MANILA, Philippines—Swarms of Chinese vessels have dumped human waste and wastewater for years in a disputed area of the South China Sea, causing algae blooms that have damaged coral reefs and threatened fish in an unfolding catastrophe, a U.S.-based expert said Monday. Satellite images over the last five years show how human waste, sewage and wastewater have accumulated and caused algae in a cluster of reefs in the Spratlys region where hundreds of Chinese fishing ships have anchored in batches, said Liz Derr, who heads Simularity Inc., a software company creating artificial intelligence technologies for satellite imagery analysis. At least 236 ships were spotted in the atoll, internationally known as Union Banks, on June 17 alone, she said at a Philippine online news forum on China’s actions in the South China Sea, which Beijing has claimed virtually in its entirety. “When the ships don’t move, the poop piles up,” Derr …