A joint research team has discovered that tuna and bonito in the seas around Japan have high concentrations of organic tin from paints used on ship hulls and material used to protect fish nets.

The team, made up of scientists from Ehime University’s Agriculture School and Kyoto University’s Fishery Experimental Station, is scheduled to announce its findings during a meeting of the Japan Society of Fisheries that opened Apr. 4 at Tokyo University of Fisheries.

The chemical concentration levels are many times that found in fish in the south Pacific and Indian Ocean, but not high enough to harm humans, according to the team. The team believes that the seas around Japan are now sources of organic tin contamination of fishery resources, and migratory fish like bonitos and tunas are believed to have absorbed the chemicals while migrating through the area.

Shinsuke Tanabe, a professor at Ehime University’s Agriculture School, and his team members collected 47 tuna and bonito from the central Sea of Japan, the waters off Kochi Prefecture, Papua New Guinea, the Indian Ocean and five other areas, from 1983 through last year. By analyzing organic tin chemicals in the livers of the fish, tuna caught in the central Sea of Japan were found to have the highest concentration of tributyl (320 nanograms — 1 nanogram is equal to 1 billionth of a gram) and two other tins.

Tuna caught off Kochi Prefecture followed with 310 nanograms, followed by 300 nanograms in bonito caught in the central Sea of Japan, according to the team. The concentration levels are comparable to those found in fish living in contaminated waters like Tokyo Bay or off the Italian coast, they said.

In the fish caught in the south Pacific or around the Philippines, the team found only 24 to 50 nanograms of tin.

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