posición relativa de la segunda aleta dorsal con respecto de la anal. Los antecedentes recopilados indican que la presencia de Isurus paucus no se restringiría a zonas de aguas cálidas, sino que también se distribuye en aguas templadas-frías.
Palabras clave: Isurus oxyrinchus, tiburón marrajo de aleta larga, Chile
The genus Isurus comprises two species: the shortfin mako Isurus oxyrinchus Rafinesque, 1809, reported world-wide in both temperate and tropical waters and the longfin mako Isurus paucus (Guitart Manday, 1966), whose distribution and habitat are poorly known. Isurus paucus has been considered an oceanic, tropical species, occurring in deep waters world-wide (Compagno et al. 2005). It displays aplacental viviparity with possible adelphophagy (Garrick 1967, Mollet et al. 2000). Size at birth ranges from 92 to 120 cm in total length (TL), with females reaching sexual maturity at 245 cm TL (Compagno 2001). This species has been reported as bycatch in tropical pelagic longline fisheries for tuna (Thunnus spp.), swordfish (Xiphias gladius), and other pelagic sharks belonging to the Lamnidae and Carcharhinidae families (Amorim et al. 1998, Castillo-Géniz et al. 1998, Senba & Nakano 2005, Marcano et al. 2005). Despite the apparently global distribution of this species, it has been poorly recorded (Compagno 2001, Compagno et al. 2005). The aim of this communication is to report the occurrence of Isuruspaucus in the southeast Pacific, off the coast of Valparaíso, with a review of the morphological characteristics that allow distinguishing it from I. oxyrinchus.
MATERIAL AND METHODS
Data reported here proceeded from a single specimen of shark captured by fishermen from Valparaíso (Portales fishing cave). A video of the captured animal (http://www.revbiolmar.cl/Sharkvideo.flv) was recorded by a witness. Besides, graphic records obtained of the shark morphology were compared with photographs of the shortfin mako captured elsewhere. The poor quality of Fig. 1f was improved with Adobe® PhotoShop® software.
The longfin mako specimen was captured 20 nm from the coast off central Chile (32º30´S-74º30´W) on March 27, 2007, by a pelagic longliner vessel targeting on shortfin mako. The individual was caught using baited hooks laid at depths between 0-20 m over an average water depth of 3000 m. After landing, the mako was sold at the fish market and only the total length (TL) and total weight (TW) of the specimen were recorded. Sea surface temperature (SST) at capture site was estimated using data collected at night over a six-day period (March 26-31, 2007), using the online PO.DAAC Ocean ESIP tool (POET-http://poet.jpl.nasa.gov). The main taxonomic characters utilized to distinguish at the species level the longfin mako from the shortfin mako were the shape of the head and snout; the length of the pectoral fins; the relative position of the first dorsal fin with respect to the posterior margin of the pectoral fins; the relative position of the second dorsal fin with respect to the anal fin.
Figure 1. Comparison of morphological features of Isurus oxyrinchus (Fig. 1a,b,c) and I. paucus (Fig. 1d,e,f). a and d, snout; b and e, pectoral fin and first dorsal fin; c and f, second dorsal fin and anal fin
Figura 1. Comparación de caracteres morfológicos de Isurus oxyrinchus (Fig. 1a,b,c) e I. paucus (Fig. 1d,e,f). a y d, hocico; b y e, aleta pectoral y primera aleta dorsal; c y f, segunda aleta dorsal y aleta anal
RESULTS AND DISCUSSION
The captured specimen was a female identified as Isurus paucus. The shark measured 310 cm in total length and weighed 200 kg. Stomach content was composed by squids, likely to be Dosidicus gigas, considering the simultaneous landings of this cephalopod and the shark in the fishing cave. A comparison of the external features between I. oxyrinchus and I. paucus are depicted in Table 1 and Fig. 1. The shape of the snout (Fig. 1a,d), the relative position of the first dorsal fin with respect to the posterior margin of the pectoral fins (Fig. 1b,e) and the insertion of the second dorsal fin with respect to the anal fin (Fig. 1c,f) were the most conspicuous morphological characteristics considered in the identification of the specimen. However, other taxonomic characteristics such as the shape of the pectoral fins and the degree of protrusion of the lower anterior teeth and body coloration could not be shown properly, due the lack of additional graphic material. On the other hand, teeth arrangement has been utilized as a diagnostic feature to differentiate within Isurus species. However, the available information indicates that in large size individuals, teeth become much alike in both species1.
Table 1. Comparison of the external morphological diagnostic features of the genus Isurus (modified from Compagno et al. 2005, Ebert 2001). The number of the corresponding figure of the specimen is indicated, when available
Tabla 1. Comparación de los caracteres morfológicos externos diagnósticos para el género Isurus (modificado de Compagno et al. 2005, Ebert 2001). Se indica el número de la figura correspondiente del ejemplar, en caso de estar disponible
The identification of this specimen from the southeast Pacific coast is interesting considering that worldwide records of I. paucus are scarce and often restricted to circumtropical waters (Compagno et al. 2005). The present record is complemented with the reports of Ebert (2001) and Queiroz et al. (2006), indicating an extension in the known latitudinal and bathymetric range of the species to more temperate and colder waters. The common assumption that I. paucus does not inhabit cold waters is no longer supported due to the evidence that longfin makos are able to regulate heat transfer with the environment, as well as other Lamnid species (Bernal et al. 2001, Goldman et al. 2004). The lack of capture records in the South Pacific Ocean could be a consequence of the capture processing system for pelagic sharks, which are often landed as headed, finned and gutted carcasses (Lamilla et al. 2005, 2008), which makes their identification difficult. Besides, I. paucus and I. oxyrinchus exhibit significant overlap in their distribution range, sharing similar ecological niches, fishing threats, and external morphological characteristics (Compagno 2001, Compagno et al. 2005). Sebastian et al. (2008) reported I. paucus by molecular DNA analysis of fins obtained from fin-storage warehouses in Chile that suggest unreported captures of this species. This background information suggests an extension of the known species range to more temperate waters, which until now was thought to be limited to tropical areas. Temperature range in the capture zone of the longfin mako female reported here differs greatly to that registered for its distribution, with a minimum of 23ºC reported by Compagno (2001) compared with the 18.5ºC in this report. On the other hand, it also points out the importance of investigating the specific composition of shark fishery captures in the southeast Pacific, to evaluate the conservation status of chondrichthyan species affected by regional fishing pressure.
This contribution was supported by Fondo de Investigación Pesquera (FIP) project Nº 2006-31 and Nº 2008-60, Dirección de Investigación of Universidad Austral de Chile (DID-UACH) and Chile Shark Conservation Program (Programa de Conservación de Tiburones, PCT, Chile). We thank two anonymous referees for their valuable observations on an early version of this manuscript. To the fishermen G. Flores, H. Flores and C. Oliva; and G. Ortiz and J. Betzhold for their valuable collaboration and provision of photographic material.
1Bourdon J. 2005. Ontogenetic convergence? Questions in differentiating large mako sharks. http://www.elasmo.com/selachin/slides/isurus_teeth/it_converge.htm. Visited on April 8, 2009.
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Recibido el 10 de octubre de 2008 y aceptado el 1 de junio de 2009