Geographical expansion of harmful algal blooms and its possible cause

Harmful algal blooms began to occur widely after the middle of 1980s. The first paralytic poisoning by shellfish contaminated with toxins produced by the dinoflagellate Pyrodinium bahamense occurred in Papua New Guinea in 1973. It spread to Indonesia, Malaysia, Brunei Darussalam and the Philippines in the last two decades. Fish mass mortality also occurred in Japan, Korea, China and Hong Kong along with the development of aquaculture. Several reasons have been suggested for the geographical expansion. These are 1) utilization of coastal area, leading the eutrophication with the change of N:P ratio; 2) increasing aquaculture operations, revealing the presence of previously unknown harmful species; 3) species transport and dispersal via ballast water, shellfish transportation, or natural mechanisms such as currents; 4) advances in methodology, leading detection of new harmful events; and 5) global long-term environmental changes such as ENSO