The largest reported fish kill of the year to date was investigated at Colgate Creek, Chesapeake Bay, on July 23 with an estimated 60,000 menhaden, thousands of bay anchovies, and individuals of a few other species found dead. Maryland Department of the Environment (MDE) in cooperation with Maryland Department of Natural Resources found deep brown waters at the site. Preliminary results from analysis of water samples showed that a bloom of the dinoflagellate Gyrodinium uncatenum was in abundance. Gyrodinium uncatenum is well known for Mahogany Tides in the Bay region but is not a toxic species,. The high pH of the water (approximately 9; neutral pH is 7 on a logarithmic scale of 0-14) and elevated dissolved oxygen concentrations (8 to 11 mg O2/L and 155% of saturation) were indicative of significant algal bloom activity. Water temperatures were 88-91 degrees Fahrenheit which can also be stressful to fish. A similar event occurred in August 2001 where elevated levels of ammonia were recorded and low levels of non-toxic Pfiesteria were found.

Water samples collected at the kill site tested negative for Pfiesteria at Dr. David Oldachs lab (University of Maryland Biotechnology Institute, Baltimore, MD). The investigation will continue with additional water testing by MDE for nutrient levels with particular interest in possible toxic levels of ammonia, a byproduct of the decomposition of such algal blooms. Fish were collected from the event and necropsies are being conducted by the Paul Sarbanes Oxford Lab.