Fish kills linked to a toxic ambush-predator dinoflagellate:distribution and environmental conditions

The toxic ambush-predator dinoflagellate Pfiesteria piscicida gen. et sp. nov. has been implicated as a causative agent of major fish kills in estuarine ecosystems of the southeastern United States. P, piscicida is stimulated by fresh fish secreta, and it was lethal to all 19 species of native and exotic flnfish and shellfish bioassayed in culture; thus far in field and aquaculture kills linked to the d~noflagellate. 13 additional fish species have been affected. Field data in combination with confirming laboratory bioassays documented toxicity at temperatures ranging from 12 to 33OC, with most outbreaks occurring at 26C or higher. P. piscicida also exhibits wide salinity tolerance; it was lethal to fish from 0 to 35 ppt in calcareous waters, with an optimum salinity for growth and toxic activity at 15 ppt. It was toxic to fish day or night (2250 toxic zoospores ml-l) without an apparent light optimum, in expenmental laboratory conditions ranging from 0.2 pEin m-2 S-' (darkness for all but 30 to 60 s at 20 p E ~n m-2 S-' per 24 h period) to 200 pEin m-2 S" (12:12 h 1ight:dark cycle). Moreover, f ~e l d fish kills have occurred in darkness and at light intensities up to 2400 pEin m -2 S -' . Through direct microscope counts of water samples, confirmed identifications with scanning electron microscopy, and confirmed toxic activity in bioassays, P. piscicida was implicated as the causative agent of 52 * 7 % of the major fish kills (affecting 103 to 10' fish from May 1991 to November 1993) on an annual basis in North Carollna estuaries and coastal waters. Since their discovery in natural hab~tat during 1991, Pfiesteria-like species also have been tracked to eutrophic sudden-death fish kill sites In estuaries, coastal waters, and aquaculture facihties from the mid-Atlantic through the Gulf Coast. Toxic ambush-predator dinoflagellates likely are widespread in warm temperate/subtropical regions, acting as significant but often undetected sources of fish mortality and disease.