6 Oct 2014 - 3:39 PM  UPDATED 13 Nov 2014 - 2:38 PM

 

What Species?

  • Southern bluefin tuna are raised in ‘sea ranches’ off the coast of South Australia
  • Southern bluefin tuna are a migratory fish. Ranching them involves catching the fish in nets and transferring them to offshore ‘sea ranches’ to be fattened up until they are big enough to sell. 

What’s the Catch?

  • Australian Commonwealth stock status reports classify southern bluefin tuna as “overfished.”
  • The latest stock assessments have found that southern bluefin tuna are at around 9% of their original level.
  • The International Union for Conservation of Nature lists southern bluefin tuna as “critically endangered” and places it on its Red List.
  • Because southern bluefin tuna migrates across international maritime borders, the global fishing industry is overseen by an international authority - the Commission for the Conservation of Southern Bluefin Tuna (CCSBT).
  • Member nations of the Commission include Australia, New Zealand, Indonesia, Japan, South Korea and Taiwan – all southern bluefin tuna fishing nations.
  • Despite being classified as endangered and overfished, and at only 9% of its original spawning stock levels, southern bluefin tuna is still being fished.
  • Southern bluefin tuna are vulnerable to overfishing, as they are a slow-growing, long-living fish.
  • Nonetheless, the scientific committee at the CCSBT raised the global quota for fishing southern bluefin tuna by 55% for the period 2011-15.
  • Southern bluefin tuna begin their lives in the warm-water spawning fields off Java, before riding ocean currents down the coast of Western Australia to the Great Australian Bight. They continue to summer in the Bight, wintering in either the Indian Ocean or Tasman sea, until they are about five, when they stop returning to the Bight. From then on they move between feeding grounds near South Africa and New Zealand before heading back to the Javanese tropics to spawn.
  • Tuna ranching, developed in South Australia in 1991, involves catching tuna in a purse seine net and transferring them to offshore ranches where they are fattened up for sale.
  • Tuna ranching value-adds to the fishing quota, allowing the Australian industry to meet its weight quota in juveniles. After about six months in the ranch, the juveniles weigh twice as much as when they were caught, and are ready for market.
  • Because they are fished before they are old enough to reproduce (southern bluefin tuna reach sexual maturity at around 12 years) the fish caught off South Australia cannot help replenish their species’ low stocks.
  • Because ranching is so new, it is not easy to determine its possible long-term environmental impacts, but one concern involves the large amount of nitrogen-rich waste produced by the ranches and its potential to contaminate the adjacent waters.
  • As a top predator, tuna play an important role in the marine ecosystem. Overfishing top predators can create what’s called trophic cascade, which means the predators’ prey become overabundant, in turn depleting stocks of marine life that are lower on the food chain.
  • Ranched southern bluefin tuna are not self-sufficient, and live on a diet of both fresh wild fish, including sardines, and manufactured fish feed (made up in part of wild fish). It takes at least 10kg of food (comprising both fresh fish and feed) to produce 1 kilo of tuna – the worst feed-to-fish ratio of any species.
  • So far, attempts to create a more sustainable industry by breeding tuna from larvae have enjoyed only limited success, and conservationists are skeptical as to whether southern bluefin tuna can ever be domesticated.
  • Most southern bluefin tuna is sold as a premium product on the Japanese market, where it is eaten as sashimi.
  • The Australian Marine Conservation Society says to say no to eating southern bluefin tuna, and the species is on Greenpeace Australia Pacific’s Seafood Redlist. 

What’s the Solution?

  • Fresh southern bluefin tuna is not a sustainable choice, but some canned tuna is. Greenpeace recommends choosing pole or line caught albacore or skipjack tuna, or Australian-caught tuna in cans.

What's the Recipe?

Blackened skipjack tuna

More information

What's the Catch pemieres 8.30pm Thursday 30 October on SBS ONE.
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