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Gymnura natalensis

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA CHORDATA CHONDRICHTHYES RAJIFORMES GYMNURIDAE

Scientific Name: Gymnura natalensis
Species Authority: (Gilchrist & Thompson, 1911)
Common Name(s):
English Backwater Butterfly Ray, Butterfly Ray, Diamond Ray, Short-tailed Ray
Taxonomic Notes: See Wallace (1967) and Smith (1991).

Assessment Information [top]

Red List Category & Criteria: Data Deficient ver 3.1
Year Published: 2006
Date Assessed: 2006-01-31
Assessor(s): Wintner, S.P.
Reviewer(s): Cavanagh, R.D., Compagno, L.J.V., Fowler, S.L. & Kyne, P.M. (Shark Red List Authority)
Justification:
A Southern Africa endemic species occurring inshore to 75 m depth from central Namibia through South Africa to southern Mozambique (and possibly also Kenya and Tanzania). Most of the available information on the species is from South Africa. In the past it has been reported as a common species, and is frequently caught by recreational shore anglers and often released. There has been no significant decrease in CPUE (shore, skiboat and estuarine angling combined) in KwaZulu-Natal (1977 to 2000) and no significant trend in catches in the Natal Sharks Board nets where annual mortality is minor. Despite a large number of animals being tagged in the South African National Tagging program, the recapture rate is very low (0.57%). Gymnura natalensis is a major bycatch of prawn trawlers on the Tugela Bank (which may be a nursery area for this species due to the small sizes caught) although annual mortality can be considered to be low. Furthermore, prawn-trawling effort on the Tugela Bank has decreased and Marine & Coastal Management has placed an observer in this fishery. In addition, a new study re-examining the bycatch of this fishery has started. It is a minor bycatch in other inshore fisheries (i.e., beach seine) but this is not significant. An update on the biology (maturity, litter size, nursery grounds, etc.) of this species is needed, including age and growth data, and pending the outcome of the new bycatch study, this species should be re-assessed. However, at present it is considered Least Concern in South Africa as current catches are likely not significantly affecting the species; and Data Deficient in the rest of its Southern Africa range. Information on the status and threats to this species (including catches in artisanal fisheries) throughout its range outside South Africa (Namibia, Mozambique, and possibly Kenya and Tanzania) is required.

Geographic Range [top]

Range Description: Endemic to southern Africa from Southern Namibia through South Africa to southern Mozambique (Wallace 1967, van der Elst 1988, Compagno et al. 1989, Fisher et al. 1990, Smith 1991, Lamberth et al. 1994, Heemstra 1995, Bianchi et al. 1999).

The genus Gymnura occurs in Tanzania but there is uncertainty regarding which species (Bianchi 1985). Species presence in Kenya is also uncertain.
Countries:
Native:
Mozambique; Namibia; South Africa
FAO Marine Fishing Areas:
Native:
Atlantic – southeast; Indian Ocean – western
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: Population size and number and size of subpopulations (if any) are unknown. The species was regarded as common in the 1940-60s (Wallace 1967, Smith 1991), but this needs to be re-examined.
Population Trend: Unknown

Habitat and Ecology [top]

Habitat and Ecology: Habitat
This species occurs close inshore, especially off sandy beaches as well as on offshore banks down to 75 m (Compagno et al. 1989, Smith 1991). Fennessy (1994) found that the species occurs more frequently in the deeper trawls (33 to 45 m) on the Tugela Bank. He observed no diel pattern for this species. Gymnura natalensis is also found in river estuaries (Wallace 1967, Compagno et al. 1989, Smith 1991) and lagoons (Wallace 1967).

Behaviour
Although normally solitary, large shoals have been spotted, often comprising animals of one sex (van der Elst 1988). Single rays tend to be found on the seabed, whereas shoals are often found in the midwater region (van der Elst 1988). Young (2001) found, between 1981 and 2001, 37 occurrences of three or more diamond rays caught in the same Natal Sharks Board (NSB) net installation in one or two days. The majority was of mixed sex and 19 groups were caught off Durban. Most of these multiple catches took place in October and November.

