Assessing the Relationship Between Pfiesteria and Estuarine Fishkills

A recently identified dinoflagellate, Pfiesteria piscicida, has been implicated as a cause of fishkills in mid-Atlantic estuaries. To date, field evidence supporting this argument has consisted of samples, analyzed for the presence of the toxic Pfiesteria forms, gathered during a fishkill. I present a probabilistic approach to examine the use of this kind of a posteriori information as an indication of cause and effect relationships. The analysis shows that the conditional probability of the presence of Pfiesteria after a fishkill has begun provides little support for Pfiesteria as a cause of fishkills, without also knowing the probability of Pfiesteria's presence under 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 Pfiesteria causes estuarine fishkills using only ‘after the fact‘ information is essentially impossible.