Anti-phytoplankton therapy of finfish:the mucolytic agent L-cysteine ethyl ester protects coho salmon Oncorhynchus kisutch against the harmful phytoplankter Chaetoceros concavicornis

The harmful diatoms Chaetoceros concavicornis and C. convolutus will kill salmonids such as rainbow trout Oncorhynchus rnykiss and coho salmon 0. kisutch if present at as few as 5 cells ml-' seawater. Our previous research has shown that the barbed spines of these diatoms become wedged between the secondary lamellae of salmonids, where they cause excessive production of mucus by goblet cells. This mucus and the diatoms accumulate on and between the secondary lamellae to such a n extent that the fish suffocate. A class of pharmacolog~cally active compounds (mucolytic agents, e.g. L-cysteine ethyl ester) exists which decrease mucus production in mammals and humans when ingested. When coho salmon Ingest this agent at up to 12 mg kg-' biomass d-l, mucus synthes~s is reduced to such an extent that an accumulation of this mater~al does not occur on and between the secondary lamellae; the fish remain viable in what would otherwise be a lethal exposure to C concavicornis cells.