Mr Alan Harris of Glen Stewart contacted the museum about a cetacean stranding on the rocks at Queensbury Bay. The animal was identified by museum scientist Kevin Cole as a Risso’s dolphin (Grampus griseus)- the largest dolphin not to be named a whale.
The stranding was reported on the 21st July 2017 and it it presumed (judging by the condition of the animal) that the dolphin had been floating at sea for a day or two before being washed up at Queensbury Bay. A reasonable amount of the delicate thin skin on the head had blistered off and there were a few scratch marks from a rocky entry to the shoreline.
The juvenile dolphin measured 2.01 meters in length and showed no signs of any major physical trauma. A reasonable amount of information has been gathered about Risso’s dolphins from mass strandings in South Africa between 1983 and 1991.
Diagnostic features for the identification of the species include a blunt head (beakless), a prominently centrally-placed erect dorsal fin (in adults) and numerous pale scratches and scars on mature animals. The calf stranded here is still uniformly coloured with a distinctive V-shaped crease from the blowhole to the tip of the rostrum.
The photo above also illustrates a whitish-grey anchor shaped mark on the chest connected by a thin mid-ventral streak with an irregular lozenge-shaped mark extending from the umbilical region to the anus. This marking persists into adulthood with the species.
Risso’s dolphins do not have teeth in the upper jaw. Erupted teeth have only been found in the lower jaw from dolphins in our waters – confined to the very front of the jaw (erupting at a body length of 1.88m). Between 2 and 6 pairs have been recorded in the species and in old animals the teeth become worn down often falling out resulting in an individual with no teeth at all.
Risso’s dolphins occur worldwide in tropical and temperate seas and can reach speeds of up to 28 km/h (normal travelling speeds are about 5.5 km/h or less). They mostly feed on squid and octopus.
Four mass strandings of the species have been recorded in South Africa – May 1983 (4 males and 4 females). April 1989 (10 males and 11 females), February 1990 (1 male and 6 females) and August 1991 (9 males and 2 females).
This is the second Risso’s dolphin record for 2017. Sean Pike of Wavecrest reported a live 1.5 meter animal which has beached close to the Nxaxo River and was released close to Sandy Point later on the same day (21st February 2017).
After the release there were no reported sightings of the animal again and it is hoped it survived.
Risso’s dolphins can reach a length of 3.5 meters in the South African waters with the front half of the animal noticeably bulkier than the body behind the dorsal fin (whale-like upfront and dolphin-like in the tail region). At birth the dolphins are brownish-grey and becomes whiter as it ages (the combination of gradually lightening pigmentation and the accumulation of scars).
Whales and Dolphins of the Southern African Subregion by Peter. B Best
Whales, Dolphins and Seals. A field guide to the marine mammals of the world by Hadoram Shirihai and Brett Jarrett