Assessing the relationship between Pfiesteria and estuarine fishkills

A recently identified dinoflagellate, Pfiesteria piscicida, has been implic ated as a cause of fishkills in mid-Atlantic estuaries. To date, field evid ence sup porting this argument has consisted of samples, analyzed for the p resence of the toxic Pfiesteria forms, gathered during a fishkill. I presen t a probabilistic approach to examine the use of this kind of a posteriori information as an indication of cause and effect relationships. The analysi s shows that the conditional probability of the presence of Pfiesteria afte r a fishkill has begun provides little support for Pfiesteria as a cause of fishkills, without also knowing the probability of Pfiesieria's presence u nder all conditions. Documenting the relative presence of toxic life stages during fishkills and under non-fishkill conditions will provide supporting evidence to assess Pfiesteria's role in fishkills. However, proving that P fiesteria causes estuarine fishkills using only 'after the fact' informatio n is essentially impossible.