The report compiles the scientific records of the 23 stocks of the major commercial tuna species worldwide - six albacore, four bigeye, four bluefin, five skipjack and four yellowfin stocks - carried out by each of the Regional Fisheries Management Organisations (RFMOs).

It uses a colour rating system to indicate varying degrees of sustainability and ranks the status and management of the 23 stocks using a consistent methodology in terms of three factors: abundance, exploitation/management (fishing mortality) and environmental impact (bycatch).

Since the last version released in September 2014, notable updates were made. Eastern Atlantic bluefin ratings improved for abundance, Western Atlantic bluefin ratings improved for exploitation rate, and Indian Ocean albacore ratings improved for exploitation rate.

But, despite improvements in multiple fisheries, ISSF says there's continued evidence that overfishing is still occurring for several stocks, notably for Pacific bluefin tuna and for the Western and Central Pacific bigeye stock.

Globally, 52% of the stocks are at a healthy level of abundance, 35% are overfished and 13% are at an intermediate level. In terms of exploitation, 52% of the stocks are experiencing a low fishing mortality rate, 9% are experiencing overfishing, and 39% have a high fishing mortality that is being managed adequately.

When viewed from the point of view of total catch, 86% of the catch comes from healthy stocks. This is due to the fact that skipjack stocks contribute more than one half of the global catch of tunas, and they are all in a healthy situation. In contrast, most bluefin stocks and two out of six albacore stocks are overfished, but combined they make a relatively small fraction of the total catch.

ISSF will now use the ratings in the report, which it says serves as a one-stop resource for comprehensive tuna stock information, to prioritise its advocacy efforts with the tuna RFMOs that are in charge of managing these stocks.