|Species: Hexanchu nakamurai (Bigeyed Sixgill Shark)
The Bigeyed Sixgill Shark (Hexanchus nakamurai) is a little known, moderately large (to 180 cm TL), primarily deepwater cow shark with a patchy distribution in tropical and warm temperate waters in the northeast and central Atlantic; the Mediterranean Sea; the northwest, western central and southwest Pacific; and the Indian Ocean. Probably mesopelagic to benthic in shelf and slope waters from 90-621 m with possible excursions to the surface. The species has often been misidentified as the larger Bluntnose Sixgill Shark (Hexanchus griseus), leading to confusion and poor knowledge of its distribution and no knowledge of any population trends. It is uncommonly taken by bottom trawl and longline gear and is of relatively small importance to fisheries. Due to insufficient information this species cannot be assessed beyond Data Deficient at present. Species-specific catch data are required to better define the distribution, population trends, if any, and threats to the species.
It may be more common than thought, but is often misidentified with smaller juveniles of its larger relative Hexanchus griseus (Ebert 2013).
Widely but patchily distributed in warm temperate and tropical seas, but possibly absent from the eastern Pacific (Ebert 1990, Compagno et al. 2005, Compagno in prep).
Northeast Atlantic: Gulf of Gascony and Bay of Biscay, off France, Spain and Gibraltar. It occurs from the Straits of Gibraltar to Italy in the Mediterranean Sea (Ebert 1990).
Uncommon to rare where it occurs (Compagno in prep, Ebert 1990). Number and size of populations is unknown. Rare in the Mediterranean Sea (Serena 2005, Ebert 1990). Misidentification with H. griseus has prevented a better understanding of this species' distribution.
Habitat and Ecology
Primarily a deepwater species, found on the continental and insular shelves and slopes from 90–621 m depth, usually on or near bottom, but occasionally moving to near the surface or inshore in the tropics (Compagno in prep, Ebert 1990). May be more restricted in habitat and distribution than H. griseus and less tolerant of conditions beyond offshore continental habitat in warmer seas (Compagno in prep.). Size at maturity is 142–178 cm in males and 123–157 cm in females (Compagno in prep., Ebert 1990). Maximum size is approximately 180 cm (Compagno in prep, Ebert 1990). Reproduction is ovoviviparous, with 13–26 pups per litter, measuring 40–45 cm TL at birth (Whitehead et al. 1984, Compagno et al. 2005, Compagno in prep, Ebert 1990). Probably feeds on small to medium-sized bony fishes, including hairtails (Trichiuridae), small tuna (Euthynnus), and crustacea (Compagno in prep, Ebert 1990).
It probably is a minor component of offshore demersal trawl and line fisheries elsewhere where it occurs, but no catch statistics are available (Compagno in prep.).
The species may be under growing pressure with the expansion of deep water fisheries and there is an urgent need to collect species-specific catch data to determine accurate population trends.
There are no species-specific conservation measures in effect or proposed at this time. Like many deeper water species more information on biology, ecology and importance in fisheries are required to assess the threat status and any future conservation needs. Where taken, catches require monitoring, particularly as deepwater fisheries expand worldwide. The development and implementation of management plans (national and/or regional e.g., under the FAO International Plan of Action for the Conservation and Management of Sharks: IPOA-Sharks) are required to facilitate the conservation and management of all chondrichthyan species in the region.
European Commission (EC) Regulation No. 1185/2003 prohibited the removal of shark fins and subsequent discarding of the body, which was binding on EC vessels in all waters and non-EC vessels in Community waters. A fin-to-carcass ratio of 5% of the shark’s whole (theoretical) weight was adopted. This Regulation also allowed Member States to issue Special Fishing Permits allowing sharks to be processed on board by removing their fins from their bodies. This loophole was eventually amended in 2013 by Regulation (EU) No. 605/2013 of the European parliament, which recognises “serious control and enforcement difficulties” associated with the fin-to-carcass ratio, and states that it “could lead to finning going undetected”. The amendment states, “The Scientific, Technical, and Economic Committee for Fisheries (STECF) acknowledges the problem of shark finning, calls for its eradication, without derogations, and advises that all elasmobranch species should be landed with their fins/wings naturally attached to their bodies. … Without prejudice to paragraph 1, in order to facilitate on-board storage, shark fins may be partially sliced through and folded against the carcass, but shall not be removed from the carcass before landing”.
Compagno, L.J.V. 1984. FAO species catalogue. Vol. 4. Sharks of the world. An annotated and illustrated catalogue of shark species known to date. Part 1. Hexanchiformes to Lamniformes. FAO, Rome.
Compagno, L.J.V., Dando, M. and Fowler, S.L. 2005. Sharks of the World. Harper Collins.
Compagno, L.J.V. In prep.. Sharks of the World. An annotated and illustrated catalogue of the shark species known to date. FAO, Rome.
Ebert, D.A. 1990. The taxonomy, biogeography and biology of cow and frilled sharks (Chondrichthyes: Hexanchiformes). Unpub. Ph.D. thesis. Rhodes University, Grahamstown.
Ebert, D.A. & Stehmann, M. 2013. Sharks, batoids, and chimaeras of the North Atlantic. FAO Species Catalogue for Fishery Purposes, 7. Rome, FAO: 523 pp.
IUCN. 2009. IUCN Red List of Threatened Species (ver. 2009.2). Available at: www.iucnredlist.org. (Accessed: 3 November 2009).
Serena, F. 2005. Field identification guide to the sharks and rays of the Mediterranean and Black Sea.
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