Seasonality
Gymnura natalensis occurs year round (common in Natal throughout the whole year; Wallace 1967) with a peak abundance in December/January (van der Elst 1988). Fennessy (1994) also found that in trawl catches diamond rays occurred more frequently during summer. This differs from the findings of Young (2001), who examined NSB net captures between 1981 and 2000. She found that catches occurred throughout the year, but peaked in October and November (40.5% of total catch) and declined sharply in December. Despite a slight increase during June and July, catches remained low for the rest of the year. Females outnumbered males during warmer months (December to May), while males were more numerous during the cooler period. However, overall both sexes were more common in catches during the cooler months and this association was significant (Young 2001). The sex ratio of NSB catches between 1981 and 2000 was 1:0.92 (m:f) (Young 2001).

Reproduction and Maturity
Wallace (1967) examined a shoal of 28 male specimens caught in Durban nets October 1964. Animals ranged between 109.7and 131.4 cm DW and appeared to be sexually mature. Gravid females (146.2 and 188.0 cm DW) with 5 to 9 embryos were recorded in Durban Bay during January, February, March, June and August. The largest embryo was 38.2 cm DW/453.6 g. A number of the glandular villi, which clothe the uterine walls of the mother, are always found inserted into the spiracular openings of the embryos, suggesting that some of the uterine "milk" is absorbed this way (Wallace 1967). Van der Elst (1988) states gestation period is one year. The smallest mature male dissected at the NSB was 96.4 cm DW and the smallest mature female was 166.9 cm DW (NSB, unpubl. data).

According to the length-age curve of van der Elst (1988), diamond rays are ~24 years old at ~250 cm DW /120 kg. Using the above maturity lengths from Wallace (1967), males mature at approximately two years (~100 cm DW/10 kg) and females at approximately six years (~150 cm DW/25 kg).

Smallest free-swimming specimen: 46.8 cm DW (Wallace 1967); 37 to 39 cm field length (NSB unpubl. data).
Largest embryo: 38.2 cm DW (Wallace 1967).
Maximum reported size: 250 cm DW (Smith 1991).
Largest observed animal: 182 cm DW (Wallace 1967); 250 cm DW, 200 cm field length (dubious!) (NSB, unpubl. data).
South Africa angling record: 89.8 kg (Wallace 1967, van der Elst 1988).

Tagged by NSB between 1996 and 2002: 25 animals, no recaptures to date (NSB, unpubl. data). Tagged by shore anglers between 1984 and 2002: 1,766 animals, 10 recaptures (0.57% recapture rate), which includes washed-up tags. Maximum distance moved: 151 km, maximum time at liberty: 330 days (Bullen et al. 2003).

Diet: flat-fish, sardine, gurnard, mole crabs, worms, crabs and squid (van der Elst 1988, Compagno et al. 1989, Smith 1991, Smale et al. 2001).
Systems: Marine

Threats [top]

Major Threat(s): South Africa

Prawn Trawl Bycatch
According to Fennessy (1994), G. natalensis is a significant bycatch of prawn trawlers. In 169 prawn trawls, 12.9% of the elasmobranch bycatch consisted of this species (= 118 specimens). He stated that the Tugela Bank may be a nursery area due to the small size of the specimens caught. The species showed 46.4% mortality. Assuming this mortality rate and using the derived estimates of catches of G. natalensis from Tugela Bank commercial prawn trawlers for the years 1989 to 1992 given by Fennessy (1994), the number of individuals killed by this fishery was 870 in 1989, 560 in 1990, 548 in 1991 and 461 in 1992. This could be considered low. The mean catch rate for 1989-1992 from Tugela Bank commercial prawn trawlers was 0.15±0.02 individuals/hour. In September 2002, during 46 surveyed trawls a total of 139 specimens were caught and in April/May 2003 a total of 44 animals were caught (46 trawls) (MCM unpubl. data). Effort in the Tugela Bank fishery has been decreasing and as of May 2004, only three operators remain in the fishery.

