The Xinhua newsagency says the government has issued warnings and ordered local marine authorities to take measures to lessen the algae's impact on the fishing industry. The algal bloom measures some 2,800 square kilometres and was found by air and marine patrols off the densely populated city of Shanghai. China's coastal regions have been ravaged by so-called 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. Experts predict that some 30 major outbreaks are expected to occur this year in China's coastal waters, posing grave dangers to fishing and aquaculture industries. Algal blooms produce poisons that paralyse and kill fish.
Fish can also suffocate from a lack of oxygen.
The tide is mainly caused by urban pollution and large-scale farming waste that produce excessive levels of nitrogen and phosphorus.