OSAKA, Japan — Shoji Kousaka always thought of Japan as a place where people knew how to dispose of their trash.

That was before he spent a morning on a fishing trawler in Osaka Bay last fall, shocked by the reams of soda bottles, plastic shopping bags, snack wrappers and drinking straws repeatedly trapped in the nets, along with the flounder and shrimp.

“Things that weren’t supposed to be there were there,” said Mr. Kousaka, a deputy chief of the Union of Kansai Government, a regional federation representing the second-largest metropolitan area in Japan after Tokyo. Based on what he saw in six hours on the boat, Mr. Kousaka estimates more than 6.1 million plastic scraps and about 3 million plastic bags sit on the floor of the bay.

Given Japan’s high collection rate for plastic waste and the country’s rigorous approach to recycling, said Mr. Kousaka, “I was surprised at how much trash was at the bottom of the ocean.”