The areas effected by phytoplankton blooms were
nauseabond and discolouration of the water was usually
observed. Red tides could cause mass mortalities in
nearby shrimp and shellfish farms. For example, major
shrimp farming areas in Samut Songkarm and Samut
Sakorn provinces were severely affected in 1977 resulting
in a sharp decline in output per hectare (Rientrairut,
1983). Green mussel larvae were also severely affected by
red tides as they were unable to settle on the wooden
poles during the outbreaks. This caused heavy losses to
the shellfish industry during the outbreaks.