Oldest estimated age for Sphyrna mokarran (Carcharhiniformes: Sphyrnidae) in the Mexican Pacific

Máxima edad estimada en Sphyrna mokarran (Carcharhiniformes: Sphyrnidae) en el Pacífico mexicano

Javier Tovar-Ávila¹ y Rodney Gallegos-Camacho²

¹ Centro Regional de Investigaciones Pesqueras Bahía de Banderas, INAPESCA. Tortuga #1, La Cruz de Huanacaxtle, Nayarit, 63732. México.

Recibido: 26 de julio de 2012.
Aceptado: 19 de agosto de 2013.

ABSTRACT

The estimated age (around 45 years) from vertebral growth band counts for the largest Sphyrna mokarran caught in the Gulf of California and Central Mexican Pacific in the last decades, locates it as the oldest elasmobranch reported to date in Mexican Pacific waters. The specimen studied was a mature female of approximately 550 kg of total weight and 424 cm of total length, caught by artisanal fishermen using longlines south of the archipelago of Islas Marías on November 11^th 2010. The distance between growth bands showed fast growth during the first 10 years. Growth bands after the age of 38 were only visible in the corpus calcareum, though difficult to separate and count. The age estimated in the present study was similar to the maximum age reported for the species in the Atlantic Ocean and Gulf of Mexico, confirming the species as long lived.

Key words: Great hammerhead shark, growth bands, maximum age.

RESUMEN

Palabras clave: Tiburón martillo gigante, bandas de crecimiento, edad máxima.

The great hammerhead shark, Sphyrna mokarran (Rüppell, 1837), is a coastal and semi-oceanic tropical species. It reaches up to 610 cm of total length (TL), though the common maximum size is around 360 cm of TL (Compagno, 1984) and the maximum reported length in Mexican waters (for a specimen caught near Acapulco, Guerrero) is 483 cm of TL (Applegate et al., 1979). The aim of the present note is to report the largest S. mokarran caught by the artisanal shark fishery in the Gulf of California and Central Mexican Pacific, at least during the last two decades (Perez-Jiménez et al., 2005). The maximum age estimation for the species in the Mexican Pacific, based on vertebral growth band counts, is reported for the first time as well. Bomb radiocarbon and marginal increment analysis indicated that growth bands are formed annually in the vertebrae of this species (Passerotti et al., 2009; Piercy et al., 2010).

The specimen studied was a mature female of approximately 550 kg of total weight and 424 cm of TL. TL was measured with a fiberglass tape to the nearest cm from the tip of the snout to the end of the tail by extending over the body axis (Compagno, 2001). The shark was caught on November 11^th 2010, by artisanal shark fishermen from La Cruz de Huanacaxtle, Nayarit, by using long-lines south of the archipelago of Islas Marias located to the west of Nayarit coast, at the outer mouth of the entrance zone of the Gulf of California (Lavín & Marinone, 2003).

Fork length (FL), estimated with a conversion equation reported for the species from the north-western Atlantic Ocean and Gulf of Mexico (TL = 1.2533(FL)+3.472, n = 24 r² = 0.98) (Piercy et al., 2010), was 334 cm.

A sample of vertebrae from the region above the branchial chamber was obtained. After removing the connective tissues with a scalpel, forceps and a 5% sodium hypoclorite solution, vertebrae were washed with tap water and air dried. Sagittal sections of the centra closest to its focus (approximately 400 p thick) were obtained with an Isomet (Buehler) low speed saw and examined with transmitted light in a compound microscope (Olympus SZ60), adapted to a video camera (Hitachi KP-D50) and a personal computer with the Image Pro Plus (v. 6.1) program. The maximum ratio of the vertebrae, from the focus to the edge of the vertebrae over the corpus calcareum, was measured with a digital caliper to the nearest 0.01 mm. Pairs of growth bands (one opaque and one translucent) were counted. The translucent band closest to the focus was assumed as the birth mark (Goldman, 2005). The estimated FL was compared to that obtained with the von Bertalanffy growth equation previously reported for females (FL = 307.8(1-exp(-0.11(t-2.86))) (Piercy et al., 2010), where t is the estimated age in years.

Although the size of the captured specimen was lower than the maximum reported size in Mexican waters (Applegate et al., 1979), its presence was interesting, considering the scarcity of large individuals. The species had been recorded during several decades along the Mexican Pacific coast (including the western coast of the Baja California Peninsula and the Gulf of California) (Espinosa-Pérez et al., 2004), but it is a rare species caught by the Mexican Pacific shark fisheries. The IUCN had catalogued it as endangered due to the concern about its low productivity and apparent population declines (Camhi et al., 2009). Sharks of such size are particularly rare in artisanal fisheries, as a consequence of selectivity of the fishing gears used.

The vertebrae analyzed (maximum ratio = 25.77 mm) pre-sented clear growth bands (Fig. 1). The estimated age was over 40 years (probably 45). Growth bands after the age of 38 were not visible in the intermedialia but they were in the corpus calcareum, though difficult to separate and count. Consistent with previous studies (Piercy et al., 2010), the distance between growth bands showed fast growth during the first 10 years, after which they became finer and tighter, as it happens in most shark species (Goldman, 2005).

Sphyrna mokarran has been categorized as a slow growing and long lived species, with one of the oldest age estimates for elasmobranchs and the oldest for temperate or tropical environments (Piercy et al., 2010). The age estimated in the present study was only one year over the maximum age estimated for the species in the Atlantic Ocean and Gulf of Mexico (44 years for a 315 cm FL female), though the estimated FL for the specimen reported here was 19 cm larger. Such differences could be related to the great variability in size-at-age reported previously (Piercy et al., 2010) and error associated to age estimation due to the difficulty to separate growth bands close to the edge in large animals. FL obtained with the von Bertalanffy equation (305 cm) with the estimated age in the present study was also lower than the converted FL from measured TL, but close to the asymptotic length reported previously for the species (307.8 cm) (Piercy et al., 2010). Despite possible errors counting growth bands in the edge of the vertebrae, the studied organism here is the oldest elasmobranch reported to date in Mexican Pacific waters and one of the oldest S. mokarran aged around the world.

AKNOWLEDGMENTS

We thank the anonymous referees for their comments and corrections, the fishermen who allowed obtaining the shark vertebrae, C. Güemez, R. Sánchez-Regaladot from CRIP Bahía de Banderas, Y. Green and A. Verde from CRIP Mazatlan, INAPESCA, A. Alatorre, A. Lizarraga and C. Guevara from ITBADBA for helping during the sample collection and processing.

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