Five out of the eight tuna species are at risk of extinction, conservationists warned today, as they called for urgent action to tackle over-fishing.
The latest assessment for the International Union for Conservation of Nature (IUCN) showed that three species are threatened with global extinction, while two more will be under threat without action to help them.
A study, published in the journal Science, which looks at all "scombrid" fish, which include tuna and mackerel, and billfishes, which include swordfish and marlins, found that seven of the 61 known species were under threat.
The study said some of the species were heavily over-fished, with little interest in conserving them because of the high commercial value of the catch.
There were also difficulties in regulating the multinational fisheries which exploit the stocks.
IUCN experts warned that all three bluefin tuna species – southern, Atlantic and Pacific – were susceptible to collapse because of pressure from fishing for the high-value fish.
Southern bluefin tuna are already critically endangered, the highest category of risk, and Atlantic bluefin are endangered, the assessment for the IUCN red list of threatened species found.
Bigeye tuna are vulnerable to extinction, while yellowfin and albacore tuna are close to being under threat, or will be threatened with extinction if conservation measures are not put in place to turn their fortunes around.
Among other species, blue and white marlin were both assessed as being vulnerable to extinction, putting them in the third of the three most serious categories for threatened species and at risk of dying out globally.
Dr Kent Carpenter, manager of IUCN's marine biodiversity unit and an author of the study, said: "All three bluefin tuna species are susceptible to collapse under continued excessive fishing pressure.
"The southern bluefin has already essentially crashed, with little hope of recovery.
"If no changes are made to current fishing practices, the western Atlantic bluefin stocks are at risk of collapse as they are showing little sign that the population is rebuilding following a significant reduction in the 1970s."
Most of the economically valuable species such as tuna are at the top of the marine food chain, and their decline could have negative impacts on other species.
They are also long-lived, with slower reproductive rates which means populations take longer to recover.
Last year, proposals were made to have Atlantic bluefin tuna listed as endangered under the Convention on International Trade in Endangered Species (Cites), temporarily stopping the trade in the species.
But attempts to list bluefin tuna, a sushi delicacy in Japan, as an "appendix I" species were defeated by a large majority of countries at the Cites meeting in March 2010.
The study published today said the only way to save southern and Atlantic bluefin tuna was to shut down the fisheries until stocks were rebuilt, although to do so would encourage illegal fishing.
Strong deterrents would be needed, such as controlling international trade in tuna through the Cites scheme, the scientists suggested.
Jean-Christophe Vie, deputy director of the IUCN's global species programme, said: "Temporarily shutting down tuna fisheries would only be a part of a much-needed recovery programme.
"Scientific finding should not be discarded in order to maintain short-term profit. Marine life and jobs for future generations are both at stake."