During 2003, routine monitoring for harmful algae, which includes the use of genetic probes developed in Dr. David Oldach's University of Maryland Biotechnology Institute laboratory specific to Pfiesteria, encountered P. piscicida on the South and Magothy Rivers of Chesapeake Bay once each in May, Magothy River in June and Trappe Creek on the Coastal Bays once each in May and June. There were 468 samples analyzed during the year from the Chesapeake and Coastal Bays with a Pfiesteria encounter rate of 1.1%, the lowest level since monitoring efforts were expanded to assess the distribution of Pfiesteria in Maryland starting in 1998. There was no suggestion of toxin presence at these locations. The extremely wet weather conditions lowering salinity in many of the tributary habitats for spring and summer coupled with Tropical Storm Isabel's impacts on turbulence and salinity distribution in autumn may have all played a role in the low frequency of encountering Pfiesteria for the year.

Between January 1 and February 23, 2004, testing was conducted on 76 water samples collected from Chesapeake and Coastal Bays tributaries. There were no detections of Pfiesteria piscicida and one positive for Pfiesteria shumwayae in a sample collected February 16th from northern Tangier Sound. Since 1998, low levels of detection for Pfiesteria in Maryland during the coldest months of the year have been typical of harmful algae monitoring results.