Global fish catches are falling three times faster than official UN figures suggest, according to a landmark new study, with overfishing to blame.

Seafood is the critical source of protein for more than 2.5 billion people, but over-exploitation is cutting the catch by more than 1m tonnes a year.

The official catch data, provided by nations to the UN’s Food and Agriculture Organisation (FAO), rarely includes small-scale, sport or illegal fishing and does not count fish discarded at sea. To provide a better estimate, more than 400 researchers around the world spent a decade finding other data to fill in the gaps.

The results, published in the journal Nature Communications, show the annual catches between 1950 and 2010 were much bigger than thought, but that the decline after the peak year of 1996 was much faster than official figures.

The FAO data indicated a catch of 86m tonnes in 1996, then a decline of 0.4m tonnes per year. In contrast, the new research estimates the peak catch was 130m tonnes, but declined at 1.2m tonnes per year afterwards.

“Our results differ very strongly from those of the FAO,” said Prof Daniel Pauly, at the University of British Columbia in Canada and who led the work. “Our results indicate that the decline is very strong and is not due to countries fishing less. It is due to countries having fished too much and having exhausted one fishery after another.”

Estimating subsistence, small-scale and illegal fishing is difficult, but Pauly said his team was confident in its results which - unlike the FAO data - includes estimates of the uncertainty. “This research is not based on a few studies here and there and then extrapolation,” he said. “It is based on the results of 200 studies we have conducted for about a decade by a network of 400 people in all countries of the world.”

The researchers used many different approaches to fill in the missing data, from hotel invoices for locally bought fish in the Bahamas to information on local fish consumption.

“This work has been carefully conducted by painstaking research into the hidden underbelly of global fishing, country by country, region by region” said Prof Boris Worm, at Dalhousie University in Canada and not involved in the new research. “This was a Herculean task that no one else has ever attempted. While the results necessarily remain uncertain, they undoubtedly represent our most complete picture yet of the global state of fish catches.”

Worm said the world’s fisheries were being over-exploited but that some stocks were being sustainably managed: “Where such measures have been taken, we find that both fish and fishermen are more likely to persist into the future.”

Global fish catches rose from the 1950s to 1996 as fishing fleets expanded and discovered new fish stocks to exploit. But after 1996, few undiscovered fisheries were left and catches started to decline. “It was never really sustainable,” said Pauly. The decline since 1996 has largely been in fish caught by industrial fleets and to a lesser extent a cut in the number of unwanted fish discarded at sea.

“The fact that we catch far more than we thought is, if you like, a more positive thing,” he said. “Because if we rebuild stocks, we can rebuild to more than we thought before.”

There has been success in some places where fishing has been restricted for a few years, for example in the Norwegian herring and cod fisheries. On resumption, catches were bigger than ever.

But Pauly said: “I expect a continued decline because I don’t expect countries to realise the need to rebuild stocks. I don’t see African countries, for example, rebuilding their stocks, or being allowed to by the foreign fleets that are working there, because the pressure to continue to fish is very strong. We know how to fix this problem but whether we do it or not depends on conditions that are difficult.”

A 2015 study showed nearly 500 Chinese fishing vessels operating off west Africa, with scores of cases of illegal fishing, according to Greenpeace. Large European vessels also operate in the region. In April 2015, the EU threatened Thailand with a trade ban over illegal fishing, while in September, Greenpeace said it had identified significant pirate fishing for tuna in the Pacific.

Prof Callum Roberts, at the University of York in the UK and not part of Pauly’s team, said: “This is a superb piece of research. Greater knowledge of what we have done and are still doing will help us to change for the better.”

“We can see more clearly now, for example, the immense value of fish to poor people in developing countries,” he said. “We can see how industrial fisheries from developing countries are robbing these people of livelihoods and food. We can also see, that in efforts to stem declines, we have been using more and more bycatch that was once thrown away.”