A red tide measuring more than 100 square kilometers drifting toward east China's coastline is threatening local fish and aquaculture industries, as it can suffocate marine life, local authorities said.

The red tide, first detected on May 3 covering 780 square km, shrank to 106.3 square km on Monday, according to the North China Sea Branch of the State Oceanic Administration (SOA).

But the shrinkage doesn't mean the red tide is fading away as it can submerge when water temperature drops, said Li Qinliang, professor with the North China Sea Environment Monitoring Center of the SOA.

The red tide, identified as Noctiluca scientillan algae, grows by photosynthesis and reserves energy under the water when sunshine is poor, Li said.

Experts said the red tide might be caused by seawater pollution and over fishing.

"Red tide can break out when creatures living on Noctiluca scientillans are overfished," said Qu Xuanzhang, researcher of the North China Sea Environment Monitoring Center of the SOA.

The red tide will continue moving northwestward to the coastline of Shandong province, according to the SOA.

Although nontoxic, Noctiluca scientillan algae may stick to the gills of fish and cause them to die.

If red tide breaks out in coastal waters, it will hurt fish and aquaculture industries as the algae takes a lot of oxygen at night and causes fish to suffocate, Li said.

The SOA has already alerted local fishermen to prepare to fight the red tide.

And if that doesn't work, fish farmers may need to move their fish to unaffected waters, Li said.