This pie chart shows the relative likelihood of observing particular other species commonly observed near Gymnura natalensis
These species are those which most commonly occur in our observation database near Gymnura natalensis. Observations favor some phyla over others. Typically Bacteria, Fungi, Protozoa, and Arthropods are more common in the field than in our records.
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.
Mozambique; Namibia; South Africa
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).
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.
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).