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Rhinobatos zanzibarensis

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA CHORDATA CHONDRICHTHYES RAJIFORMES RHINOBATIDAE

Scientific Name: Rhinobatos zanzibarensis
Species Authority: Norman, 1926
Common Name(s):
English Zanzibar Guitarfish
Taxonomic Notes: Revision of the large genus Rhinobatos is proposing to elevate subgenera Acroteriobatus to generic status, thus Rhinobatos zanzibarensis will be referred to as Acroteriobatus zanzibarensis (L.J.V. Compagno pers. comm.). However, these changes are not yet published and until such time as they are the species should continue to be referred to as Rhinobatos zanzibarensis.

The taxonomic validity of this species is in question and requires review. However, until such time as this problem is solved it is treated as a valid species.

Assessment Information [top]

Red List Category & Criteria: Near Threatened ver 3.1
Year Published: 2006
Date Assessed: 2006-01-31
Assessor(s): Burgess, G.H. & Marshall, A.D.
Reviewer(s): Heupel, M.R., Simpfendorfer, C.A. & Kyne, P.M. (Shark Red List Authority)
Justification:
An apparently rare coastal rhinobatid endemic to Zanzibar, Tanzania, but known only from the type specimens. A large species of guitarfish, reaching at least 205 cm total length. Survey work has failed to locate other individuals in and around Zanzibar, the adjacent Tanzanian coast and the wider East African region. Coastal rhinobatids are susceptible to capture in a variety of fishing gear including trawl, gillnet, line and seine net and their occurrence along inshore areas of the continental shelf makes these rays an easy target for such fisheries. Target fisheries for guitarfish currently exist in several countries particularly in the Indo-West Pacific and declines in some species have been documented. When taken, flesh is utilized and the fins of larger species (possibly including R. zanzibarensis) enter the international fin trade for the Asian market. Coastal waters of Zanzibar are consistently fished by artisanal fishers mostly using traditional vessels. A wide variety of gear is employed including drag nets, spear, handline, seine nets and fish traps. Rays are known to be caught and retained and shark fin is marketed. If R. zanzibarensis is restricted to the inshore continental shelf around Zanzibar, the majority of its available habitat would be fished by a variety of gear. Various regulations govern gear use in Zanzibar, but there are issues with enforcement and so illegal and sometimes destructive fishing activities still occur. The present level of fishing in coastal areas of Zanzibar is of concern for this large, rare and limited range guitarfish. If however, the species is found to have a wider range into deeper waters, it may presently persist due to the limited range of traditional fishing vessels. Fishing pressure outside coastal areas is minimal at present in Tanzania. Although a little known species, given its large size, rarity, and restricted range as presently known, the species is assessed as Near Threatened. There is an equal possibly that the species may no longer exist in the wild, or that remnant populations presently exist beyond the range of traditional fisheries operating around Zanzibar. Further research (including continued market and fishing surveys) is required to document habitat, distribution and to locate any remnant populations.

Geographic Range [top]

Range Description: Extremely limited range in the Western Indian, only reported off Zanzibar Island, Tanzania. Narrow range supported by the fact that efforts to find additional specimens in fish markets and in underwater surveys in other areas have failed to produce any other individuals.
Countries:
Native:
Tanzania, United Republic of
FAO Marine Fishing Areas:
Native:
Indian Ocean – western
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: As presently known, an extremely rare species, known only from a few specimens. Survey work has failed to locate other individuals in and around Zanzibar, the adjacent Tanzanian coast and the wider East African region. There is no knowledge of population size or structure.
Population Trend: Unknown

Habitat and Ecology [top]

Habitat and Ecology: A rare, little known coastal species. A large species, Norman (1926) reporting a female of 205 cm TL.

Like other rhinobatids, aplacental viviparous, but biology is unknown. Studied Rhinobatos species have an annual reproductive cycle with litter sizes of 2 to 16 pups.

Life history parameters
Age at maturity (years): Unknown.
Size at maturity (total length cm): Unknown.
Longevity (years): Unknown.
Maximum size (total length): 205 cm TL (Norman 1926).
Size at birth (cm): Unknown.
Average reproductive age (years): Unknown.
Gestation time (months): Unknown.
Reproductive periodicity: Unknown.
Average annual fecundity or litter size: Unknown.
Annual rate of population increase: Unknown.
Natural mortality: Unknown.
Systems: Marine

Threats [top]

Major Threat(s): Coastal rhinobatids are susceptible to capture in a variety of fishing gear including trawl, gillnet, line and seine net and their occurrence along inshore areas of the continental shelf makes these rays an easy target for such fisheries. Target fisheries for guitarfish currently exist in several countries particularly in the Indo-West Pacific and declines in some species have been documented. When taken flesh is utilized and the fins of larger species enter the international fin trade for the Asian market. It is probable that the fins of R. zanzibarensis enter the international market, as the large wedgefish Rhynchobatus djiddensis is commercially exploited for its fins in Tanzanian waters in bottom-set gillnets, when taken in prawn trawls and possibly also by spearfishermen (Barnett 1997), suggesting that R. zanzibarensis, as a large guitarfish, is also a desirable catch.

Coastal waters of Zanzibar are consistently fished by artisanal fishers mostly using traditional vessels (Jiddawi and Öhman 2002). A wide variety of gear is employed including drag nets, spear, handline, seine nets and fish traps (Jiddawi and Öhman 2002). Rays are known to be caught and retained and shark fin is marketed (Jiddawi and Öhman 2002). If R. zanzibarensis is restricted to the inshore continental shelf around Zanzibar, the majority of its available habitat would be fished by a variety of gear. Various regulations govern gear use in Zanzibar, but there are issues with enforcement and so illegal and sometimes destructive fishing activities still occur (Jiddawi and Öhman 2002). The present level of fishing in coastal areas of Zanzibar is of concern for this large, rare and limited range guitarfish. If however, the species is found to have a wider range into deeper waters, it may presently persist due to the limited range of traditional fishing vessels. Fishing pressure outside coastal areas is minimal at present in Tanzania (Jiddawi and Öhman 2002).

Conservation Actions [top]

Conservation Actions: Further research (including continued market and fishing surveys) is required to document habitat, distribution and to locate any remnant populations. Elasmobranch catches in Zanzibar artisanal fisheries need documenting, including the degree of finning.

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 sustainable management of all chondrichthyan species in Tanzania. At the time of writing, Tanzania had not made progress towards developing a National Plan of Action.

Citation: Burgess, G.H. & Marshall, A.D. 2006. Rhinobatos zanzibarensis. The IUCN Red List of Threatened Species. Version 2014.3. <www.iucnredlist.org>. Downloaded on 27 November 2014.
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