A local fisherman alerted the museum this week (10 July 2018) of some wild fish activity off the reef at the Nahoon Point Nature Reserve. Large bait balls of sardines were being pursued by hundreds of skipjack tuna (Katsuwonus pelamis). Retired museum colleague, Greg Brett, confirmed the identity of the species and commented that it was a good record for the region.
Documenting the event it was noted that the ‘boiling’ water off the reef had large fish surfacing as they preyed upon the smaller sardines.
The skipjack tuna are spindle-shaped with a silver belly and 4-6 horizontal stripes. They can grow to 1 m and weigh up to 35 kg. They mostly occur off the east coast in summer, so this record for the winter months (so close inshore) is rare. They feed on pelagic fish and squid (and occasionally juvenile skipjack).
They generally occur in large schools near the surface in offshore waters and can be associated with sharks, whales and other tuna species. There was a lot of whale activity in the area on the day they were documented at Nahoon. The genus of the scientific name Katsuwonus is derived from Katsuwo, the Japanese name for the fish and pelamis (white wax). A silvery wax-like pigment can be scraped from the chest region and is used in high quality paints by taxidermists. The common name is in reference to its habit of ‘skipping’ along the surface (as illustrated below).
This fish is able to maintain swimming speeds of about 40 km/h due to a specialized ‘warm-blooded’ vascular system which keeps the muscles well supplied with oxygen. They occur in water temperatures ranging from 19-30 degrees Celsius.
A guide to the common sea fishes of southern Africa by Rudy van der Elst
Coastal fishes of southern Africa by Phil and Elaine Heemstra
Two oceans – a guide to the marine life of southern Africa by G Branch, C Griffiths, M Branch and L Beckley