Another massive red tide has appeared off the mainland coast in the Bohai Sea, with scientists alarmed at the growing frequency of the toxic slicks. Two red tides of algae have engulfed more than 5,000 sq km of the northeast sea, forming the largest toxic slick in its history and threatening to contaminate important fishing waters. 'We call these red tides a biological cancer because they threaten environmental quality, aquatic life forms and even human life,' said Zhao Zhangyuan, of the Chinese Research Academy of Environmental Sciences. The toxic slicks consist of a densely populated algae bloom that breeds in abundance, sapping the water of oxygen while producing toxins that can paralyse fish and contaminate seafood. 'The red tide in the Bohai Sea shows that the situation is deteriorating. It's getting worse and worse,' said Mr Zhao. One tide started on Friday near the mouth of the Yellow River, the mainland's second longest, affecting an area of 1,850 sq km. The other tide started on Saturday in the middle, east and north of the Bohai Sea, affecting 3,200 sq km, the State Oceanic Administration said on Tuesday. The administration urged authorities in Liaoning , Hebei , Tianjin and Shandong , which all lie along the Bohai coast, to monitor the algae and ensure all seafood was free of toxins before being sent to market. 'Red tides are just one of the environmental problems the mainland is facing. It will lead to an unimaginable disaster if no clever action is taken quickly ... and the government has been slow to act,' said Mr Zhao, adding that it was hard to say when the mainland would be free of the toxic algae. The algae mainly consumes urban pollution, industrial discharge, farm waste and fertiliser run-off that flows into coastal waters from rivers and streams.