At least 67 unregistered Chinese vessels fishing near Japan’s northern exclusive economic zone violated a multinational agreement in 2016 and raised further concerns about depleted mackerel stocks, a government survey showed.

The survey, released by the Ministry of Agriculture, Forestry and Fisheries on Jan. 24, found a total of 288 Chinese fishing vessels operated in the Pacific Ocean near the EEZ off Hokkaido and the Sanriku region last year, up by 50 percent from 2015.

The 2016 survey was the first to count unregistered boats, and it found some of the vessels used illegal fishing methods.

Japan has been asking China to take action against these unregistered fishing boat operators.

Japan, China, Canada, Russia, South Korea and Taiwan agreed on the North Pacific Fisheries Commission (NPFC) in 2016 to prevent overfishing of mackerel, saury and other fish in the northern Pacific Ocean.

The participating governments are required to register with the commission all fishing vessels authorized to operate in the open sea.

The ministry concluded that the 67 unregistered boats were Chinese based on their flags and radio responses.

Most of the unregistered boats operated under fake names. In one case, two different boats used the same name. In other cases, the same boat showed different names at different times. One vessel simply hid its name painted on the side.

Many of the registered and unregistered boats used a Chinese fishing method in which one net can catch an entire school of fish. One boat was seen using drift netting, a method banned by the United Nations.

Fishing boats using these methods were rarely seen near Japan’s EEZ until 2013. Their number has sharply increased since 2014, including newly built boats and a large vessel of the 10,000-ton class.

The area surveyed is prime fishing grounds for mackerel. Overfishing there could affect the size of catches for Japanese fishermen operating within Japan’s EEZ.

Mackerel resources in the Pacific Ocean were estimated at more than 3 million tons in the 1970s, but the figure once fell as low as 150,000 tons.

After fishing regulations were implemented, the number recovered to 1.47 million tons by 2014.