The tuna long line fishery is one of the Japanese traditional fishing methods, which started in the 18th century. This fishing method was expanded to foreign countries, particularly other Asian countries, to export tunas to the Japanese sashimi tuna market and now it has become an international fishery.
The tuna long line fishery operates once per day. The number of operations per year ranges from 250 to 270.
About 250 to 300 metric tons of tuna are caught by one vessel annually.
Longline vessels set out a mainline supported by floats that may extend for miles beneath the surface of the ocean. Hanging at regular intervals from the mainline are the gangins with baited hooks which can be adjusted to fish at varying depths. As a passive (stationary) type of gear, a long line’s chance of success is based on the target species’ demand for food. Fish are caught on longlines because they are attracted to their baited hooks.

The hook is coated for anti-corrosion and 6 cm long. The baits are frozen squid, sardine, mackerel, Pacific saury, mackerel scad, etc. This prevents catch of juvenile tunas.

After being retrieved onto the deck, each fish is gilled, finned and gutted. After flushing blood, the fish is cleaned with sea water. These must be done quickly to keep the freshness of the fish. The fish is frozen in the quick freezing room (minus 60 degrees Celsius). On the next day, it is soaked in a water tank so that the body is coated with ice. Then, it is kept in a storage room under minus 60 degrees Celsius until landing. Tunas kept under minus 60 degrees Celsius are as fresh as fresh tunas (not frozen) for two years.

It varies, depending on the size of the vessel and tunas. The average is 200 to 300 metric tons or 3,000 in number.