Update on the July 1999 Fish Kill on
the Lower Pocomoke River

August 1, 1999 update.

On July 22, 1999, Maryland Departments of Natural Resources (DNR) and the Environment (MDE) responded to reports of dead fish on the lower Pocomoke River. Dead fish were observed in the main portion of the river between Shelltown and Fair Island, but the core of kill was in Bullbegger Creek, located in the Virginia portion of the lower Pocomoke. The kill was estimated by MDE to affect approximately 500,000 young-of-the-year menhaden. No lesions were observed on the fish. MDE estimated that the fish had been dead for at least 18 hours. Other fish in the area were healthy, suggesting conditions that caused the kill had ceased.

Evidence indicates that the cause of the kill was low dissolved oxygen.  Dissolved oxygen levels naturally decrease at night as algae switch from producing oxygen (photosynthesis) to consuming oxygen (respiration).  Measurements of dissolved oxygen levels in the area since the kill have shown very low levels.  It is hypothesized that a large school of menhaden moved into Bullbegger Creek to feed or avoid predators.   As oxygen levels naturally decreased after dark, the large amount of fish in the relatively confined waters of Bullbegger Creek further reduced the oxygen concentrations to lethal levels; resulting in the fish kill.  Smaller fish kills due to low dissolved oxygen are not uncommon during the summer, and are exacerbated by nutrient enrichment and drought conditions.

As a precaution, water samples were collected and sent to seven laboratories for Pfiesteria analysis.  As of August 1, preliminary results have been received from five of the laboratories.  All results to date indicate that Pfiesteria was not involved in the Bullbegger Creek kill

Maryland DNR and MDE are working with authorities in Virginia to monitor the situation and collect additional information as needed.

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