One of the tides began on Friday near the mouth of the Yellow River, the second longest river in China, affecting an area of 1,850 square kilometers (740 square miles), the State Oceanic Administration (SOA) said.

Another red tide was first spotted Saturday near Tianjin, a major port city in north China, covering an area of 3,200 square kilometers.

"A large number of remains of mosquitoes and flies could be seen floating on the surface of the affected water, but there is no sign of massive deaths of shellfish," said Chen Lianzeng, deputy director of SOA.

"The red tides are continuing and could expand in the coming days," he was quoted as saying by Xinhua news agency.

SOA issued an urgent circular urging localities around Bohai to closely monitor the tides and ensure that all shellfish and other seafood is toxin-free before being sent to market.

A "red tide" is a densely populated algae bloom that breeds in abundance and suffocates fish by sapping the water of oxygen while also producing toxins that can paralyze fish and contaminate seafood.

The algae mainly feasts on urban pollution, industrial discharges, farming wastes and fertilizer run-off that are rich in nitrogen and phosphorus and which flow into coastal waters from rivers and streams.

China's coastal regions have been ravaged by springtime red tides in recent years, especially in the northern Bohai Sea, the East China Sea and the South China Sea near the mouth of the Pearl River.

TERRA.WIRE