IDENTIFICATION
Yellowfin tuna are large robust fish with a spindle shaped body. The back is a dark metallic blue, fading into yellow along the sides with a silver-white belly. The dorsal and anal fins are yellow. They have very powerful red muscles which allows for high speed sprints and also for high speeds over long distances.

DISTRIBUTION
Yellowfin tuna are distributed throughout the world in tropical and subtropical seas. They inhabit waters with temperatures between 18 degrees centigrade and 31 degrees centigrade and are usually found near the surface, but can also be found at great depths. Each stock has its own feeding, spawning and migration routes.

FEEDING
An opportunistic feeder, yellowfin tuna feed on small fish, mantis shrimps, crabs and crayfish larvae. They also feed on small fish which hide under floating objects. Tuna probably need to eat up to 15% of their body weight in food per day.

REPRODUCTION
Spawning occurs throughout the year in the core areas of distribution but peaks during the summer months. Indian Ocean stocks spawn off Madagascar while Atlantic Ocean stocks spawn off Brazil. The larvae are pelagic and are widely distributed by ocean currents. Sexual maturity is reached at a length of 120 cm.

GROWTH
Yellowfin tuna can attain 210 cm in length but in Kwazulu-Natal they rarely exceed 100 cm in length. The South African angling record is 107 kg.

FISHERY
A number of countries exploit yellowfin tuna and it accounts for about 23% of the world tuna catch. The flesh is excellent and is used for canned tuna.

Yellowfin tuna are caught by commercial and recreational fishers. The commercial fishing industry uses three methods, namely longlining, poling and purse seine netting. Recreational skiboat anglers either troll or use live bait.

Longlining is mainly undertaken by foreign vessels in the south Atlantic and south Indian Oceans and about 15 000 tons of fish are caught annually.

In 1960, tuna became commercially exploited in South Africa and were caught by means of longlining. Annual catches were low and fetched a poor price in the market. By 1964, the industry was not commercially viable. Renewed interest in commercial fishing began in the early 1970's due to improved catching methods and sport anglers also started targeting the species with reasonable success. In 1979, a massive run of yellowfin tuna was recorded and by November of that year, 115 commercial vessels were targeting tuna resulting in over 6 000 tons being landed. In 1980, large shoals failed to appear and the new industry virtually collapsed, but this effort was redirected to other tuna species such as longfin, bigeye and skipjack. This redirected effort provided the bulk of South Africa's tuna catch of about 3000 tons per year.

MANAGEMENT

• Yellowfin tuna are classified as exploitable.
• There is no closed season.
• A weight restriction of 3.2 kg per fish is applicable.
• There is a bag limit of 5 per person per day.

Huge catches of juvenile yellowfin tuna are taken by purse seine vessels in tropical waters. Fishing of the juveniles will have an effect on the abundance in South African waters and needs to be carefully managed. Marine reserves are not effective in conserving this species as it is oceanic and highly migratory.

Rudy van der Elst and Bruce Mann are acknowledged for the scientific information on which this web page is based.