Fish kills related to Prymnesium parvum N Carter (Haptophyta) in the People's Republic of China

Prymnesium parvum has been known to cause mass mortality of fish in PR of China since 1963. It usually occurs in brackish waters and inland high-mineral waters. The fish-breeding industry (mainly species of carp) in these regions of the PRC has been threatened by this microalga. Electron microscopic examination of isolates from Dalian and Tianjin revealed that the isolates were P. parvum, based on specific scale patterns and two kinds of scales. The symptoms of the poisoned fish and the control of this toxic alga are also discussed. The addition of ammonium sulfate, copper sulfate, mud, reduced salinity and organic fertilizer to fish ponds has been partially successful in controlling blooms of this toxic alga. Adding 50-70 kg ha(-1) day(-1) manure (dry weight) to the fish pond to inhibit P. parvum from becoming the dominant species in the fish pond is recommended. A reduction in salinity to less than 2% is the easiest way to save freshwater fish from being poisoned by P. parvum. Use of ammonium sulfate is an efficient, economical and safer method to control P. parvum than copper sulfate or mud.