Shenandoah and James River Fish Disease and Mortality Investigation

Spring 2010 Update As Of June 1

Angler reports of small numbers of dead and diseased smallmouth bass began in the South Fork Shenandoah River in early April. DGIF verified that light mortality and disease of smallmouth bass and sunfish was occurring throughout the South Fork Shenandoah River from Port Republic to Front Royal. DGIF also sampled throughout the North Fork Shenandoah and Main stem Shenandoah River and did not observe any disease or fish mortality.

Things have been fairly quiet this spring on the Cowpasture, Jackson, and upper James River. Angler reports of dead or diseased fish in these rivers were almost non-existent this spring. DGIF sampled fish from various locations in the upper James River, and at locations on the lower Jackson and Cowpasture River. A few dead fish have been observed in the upper James and Cowpasture by DEQ while taking water samples, and DGIF observed a high percentage of the rock bass and a few smallmouth bass with lesions or other signs of disease while collecting fish in late April. The number of dead and diseased fish appears to be lower than previous years. DGIF will continue to watch these rivers.

Biologists monitored fish in the Maury River throughout March and April this spring. The Maury River continues to avoid these fish mortality and disease events, as all fish appeared healthy.

This year's spring fish mortality event on the South Fork Shenandoah appears to be on the light side with anglers reporting seeing a few (2-10 on avg.) deceased adult smallmouth bass over a several mile section of river. This is similar to what was observed in spring 2009. The percentage of smallmouth bass and sunfish showing signs of disease (10-25%) at individual locations is also similar to previous years.

Impacts from this year's mortality/disease events are expected to be light. However, DGIF biologists will not obtain a full assessment of the fishery in the Cowpasture, James, and Shenandoah Rivers until later this fall. In the meantime, DGIF will continue to monitor the fish population in these rivers.

Chronic spring-time fish mortality and disease events have occurred in the Shenandoah River 2004-2009, and in the upper James River 2007-2009 (Figure 1). These episodes have not been uniform in location or severity over these time periods. 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 2). Some dead and dieing fish have no visibly external abnormalities. Other external symptoms include: dark patches of skin, raised bumps, loss of scales, split or eroded fins, and discolored/eroded gills. See photos.

Determining the cause of these mortality and morbidity events has proven to be extremely difficult. Scientists have conducted in-depth studies on fish health, pathogens, water quality, and contaminant exposure. The fact that these events have occurred in two separate watersheds that differ in many ways has added to the complexity of understanding the cause.

Location of fish mortality and disease events 2004-2009.

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 were directly responsible for these fish mortality/morbidity events (Figure 3). Contaminant levels were measured in the rivers affected as well as some rivers where these fish mortality/disease events were not occurring. Contaminant levels were measured at both base-flow and during runoff events (Figure 4). However it must be noted that not every possible chemical compound was measured, and that the toxic concentration 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 or by visiting

Some chemical compounds and heavy metals have been shown to suppress the immune system 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 could 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 or James 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.

  • Taking a water sample for analysis.
  • Placing a passive chemical sampler in the river.

Fish health investigations to date have included: histopathology (Figure 6), parasitology, bacteriology, virology, and blood analysis (Figure 5). 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 significant amount 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).

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

Researchers 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))

DGIF and DEQ have recently (2008-2010) been focusing efforts on learning more about a particular biological pathogen (bacteria) that has been cultured from fish in the affected rivers. Researchers are focusing on this pathogen to better understand what is happening to these fish, as it may prove to be a significant part of the disease/mortality episodes. The bacterium Aeromonas salmonicida (Figure 7 & 8) is the only variable common to all the fish mortality/disease locations. The bacteria has been cultured from adult and juvenile smallmouth bass and several other fish species from the affected rivers. It has not been found on fish in other Virginia rivers where the disease episodes are not occurring. The bacteria is considered a "cold-water" fish pathogen since it cannot survive water temperatures > 74° F. The bacteria has been cultured from multiple fish species throughout the world, but it most commonly causes disease in trout and salmon. Bacteriologists with the United States Geological Survey (USGS) have determined that this bacteria can act as a "primary" pathogen and does not necessarily require the fish to be stressed from other factors before becoming diseased. However, as mentioned earlier, other researchers are investigating immune function viability in fish and whether certain environmental variables can affect the virulence of the bacteria. 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.

While scientists conclude that they will probably 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. Work is ongoing in 2010 to determine how the bacteria gets distributed throughout the river, the number of potential reservoirs, and how much river (distance) can be affected by a single reservoir of bacteria. Additional questions that researchers hope to answer concerning this bacteria include:

  1. What is the spatial distribution of the disease in these rivers?
  2. Why are certain species of fish more susceptible to the disease than others?
  3. What is the main vector of disease transmission (fish to fish contact or through water/fish contact)?
  4. Why is disease not as prominent in juvenile fish as it is in adults?
  5. Are fish becoming more resistant to the bacteria over time?
  6. Do certain environmental parameters influence the virulence of the bacteria?
  7. What is the average percentage of smallmouth bass and sunfish (throughout the river) that are carrying the bacteria and becoming diseased?
  • Swabbing fish for bacteria.
  • Culture of Aeromonas salmonicida.

