Can algicides be used to kill the toxic dinoflagellate, Alexandrium minutum, in aquaculture ponds?

Abstract

Alexandrium minutum is a toxic dinoflagellate that causes paralytic shellfish poisoning (PSP). It is distributed in the coastal waters of southwestern Taiwan and often also occurs in fish ponds in Pingtung county. It caused the PSP incidents which resulted in human death in 1986 and illness in 1991. Moreover, mass mortalities of various species such as, culture grass prawn (Penaeus monodon) in 1989, milkfish (Chanos chanos) in 1992 and 1995, gray snapper (Lutjanus argentimaculatus) in 1992, and long dorsal fin pompano (Trachinotus blochii) in 1993 were also observed when this particular algae bloomed. Hence, controlling its occurrence as well as reducing its density in aquaculture ponds has become extremely important for proper management of water quality. Chemicals which are commonly used as algicides in aquaculture ponds were first selected to test their effectiveness toward destruction of A. minutum. The effectiveness of chemicals such as BKC, (NH4)2SO4, CuSO4 5H2O, KMnO4, formalin, methylene blue and malachite green which are commonly used as algicides to destroy A. minutum depends on both the dosage as well as algal density. Amongst the tested algicides, NH4-N (5-14 ppm) and BKC (0.5-2 ppm) were found to be the most effective, followed by 2 ppm methylene blue and 0.2 ppm malachite green. CuSO4 5H2O (0.5-10 ppm), KMnO4 (1-10 ppm), and formalin (20-100 ppm) were, however, less effective. It may hence be inferred that, NH4-N and BKC can be effectively used to destroy A. minutum. However, 14 ppm NH4-N is harmful to certain species of fish such as red drum Sciaenops. ocellatus. Besides, addition of NH4-N or BKC can enhance the toxic effect of pond water due to the toxins released from the destroyed algal cells. Thus, the best method to control the bloom of A. minutum in culture ponds is to exchange a large volume of pond water either with fresh seawater or some other pond water which does not contain any toxic algae.