Beach Seine Bycatch
A small number are caught in the beach seine fisheries in the Western Cape, e.g., between January 1991 and December 1992 (311 beach seine hauls) 18 immature diamond rays (0.002% of total catch) were caught and retained (Lamberth et al. 1994).

Recreational Angling
The species is also caught by anglers, mainly shore-based. Based on shore angling competition data (all areas in South Africa) only, catches (metric tons) and effort (mt/1,000 hours) fluctuated between 1.42-38.4 and 0.29-0.58, respectively (1990-1994) (Smale 1997). Pradervand and Govender (2003) report the capture of 132 animals (0.7% of total catch) in the border region based on competitive shore angling catches between 1982 and 1998. For the Transkei region Pradervand (2004) reports 121 animals (0.7% of total catch) based on Natal Coast Anglers Union competition data between 1977 and 2000. In KwaZulu-Natal, Pradervand (1999) gives a total of 1,197 diamond rays caught between 1995 and 1998, based on catch cards, competitions and inspections of shore, skiboat and estuarine anglers. In the KwaZulu-Natal shore-based competition fishery between 1977 and 2000, 2% of the total catch by number and 19% by mass was G. natalensis (Pradervand 2003). CPUE varied between 0.0012-0.011 diamond rays per angler per hour. There was a significant increase in CPUE over the time period (Pradervand 2003). This is likely to be the result of increased catch card returns and inspection frequency over the time period. CPUE by mass followed the same pattern as above with a significant increase (Pradervand 2003). With the above data one has to keep in mind that this does not represent total angling catch, but only reported catch. In addition, anglers only report when they have caught fish, thus CPUE is always slightly exaggerated. This affects mainly the catch card portion of the data.

Although the majority of angling catches is released, release mortality is unknown and could be substantial due to the angling practices of gaffing, weighing etc. (Wintner pers. obs.). It is interesting to note that the recapture rate of G. natalensis tagged in the South African National Tagging program is only 0.57% (Bullen et al. 2003).

NSB Shark Nets
According to Young (2001) diamond rays (mainly juveniles) were caught on a regular basis between 1981 and 2000 in the NSB nets. During this period, G. natalensis catches represented the second highest batoid catch (12.6% of total batoid catch, 890 animals). The mean annual catch was 44.5 individuals; the mean catch rate (no. animals/km net/yr) was 1.08. No significant trend in catch was found for this species. There was, however, a significant increasing trend in size caught during the period. Mortality levels were low (21.6%, mean = 9.6 animals per year).

Begg (1993) considered diamond ray catches in the NSB nets at a level too high relative to stock size to be sustainable, as this species is also caught by anglers and by prawn trawlers (Fennessey 1994). His annual mortality estimates, however, seem to be an overestimate when compared to the results of Young (2001) and Fennessy's data (1994).

Outside South Africa

Information on threats is required from the other countries in this species' range: Namibia, Mozambique, possibly also Tanzania and Kenya where the species may occur. It is likely that, where it occurs, the species would be taken by artisanal fisheries in the East African countries.

Conservation Actions [top]

Conservation Actions: No specific conservation measures in place. These rays are released if alive when caught in the NSB nets, and are generally released alive when taken by anglers (but not all the time, so this should be encouraged further). Effort reduction in the NSB nets is a conservation action recommended for KZN marine species in general.

Bycatch in all fisheries which interact with this species should be monitored, and the situation outside of South Africa examined (Namibia and Mozambique). The species' occurrence and distribution in Kenya and Tanzania requires examination. Effort has decreased in the Tugela Bank prawn trawl fishery and a 4 month closed season has been put in place.

The recreational line fishery in South Africa is managed by a bag limit of one/species/person/day for unspecified chondrichthyans, which includes G. natalensis.

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 the region. See Anon. (2004) for an update of progress made by nations in the range of G. natalensis.

Citation: Wintner, S.P. 2006. Gymnura natalensis. The IUCN Red List of Threatened Species. Version 2014.3. <www.iucnredlist.org>. Downloaded on 28 November 2014.
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