Docked at a Canadian port, crew members returned from a test run of the Ocean Cleanup’s system to rid the Pacific of plastic trash were thrilled by the meager results — even as marine scientists and other ocean experts doubted the effort could succeed. The non-profit, launched in 2013 amid buoyant media coverage, hopes to clear 90% of floating plastic from the world’s oceans by 2040. But the group’s own best-case scenario — still likely years away — envisions removing 20,000 tonnes a year from the North Pacific, a small fraction of the roughly 11 million tonnes of plastic flowing...