MITO, Japan >> Japan has proposed raising its catch quotas for Pacific bluefin tuna, a fish so highly prized for sushi and sashimi that its population is at less than 5% of historical levels.

An online meeting of countries that manage the Pacific bluefin that began Tuesday is studying the proposal to raise Japan’s catch limits for both smaller and larger bluefin tuna by 20%.

A slight improvement in the spawning population for the fish has raised confidence that it can recover from decades of overfishing. But conservation experts say increasing catch limits too soon could undo progress toward restoring the species.

Increasing harvests of such fish could also drive prices lower, making the industry less profitable in the long run, the Pew Charitable Trusts said in a report issued Tuesday.

The report, Netting Billions 2020: A Global Tuna Valuation, put the market value of seven tuna species including bluefin at $40.8 billion in 2018. Despite increased catches, that was a decrease from $41.6 billion in 2012.

“Just because increasing catch is sustainable does not mean it is always the right thing to do,” said Grantly Galland, an officer in Pew’s international fisheries team.

Prices for most species of tuna have fallen due to oversupply of caught fish, he said.

The meeting of the Western and Central Pacific Fisheries Commission and the Inter-American Tropical Tuna Commission includes more than two dozen countries that collaborate to manage fisheries on the high seas and curb illegal and unauthorized fishing and other activities that endanger highly migratory species such as the Pacific bluefin.

The countries participating in management of the Pacific bluefin committed in 2017 to reducing their catches to help return the species to 20% of its historic size by 2034.

Japan plays a critical role in the survival of the species not just because of its huge appetite for the fish. The Pacific bluefin spawns almost entirely in seas near Japan and Korea. Japanese fishermen also capture small tuna to be farmed to maturity, although the number of traditional artisanal fishermen has fallen in recent years as younger Japanese choose not to engage in such dangerous and difficult work.

The latest data show the spawning stock biomass of the Pacific bluefin, an indicator of the fish’s ability to reproduce at a sustainable level, rising to about 28,000 metric tons in 2018 from 10,837 metric tons in 2010.

That is still less than half the estimate for 1995 of a spawning stock biomass of 62,784 metric tons. It puts the species at about 4.5% of the baseline level it would be at if there were no fishing at all, up from 4% several years ago.

Demand for bluefin tuna is such that any progress draws pressure for bigger catches. Last week, the Marine Stewardship Council granted certification for a Japanese fishery’s Atlantic bluefin, over conservationists’ objections that such a move might hinder its long-term recovery.

In 2019, Japan reported a catch of 3,757 tons of smaller tuna and 5,132 of larger tuna, according to documents prepared for the online meeting. To comply with its limits, it transferred 250 tons of its catch quota for smaller Pacific bluefin to its limit for the larger ones, according to documents prepared for this week’s online conference.

The proposal to raise its catch limits would enable Japanese fisheries to catch 801 more tons of smaller fish, which weigh less than 30 kilograms, per year and 976 more tons of larger ones.

A similar proposal was rejected last year.

A key issue is the unpredictability of each year’s rate of “recruitment,” or reproduction for bluefin, among other factors. For this reason many experts favor a shift toward a more systematic method of managing fish catches that would use complex computer modeling to target specific objectives.

Pew and other conservation groups are promoting this approach, which has been adopted for managing southern bluefin tuna, another threatened species.