We had an interesting visitor into the Knysna lagoon recently, a whale shark Rhincodon typus. Whale sharks are very large, slow-moving filter-feeders which have a circumpolar distribution through all tropical and warm water, between 30°N and 35°S. The distribution of whale sharks is temperature dependent, and they are rarely sighted in areas with a surface water temperature less than 21°C. This is probably why the whale shark ended up in Knysna lagoon, it was searching for warmer water. Whale sharks have a unique colouration of white or yellow spots, and horizontal and vertical stripes on a grey-blue upper surface. The underside is white or yellowish in colour. The largest whale shark ever recorded was 20m in length and weighed 42 tons! Most whale sharks are a smaller, with 12m being a more average length, with males reaching maturity between 7-9m while females are sexually mature at roughly 9m in length. The whale shark that stranded in Knysna was roughly 4m and at this size would have been an immature individual. As with other shark species, male whale sharks have claspers while females do not. Little is known about the reproduction of whale sharks, but a mature female caught in Taiwan had roughly 300 young in her uterus at varying stages of development! Size at birth is between 55 and 64cm, although a free-swimming individual of 46cm has been recorded. Whale sharks are, despite their large size, relatively harmless. They do not act aggressively towards humans when approached, and there are many ecotourism ventures focused on whale shark interactions. Whale sharks are listed as Endangered according to IUCN, with a decreasing population. Vessel strikes, bycatch by fisheries, and focussed fisheries for meat and fins, even tourism if incorrectly managed all play a part in a drop in whale shark numbers.
Written by: Minke Witteveen
For further reading:
- Pierce, S.J. & Norman, B. 2016. Rhincodon typus. The IUCN Red List of Threatened Species 2016. Accessed: 2017-03-11. URL: http://www.iucnredlist.org/details/19488/0.