Are tuna stocks being overfished?

Humans have been catching tuna for many thousands of years, but industrial fisheries for these large ocean predators, a result of increasing demand for tuna in canneries, did not start until the 1940s and 1950s. The total world catch of the major commercial species of tunas (albacore, bigeye, bluefin, skipjack, and yellowfin) has increased continuously and tenfold during the last 50 years (from 0.4 to over 4 million tonnes).

Skipjack is by far the main tuna species caught, followed by yellowfin, bigeye, albacore and the two bluefin species. The main commercial tuna fishing nations are Japan, Taiwan and Spain.

Today, many stocks of the major commercial tuna species are fully or over-exploited. Of all the principle market species, bluefin have suffered the most from the ravages of overexploitation, for two main reasons: their slow reproductive rate, and their exceptionally high value in the sashimi market, where a single fish can be worth up to US$100,000. Most seriously overexploited is the southern bluefin, which is now listed by the IUCN (the world conservation union) as being Critically Endangered.

The Atlantic Ocean currently accounts for about 14% of world tuna production. The principal species caught in the Atlantic, in terms of quantities landed, are skipjack and yellowfin, with nearly 80% of the landings coming from the eastern Atlantic. Most of this catch is made by large purse-seine vessels, which also catch bigeye.

According to the latest assessments carried out by ICCAT, the organisation responsible for managing tuna fishing in the Atlantic Ocean, virtually all Atlantic tuna stocks are heavily exploited, some unsustainably so, and catches are, in many cases, in decline.

Bigeye - Catches of bigeye peaked in 1994, but had plummeted by almost 50% by 2004. The large quantities of illegal catches for many years mean that scientific assessments of the abundance of the stock are very uncertain. An analysis conducted in 2004 indicated that the bigeye stock had declined significantly due to large catches being made since the mid-1990s which were around, if not below, the presumed sustainable level. In effect, the number of fish taken was too great during that time period.

Yellowfin - There has been a steady decline in overall catches of yellowfin in the Atlantic since 2001. According to the most recent ICCAT assessment, the stock of yellowfin is being very heavily fished, and is possibly over-exploited, meaning that the stock is smaller than it should be and fishing should be reduced to allow the stock to increase.

Albacore - Although the South Atlantic albacore stock is thought to be relatively healthy, the North Atlantic stock of albacore is considered over-fished, with the spawning stock 30% below the estimated sustainable level (i.e. there needs to be more fish of reproductive age if the population size is to be maintained in the future).

Bluefin - The Western Atlantic population has decreased by more than 80% since the 1970s when stock assessments were first made. Despite uncertainties in the data, it is also clear that the Eastern Atlantic population has declined during this time, and current levels of exploitation are unsustainable.

Skipjack - The characteristics of Atlantic skipjack tuna stocks and fisheries make it extremely difficult to conduct stock assessments. ICCAT has not conducted a stock assessment for Atlantic (West or East) skipjack tuna since 1999, and there are no definitive conclusions available on the status of the stocks.

ICCAT also considers the populations of blue marlin, white marlin, and North Atlantic swordfish to be over-fished.

Consumption: canned or fresh

Tuna species are a significant source of food in many countries. They are very important economically, with world trade in tuna worth in excess of US$5 billion. In terms of the global trade in fish commodities, tuna accounts for around 8 per cent of imports by value and quantity.

Tuna is commonly consumed in two forms: either fresh, or canned:

Sashimi is prepared from fresh, high quality raw tuna meat, or from tuna frozen at temperatures below -40ºC immediately after capture. Traditional sashimi is prepared from the three species of bluefin, bigeye and yellowfin tunas. It is served in thin slices, dipped in soy sauce and/or wasabi (Japanese horse radish) and consumed raw. Sushi is a cylinder of rice combined with vinegar, with a slice of sashimi on the top or in the middle.

Tunas that are not acceptable for sashimi are sold in the steak market, generally in Europe and the United States. Steaks are generally prepared from bigeye, yellowfin and albacore tunas, mostly fresh but also frozen.

Japan is the world's largest producer and market for fresh and frozen tuna and tuna based products (excluding canned tuna), although tuna in the form of sashimi and steaks is becoming increasingly common within the European and North American markets.

The major importers of canned tuna are the United States, the United Kingdom and France, in terms of both quantity and value. Skipjack, yellowfin and albacore are the species principally used for canning. The fish, which are generally frozen, are cut up, cooked and then canned in brine or oil.

In the UK, tinned tuna dominates the canned seafood sector, with sales estimated to be worth in excess of £250 million per year.