Japanese tuna longliner. (Photo: Stock File)

Tuna stock levels higher than most people believe

The findings of a global renowned fisheries scientist who presented a paper in the journal Nature about seven years ago have been strongly contested. The study postulated that the Pacific tuna stocks were all endangered while citing that the nominal catch per unit of effort (CPUE) of the Japanese longline fishery declined steeply in the 1950s.

However, the CPUEs used were simple values of reported catches divided by reported efforts and failed to include scientific adjustments for changes in the real fishery, as it was explained by Dr. Peter Miyake in the column he writes on the Website that belongs to the Organisation for the Promotion of Responsible Tuna Fisheries (OPRT) . 

Many fishery scientists have therefore expressed concerns about the paper’s errors and erroneous assumptions. When these scientists sent the journal a joint protest, that opinion was only posted in the column of the “comments from public.”

Miyake states that while it is true that the CPUE of Japanese longline fishery plummeted in the 1950s, this phenomenon may be often observed at the start of the exploitation of populations which had never been fished. The Japanese fleet only started fishing tuna more expansively in 1952, which means the initial catch rates were excessively high and did not mirror the real abundance of the stock.

Also, while the fleet’s original goal was to fish as much tuna as possible regardless of the species, it later changed to obtaining maximum values by targeting high-priced fish. This would have led to an apparent decline in the overall hook-rate.

Current tuna catches in the Pacific Ocean are more than 20 times those of the 1950s. If the paper in question were correct, there would be no way fleets would be catching that much fish today and tuna stocks would have been completely depleted.

In contrast, most of the Pacific tuna stocks are still at proper sustainable levels, which proves the paper wrong.

Still, it is still very frequently cited as scientific basis claiming that tuna stocks are endangered.

Dr Peter Miyake is a leading tuna researcher who has worked for many international tuna fisheries management organisations, including the Inter American Tropical Tuna Commission (IATTC) and the International Commission for the Conservation of Atlantic Tunas (ICCAT). He now participates in the scientific meetings as a visiting researcher at National Research Institute of Farseas Fisheries of Japan.

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