A Decade of Fish Disease and Mortality Investigations

This update posted December 2014

In April and May of 2014 a few anglers and concerned citizens reported small numbers of dead and diseased smallmouth bass in the Shenandoah River. DGIF verified that there were a relatively high percentage of smallmouth bass and redbreast sunfish with lesions and other abnormalities on the South Fork Shenandoah River from Port Republic to Front Royal. DGIF also sampled throughout the North Fork Shenandoah River and observed fish with the same abnormalities, but the percentage was lower.

Things were fairly quiet in 2014 on the Cowpasture, Jackson, and upper James River. Angler reports of dead or diseased fish in these rivers were almost non-existent.

Impacts from this year's mortality/disease events on the Shenandoah were fairly heavy. However, DGIF sampled the fish community in the fall of 2014 and found an abundance of 9 to 11 inch fish and a lot of very young smallmouth bass. Barring any future disease outbreaks the 9 to 11 inch fish should grow into the 11 to 15 inch range over the next two years.

It is common for a few fish in a population to exhibit some type of abnormality such as lesions, dark patches of skin, raised bumps, loss of scales, split/eroded fins or discolored/eroded gills See photos. When twenty percent or more of the fish in a population are exhibiting one or multiple abnormalities of this type it becomes more of a concern. Chronic spring-time fish mortality and disease events have occurred in the Shenandoah River over the last decade, and were present in the upper James River from 2007-2010. These episodes have not been uniform in location or severity. Adult smallmouth bass, redbreast sunfish and rock bass have been the primary fish affected. However, several additional species have also been inflicted. Affected fish typically exhibit open sores or "lesions" on the sides of their bodies (Figure 1) and some dead and dying fish have no visibly external abnormalities. These events have not occurred annually and have been less common since 2010. However, angler reports and annual fall sampling seems to indicate another disease episode occurred during the spring of 2014 effecting a significant number of larger smallmouth bass and redbreast sunfish.

Determining the cause of these mortality/disease events has proven to be extremely difficult. Scientists have and continue to conduct in-depth studies on fish health, pathogens, water quality, contaminant exposure and recently have begun looking at possible toxins released by bacteria. The fact that these events have occurred in multiple watersheds that differ in many ways has added to the complexity of understanding the primary cause.

Fish health investigations to date have included: histopathology (Figure 2), parasitology, bacteriology, virology, and blood analysis (Figure 3). This information has been collected from the affected rivers, over multiple years, and also from "reference" rivers where these mortality/disease events have not been occurring. Fish health samples have been analyzed by several Universities, the United States Fish and Wildlife Service's Northeast Fish Health Lab, and the United States Geological Society's Eastern Fish Health Lab. While researchers have collected a plethora of fish health data, linking the disease and mortality episodes to a single cause has been elusive. Detailed research findings are described in the Virginia Tech University final report "Investigation Into Smallmouth Bass Mortality in Virginia's Rivers" (Orth et al. 2009) (PDF).

Adult smallmouth bass with lesion.

From the research and monitoring conducted to date, there has not been any conclusive evidence that water quality variables or chemical contaminants are directly responsible for these fish mortality/disease events (Figure 4). Contaminant levels were measured in the rivers affected as well as some rivers where these fish mortality/disease events are not occurring. Contaminant levels were measured at both base-flow and during runoff events (Figure 5). However it must be noted that not every possible chemical compound was measured, and that the toxic concentration to fish of many chemical compounds are unknown. It is also not well understood how some chemical compounds could "interact" with one another and become toxic to fish. More research is needed in this area. Detailed findings from water quality and contaminant monitoring projects can be obtained from the Virginia Department of Environmental Quality's Valley Region Office.

Some chemical compounds and heavy metals have been shown to suppress the immune system and influence development of certain aquatic organisms. These contaminants are referred to as "endocrine disruptors". Natural and synthetic forms of the hormone estrogen also fit into this category. Estrogenic activity was measured in water samples taken throughout the Shenandoah River and its tributaries at levels that may cause biological effects in fish. However, at this time there has been no definitive or conclusive evidence that chemicals are negatively affecting the immune system of fish in the Shenandoah River and contributing to the mortality/disease events. Researchers with the United States Geological Survey are still actively engaged in understanding how certain contaminants may influence the immune system of fish. This research includes fish taken from Virginia rivers as well as other rivers in the Chesapeake Bay Watershed. VDGIF continues to work with these scientists by providing fish samples.

  • Collecting histopathology samples from a smallmouth bass.
  • Taking a blood sample from a live adult smallmouth bass.

DGIF and USGS have been focusing on a particular biological pathogen as a possible cause of the disease/mortality episodes. Smallmouth bass, redbreast sunfish and rock bass were collected before, during and after the April/May mortality period from different rivers and analyzed for the presence of pathogenic bacteria from 2008 to 2012. The pathogenic bacterium Aeromonas salmonicida was present and typically the most abundant on fish sampled during the fish kill period. This bacterium was not present on fish in the Maury River during the fish kill period and there have not been any fish kill issues or reports in the Maury River. A. salmonicida was not present on fish before or after the fish kill period. Although this bacterium is present and has the ability to greatly impact fish health we are not aware of why it may be impacting the fish population. A. salmonicida is present in multiple aquatic systems around the world. The simple presence usually doesn't cause such impacts on bass and sunfish populations. It most commonly causes disease in trout and salmon. Environmental factors such as temperature, flow and eutrophication may also play a role in its ability to flourish. The bacteria is considered a "cold-water" fish pathogen since it cannot survive water temperatures > 74° F. USGS researchers have identified that coldwater tributaries entering the river and large springs upwelling in the river are "reservoirs" of this bacteria where it can survive year-round. A. salmonicida is a very virulent bacterium that may influence populations with only its presence. However, if any additional environmental, behavioral or chemical stress is added to the population while A. salmonicida is present in the river then it would have a higher probability of having a detrimental impact on the population. Although it seems A. salmonicida may be a major contributor to the mortality/disease events we now ask why has it only impacted the fishery during the last decade and what may be stressing the population to let A. salmonicida thrive?

While scientists conclude that they may never be able to determine where specifically this bacteria came from nor when it may have been introduced into these rivers, learning more about this pathogen could lead to understanding the root cause of the problem. Additional questions that researchers hope to answer concerning this bacterium include:

  1. What is the spatial distribution of the disease in these rivers?
  2. What is the main vector of disease transmission (fish to fish contact or through water/fish contact)?
  3. Why is disease not as prominent in juvenile fish as it is in adults?
  4. Are fish becoming more resistant to the bacteria over time?
  5. Do certain environmental parameters influence the virulence of the bacteria?
  6. Are there other stressors such as blue-green algae toxins or endocrine disruptors that are impacting the immune system of fish which allows A. salmonicida to impact the population?
  • Taking a water sample for analysis.
  • Placing a passive chemical sampler in the river.

Researchers also have looked to aquatic insects as a possible way to understand the cause of the problem in the Shenandoah River Watershed. The Entomology Department at Virginia Tech was contracted by DGIF in 2006 to conduct a comprehensive evaluation of the aquatic macroinvertebrates in the Shenandoah River Watershed. Unfortunately, the study did not detect the cause of the fish mortality and disease problems. However, the main finding was that the Shenandoah River's aquatic insect community is indicative of a agricultural based watershed, is more vibrant than the New River in Virginia and the Susquehanna River in Pennsylvania, and is more diverse and healthy than it was back in the 1960's. (Shenandoah Macroinvertebrate Study Report (PDF))

  • Swabbing fish for bacteria.
  • Culture of Aeromonas salmonicida.