The Agriculture, Fisheries and Conservation Department (AFCD) has implemented a long-term red tide/phytoplankton progamme in Hong Kong since 1975.  From 1975 to 2023, 995 red tide incidents were recorded in Hong Kong waters.  Among these red tide incidents, 17.5% (viz. 24 species) is considered as harmful algal blooms (HAB) that may cause fish / shellfish mortality, seafood contamination and even human intoxication. Prorocentrum cordatum, Heterosigma akashiwo and Phaeocystis globosa are the most dominant HAB species recorded in Hong Kong as these 3 species accounted for approximately 50% of the HAB incidences in Hong Kong.  While it is known that P. cordatum is venerupin (hepatotoxin) producing and the latter two species are known fish-killing species, these species did not result in major negative impacts to the aquaculture industry, human health or natural environment in Hong Kong. On the other hand, substantial loss of cultured fish was caused by other less frequently occurring species such as Karlodinium digitatum and Karenia mikimotoi in 1998 and 2015 respectively. 

Geographically, the highest HAB occurrence (49.2%) was recorded in Tolo Harbour whereas the lowest (3.2%) was recorded in the eastern part of Hong Kong waters. The particularly high occurrence of HAB in Tolo Harbour might be attributed to its topographic feature which is a sheltered area with limited tidal flush compared with other water regions. Yet, HAB occurrence along the Tolo Channel has reduced significantly in recent years as the marine water quality has been much improved as a result of the implementation of various sewage treatment schemes in the region.  AFCD is committed to continue its effort in red tide and HAB monitoring in Hong Kong so as to provide timely alerts to various stakeholders as well as to support the mariculture industry.

harmful algal blooms (HABs), Hong Kong,red tide, long-term phytoplankton monitoring