The phenomenon of 'red tides', which can kill marine fishes due to toxin release, is caused by the rapid multiplication
According to NCERT Class 11 Biological Classification, red dinoflagellates, such as Gonyaulax, often undergo such rapid and explosive multiplication that they make the sea appear vibrantly red (red tides). These extensive blooms are not just visual; they release potent chemical toxins in large quantities that can severely poison and kill numerous marine animals, including massive populations of commercial fishes.
Ref: NCERT Class 11 Biology > Chapter 2: Biological Classification > 2.2 Kingdom Protista