The Scalloped hammerhead shark, Sphyrna lewini (Griffith & Smith, 1834) is a coastal and pelagic circumglobal species that resides within coastal warm temperate and tropical seas (Bigelow, & Schroeder, 1948; Compagno, Krupp, & Schneider, 1995). In the Eastern Pacific, the species ranges from southern California and the Gulf of California to Panama, Ecuador, Peru and northern Chile (Compagno et al., 1995; Bustamante, 2014).

Sphyrna lewini is highly impacted by anthropogenic activities, including overfishing and as bycatch in fisheries (Miller et al., 2013). Baum et al. (2003) reported a rapid decline in large coastal and oceanic shark populations in the Northwest Atlantic, and estimated that scalloped hammerhead sharks declined by over 75 % in the past 15 years. As noted recently, abundance of S. lewini in Isla del Coco Nationa Park, Costa Rica, has decreased by approximately 45 % during the last 21 years (White, Myers, Flemming, & Baum, 2015). In 2008 the IUCN Red List classified S. lewini as Endangered, and in 2013 it was included on the Appendix II of CITES.

Sphyrna lewini is caught by different fisheries in oceanic (longlines) and coastal waters (artisanal), as consequence of the strong spatial segregation between adults and juveniles. In fact, pups and juveniles live in nursery areas located in coastal waters of estuaries, bays and mangroves, where rich nutrient waters provide food and protection from predators (Clarke, 1971; Branstetter, 1987; Castro, 1993, Duncan, & Holland, 2006). Springer (1967) defines nursery areas as “coastal areas that are geographically separated from adult feeding grounds”. However, Heupel, Carlson & Simpfendorfer (2007) propose a new definition, which “requires three criterion for an area to be identified as a nursery: (1) sharks are more commonly encountered in the area than other areas, (2) sharks have a tendency to remain or return for extended periods, and (3) the area or habitat is repeatedly used across years”.

The adults migrate to open waters, returning to nursery areas for mating and pupping (Clarke, 1971; Duncan, & Holland, 2006). Schools of females S. lewini are known to inhabit areas near several oceanic islands within the Eastern Tropical Pacific, such as Isla del Coco, Galápagos Islands and Malpelo (Hearn, Utreras, & Henderson, 2010). In Isla del Coco, the schools of adults females S. lewini are reported from sites that host mutualistic fish cleaning stations (Sibaja-Cordero, 2008; Nalesso, 2014). On the north side of Isla del Coco is located Wafer Bay (5°32’56.33” N - 87°03’44.96” W and 5°32’47.35” N - 87°03’27.48” W). The bay is shallow ranging from 15-20 m in depth with sand channels, suspended sediments and turbid waters. The Genio River, the biggest river on the island, leads into this bay, which is the major basin of the island (Sibaja-Cordero, & Cortés, 2010; Sibaja-Cordero, Troncoso, Benavides, & Cortés, 2012).

In Wafer Bay, on December 3, 2014 at approximately 19:00 hr, four young-of-the-year S. lewini were caught with hand line and a modified size five circular hook (5°32’42.4” N - 87°03’45.3” W), at 12 m deep with a sea surface temperature of 29ºC (Fig. 1). Identification of the species was attained by analysis of the following distinctive characters outlined by Compagno et al. (1995) and Compagno, Dando & Fowler (2005): 1-a indentation located in the center of the margin of the head and two lateral slits respect to the principal; 2- the dorsal surface and flanks are gray-brown and the tips of the pectoral fins are dark, almost black; 3- the second dorsal fin is elongated almost until the beginning of the precaudal peduncle.

A total of three males (total length (TL): 73, 73, 76 cm) and one female (TL: 75 cm) were caught. The variation of size versus age of several populations of S. lewini has been documented. In Michoacán, Mexico, it was estimated that near term embryos are 44-50 cm TL (Anislado, 2000). For males, size from birth to year one is 48-74 cm TL and females 52-63 cm TL (Anislado, Gallardo, Amezcua, & Mendoza, 2008; Kotas, Mastrochirico, & Petrere, 2011). This is similar to the mean (74.3±17.4 cm TL) of 315 hammerhead juveniles analyzed within Golfo Dulce, in the South Pacific of Costa Rica (Zanella, & López-Garro, 2015). Zarate (2010) reported age and growth of S. lewini in Oaxaca, Mexico, and estimated the first year of growth in females (34.8 cm) and males (33.1 cm).

Based on previous studies, we conclude that the S. lewini captured in Wafer Bay most likely have not completed the first year of life. The presence of these individuals at Isla del Coco suggests that a pregnant female gave birth at Wafer Bay, which may be used as a nursery ground for S. lewini. This is the first record of young-of-year hammerhead sharks from Isla del Coco National Park (Bussing, & López, 2005; Garrison, 2005; Sibaja-Cordero, 2008; Zanella, López-Garro, Golfín-Duarte, & Saenz, 2012; Nalesso, 2014).

Although this report is not consistent with the traditional concept of nursery areas, because Wafer Bay is not located in coastal waters, the estuarine ecosystem offered by the Genio River could be used as nursery ground for pups and juveniles of S. lewini. In conclusion, we recommend further study to evaluate the presence of young-of-the-year S. lewini in Wafer Bay to clarify if this was an unusual event or if the bay is a nursery ground for the species.

ACKNOWLEDGMENTS

We thank the personnel of Isla del Coco National Park and Isla del Coco Marine Conservation Area for their assistance. We also acknowledge the Fundación Amigos de la Isla del Coco for supporting the expeditions.

RESUMEN

El tiburón martillo, Sphyrna lewini, es una especie costera y pelágica semioceánica, que reside en aguas tropicales y subtropicales. Presenta una fuerte segregación intraespecífica: neonatos y jóvenes viven en aguas costeras protegidas (áreas de crianza), mientras que los adultos migran aguas afuera, y vuelven a las aguas costeras y protegidas para fines reproductivos. El 3 de diciembre 2014, alrededor de las 19:00hr horas cuatro jóvenes (de un año o menos) de S. lewini fueron capturados con una cuerda de mano, utilizando un anzuelo circular 5 modificado, en Bahía Wafer, Isla del Coco (5°32’42.4” N - 87°03’45.3” W). Fueron registrados un total de 3 machos (longitud total 73, 73, 76cm) y una hembra (75cm). La presencia de los individuos en la Isla del Coco sugiere que una hembra grávida parió en Bahía Wafer, y que posiblemente esté siendo utilizada por S. lewini como área de crianza. Recomendamos evaluar la presencia y los movimientos de los jóvenes de S. lewini en Bahía Wafer para conocer si fue un evento aislado o si realmente es un área de cría.

 

Palabras claves: isla oceánica, área de crianza, segregación intraespecífica, tiburones, Sphyrna lewini.

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