The museum was alerted on Monday 9 Sept 2019 (via the aquarium – Ms Siani Tinley) that a fisherman Juan Wessels had come across a live dolphin in the surf zone west of Cove Rock on Sunday 8 Sept 2019. A video of the dolphin indicated that the animal was in distress. With some effort he managed to get the dolphin to deeper water and it swam away.
Museum scientist Kevin Cole made a comment to Juan Wessels after viewing the video that its chances of survival were slim and that he thought it may strand within a day or two. Juan advised where he thought the animal may come out (somewhere west of the location where he had found it).
Mrs Jacky Biller of Palm Springs called to report that a dolphin had stranded west of the Kidd’s Beach Village (Tuesday 10 September 2019). Her husband Sean Biller is also thanked for securing the specimen overnight so that it would not be lost to the sea at the high tide. An investigation revealed that it was a deep water 2.96 m female Risso’s dolphin.
This is the largest dolphin not to be called a whale and anatomically has a very ‘top-heavy’ head section. Their appearance presents with a blunt head (carrying a very distinctive vertical furrow in its face) and a very erect centrally placed dorsal fin. The colour of the animal changes from birth to adulthood and most notably in adulthood it has an anchor shaped mark on the chest. As they grow older, they accumulate characteristics body scarring which was very visible on the stranded specimen. They only have teeth on the front of the lower jaw in our waters. A robust, conical tooth was collected from the animal with a diameter of 7.2 mm at the gum base.
A necropsy was undertaken on Wednesday 12 September 2019 by Kevin Cole with the kind assistance of Kidd’s Beach resident Barry Marshall. All internal organs were examined and the stomach removed for a detailed investigation.
No food or plastic was found in the latter and all organs (liver, lungs, lower intestine, ovaries and kidneys) appeared normal with the exception of the 1.653 kg heart. A hard chestnut sized lump was detected and it was removed and preserved for an investigation by Bayworld and Nelson Mandela University scientists. Other samples included blubber, skin, blood and muscle.
These dolphins can grow to a length of 3.41 m (males) and 3.18 m (females). They normally move in small groups (10 – 30 individuals) and occur worldwide in temperate and tropical seas. In our seas they are associated in waters of 350 – 600 m of the upper continental slope. Their main diet consists of squid and it is presumed they feed mostly at night.
This is the 4th Risso’s dolphin record for the museum since 2011 with the last being at Morgan Bay (March 2019). 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). There still a lot we need to learn about these cetaceans and they are listed as data deficient in the South African Red Data Book.
Reference: Whales and dolphins of the southern African subregion by Peter B. Best