Age and growth of the smooth hammerhead shark,Sphyrna zygaena, in the Eastern Equatorial Atlantic Ocean, using vertebral sections

Rui Coelho,Sérgio Amorim,Miguel N Santos,Joana Fernandez-Carvalho

doi:10.1051/alr/2011145

Abstract

The smooth hammerhead shark Sphyrna zygaena (Sphyrnidae) is regularly caught as bycatch in pelagic longline fisheries, but is one of the least studied of all pelagic sharks. Recently, ICCAT (International Commission for the Conservation of Atlantic Tunas) issued recommendations underlining the need for more studies on the life history parameters of this and other pelagic shark species. To this end, the age and growth of S. zygaena were studied in the Eastern Equatorial Atlantic Ocean, in an area where growth parameters were not yet available for this species. Data from 139 specimens, caught between June and September 2009, ranging in size from 136 to 233 cm fork length (FL), were analysed. Preliminary trials were carried out to assess the most efficient growth band enhancement technique. These indicated that sectioning the vertebrae into 500 μ m sections followed by staining with crystal violet produced the best results. Growth models were fitted using the traditional von Bertalanffy growth equation and a modification of this equation using a known size at birth. Growth models were compared using the Akaike information criterion (AIC). The von Bertalanffy growth equation seemed to be the most adequate model to describe growth in this species, with resulting growth parameters of L inf = 272 cm FL, k = 0.06 year for males and L inf = 285 cm FL, k = 0.07 year for females. In the first four years of life, S. zygaena grows 25 cm per year on average, but its growth slows down in later life. Future stock assessment models should incorporate these age and growth parameters for species management and conservation.

Highlights

Elasmobranch fishes have gained increased importance as fishery resources in recent years (Barker and Schluessel 2005)
As with other pelagic shark species, S. zygaena is commonly caught as bycatch by pelagic longlines targeting swordfish in the Eastern Equatorial Atlantic, even though it is caught in much lower numbers than the considerably more common blue shark (Prionace glauca) and mako shark (Isurus oxyrinchus) (Buencuerpo et al 1998)
Females ranged in size between 140 and 233 cm fork length (FL), while males ranged in size between 136 and 230 cm FL (180.8 ± 15.7 cm)

Summary

Introduction

Elasmobranch fishes have gained increased importance as fishery resources in recent years (Barker and Schluessel 2005). These fishes are exploited by directly targeted fisheries and caught as bycatch in fisheries targeting other species (Stevens et al 2000). Elasmobranchs generally have Kstrategy life cycles characterized by slow growth rates and low reproductive potential (Cortés 2000). These life history traits make them extremely vulnerable to fishing pressure, with overexploitation occurring even at relatively low levels of fishing mortality (Smith et al 1998). As with other pelagic shark species, S. zygaena is commonly caught as bycatch by pelagic longlines targeting swordfish in the Eastern Equatorial Atlantic, even though it is caught in much lower numbers than the considerably more common blue shark (Prionace glauca) and mako shark (Isurus oxyrinchus) (Buencuerpo et al 1998)

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Conclusion