Nearly 80 percent of Japanese anchovy caught during a survey of Tokyo Bay had plastic waste inside their digestive systems, according to the research team who conducted the study.
Eating the fish is unlikely to cause major health problems, but the finding indicates worrying levels of pollution in the oceans around the nation, according to the group led by professor Hideshige Takada of the Tokyo University of Agriculture and Technology.
The team examined 64 Japanese anchovies caught in the bay in August last year and found 150 micro-plastic particles, or plastic pieces up to 5 millimeters, in the digestive tubes of 49 of them.
Of the 150 plastic pieces, about 80 percent were 0.1 to 1 mm in size, the team said.
The micro-plastic is believed to come from waste such as plastic bags and containers broken down by waves and ultraviolet light, and also from micro-beads widely used in cosmetics and toothpaste.
Measures should be taken to curb the inflow of micro-plastic into the sea as they may affect the ecosystem, Takada said.
Micro-plastic litter absorbs hazardous chemicals such as polychlorinated biphenyl and could become concentrated in birds and fish, he said.
The study marks the first finding of micro-plastics in fish in the nation's waters, the professor said.
Micro-plastics have so far been found in fish in the United States, Britain and Indonesia, as well as in sea turtles, whales, clams and seabirds, with another study suggesting 90 percent of seabirds have ingested such small plastic pieces.
The world leaders' joint declaration adopted at the Group of Seven summit last year in Elmau, Germany, called for "increased effectiveness and intensity of work" to address plastic litter pollution.