Shenandoah and Upper James River Fish Kills and Aeromonas Salmonicida, April 28, 2009

Ongoing research into fish kills in western areas of Virginia indicates that fish bacteria are a potential cause of lesions and death among smallmouth bass, redbreast sunfish, and rock bass that occurred in 2007 and 2008 within the upper James, Jackson, Cowpasture, and the Shenandoah rivers. 

The specific bacterium is a fish pathogen called Aeromonas salmonicida.  It most often is reported to cause a disease known as furunculosis among trout and salmon, but also can cause a diverse number of health problems in a wide range of warm-water fish and some marine fish. 

Research into the bacterium and its impact on fish is being conducted in controlled experiments at the National Fish Health Lab in Leetown, W. Va., to determine a definitive cause.  Initial results show:

• The bacterium can induce lesions and kill smallmouth bass even when the fish are living in clean water.
• The bacterium transmits easily between fish and from infected water to fish.

Extensive field work and research continue in the main rivers as well as within cold-water streams that flow into both the upper James and Shenandoah rivers. Bacteriologists with the U.S. Geological Survey are investigating how the bacterium began to affect fish in Virginia rivers. Typically, it does not survive warm summer water temperatures.

As a result, research is concentrating on whether the organism is reintroduced into the rivers annually or whether “cold water” spots exist near or in the affected rivers that enable the bacterium to survive from one year to the next. Existence of such locations could create local areas of infection among the fish that gather there. Research this year will focus on:

• Understanding how the bacterium is transmitted throughout the rivers.
• Explaining why certain species of fish are more susceptible than others.
• Testing environmental and biological variables that may increase the ability of the bacterium to thrive.
• Using fish samples that already have been collected this spring to establish the health of fish before any fish kills.

Fish samples will be collected if and when fish kills occur in these rivers, and also during a post-fish kill period in summer 2009.  

In addition to the research on Aeromonas salmonicida, studies by state and federal scientists and university researchers have centered on water chemistry, fish diseases, and general health of fish and other aquatic life. Water quality studies have not identified any specific chemicals at levels that would be expected to cause the fish kills.

Many of the contaminants found in the rivers have not been studied, and there is no information on the levels at which they could affect aquatic organisms. A second group at USGS is studying whether combinations of contaminants could increase their strength, or whether they cause problems with immune system function that could help explain the fishes’ susceptibility to diseases. Scientists also are trying to determine whether there is a correlation among contaminants that could cause the intersex condition that is prevalent among smallmouth bass and sunfish.