Prymnesium parvum, an Ichthyotoxic Alga in an Ornamental Fish Farm in Southern Israel

The haptophyte alga Prymnesium parvum (Prymnesiophyceae) caused “creeping” mortality in ornamental fish, molly (Poecilia sp.) and koi (Japanese carp, Cyprinus carpio), farmed in the Arava Valley in southern Israel. The toxicosis occurred when the water system was changed from flow-through to closed circulation. A moderately high temperature, three-fold increase of salinity, and probable rise in eutrophication created conditions suitable to P. parvum blooms. The system was treated with 10 ppm ammonium sulfate and fish mortality ceased.