Status of the Fishery (November 2009)

Smallmouth Bass

The one question that anglers and concerned citizens have asked since the beginning of these fish mortality and disease episodes is what has been the impact on the fish population. In the initial years of these events there was higher mortality observed and biologists estimated that fish losses were quite high. Fish biologists stressed that these were estimates and that the severity of the mortality and disease was not uniform throughout the rivers that were affected. However, several factors have allowed these fish populations to recover faster than anticipated. The most significant of these being excellent smallmouth reproduction between 2004 and 2007 (Figure 9). The years 2004 and 2007 were two of the best spawning years in the past decade in the Shenandoah River. Virginia biologists have documented that river flow in the spring/early summer is what determines the success of the smallmouth bass spawn. It also only takes a small number of successful spawning fish to keep the population viable. While researchers have recently verified that juvenile smallmouth bass are carrying the bacteria Aeromnas salmonicida, these fish do not appear to show signs of disease. Biologists have also not been able to document baby bass mortality associated with these episodes. Natural reproduction is what "drives" the river smallmouth bass populations in Virginia. The relative abundance of larger fish in the population is directly related to spawning success in previous years (Figures 10 & 11). Anglers can "ride the wave" of a strong spawn for several years as these fish grow into desirable sizes. Two and three year old smallmouth are the bass most frequently caught by anglers fishing the Shenandoah and James River. On average it takes a smallmouth bass five years to reach 14 inches in the South Fork Shenandoah River. This explains the lag of a few years following a strong spawn when the numbers of larger fish increase in the population. This is easier to see in the Shenandoah graph (Figure 10) than in the James River graph (Figure 11). The opposite is true when there are several years in a row with below average spawning success. 1999-2002 were drought years and 2003 was an extremely wet year. This five year time period produced very few smallmouth bass. Because of the lack of fish entering the population the numbers of larger fish started to decline in 2003 and 2004. When the first fish mortality event hit the South Fork Shenandoah in 2005 adult smallmouth bass numbers were already on a decline. Focus on how the adult smallmouth bass population responded to the fish mortality episode on the SF Shenandoah in 2005 (Figure 10) and the James River in 2007 (Figure 11). One can see that both adult populations plummeted during the worst mortality years. However, notice how these populations have recovered and are near or well above the average over the last 10-15 years. Another important thing to take away from these data graphs is the consistent low proportion of large smallmouth in the population. The impacts of the fish mortality can be seen in the bigger fish sizes, but it is much less noticeable. Readers may also notice that 2002 was the best year to catch larger smallmouth bass in both the James and the Shenandoah River. This is due to consistent spawning in the early 1990's and a mega spawn in 1997. While electrofishing catch rates of larger smallmouth bass are quite variable on both the Shenandoah and James River, biologists have estimated that the mortality events have taken about 5-10% of the larger bass from the population per year.

Angler-creel survey data is also used to validate what biologists see in their electrofishing data. DGIF conducted a creel survey on the South Fork Shenandoah River in 2008 and the angler catch rate for smallmouth bass was 2.7 fish per hour. Comparing this to a catch rate of in 1.6 fish per hour in 1997 one can see how the fishery changed in a decade. Angler satisfaction was also high (75 %) for the 101 anglers surveyed on the South Fork Shenandoah River in 2008. The electrofishing data suggests that the overall smallmouth bass population in the Shenandoah River has only been marginally affected by the recent fish mortality and disease episodes. The most noticeable difference to anglers would be a modest reduction in large bass (>16"). However, the size structure of the smallmouth bass population in the South Fork Shenandoah River during fall 2009 was excellent with a high proportion of fish being of angler preferred size (Figure 12). Excellent reports from anglers in 2009 also were an indication that the abundance of quality size smallmouth bass was on the increase in the South Fork Shenandoah River. It must be noted that this electrofishing data represents the smallmouth bass population as a whole. This information was generated by combining electrofishing data from multiple sites throughout these rivers. Smallmouth bass population statistics can vary for different individual reaches of river.

Biologist predict that anglers will see a modest decline in the number of larger smallmouth bass this year particularly in the James River upstream of where the Maury River enters at Glasgow. The catch rates of larger smallmouth on the South Fork Shenandoah River this summer could also be slightly lower than last year due to this spring's disease episode. Fishing in the North Fork Shenandoah. mainstem Shenandoah, and cowpasture River is expected to be similar to last year. DGIF biologists want to remind anglers that not every fish that develops a lesion will die. Biologists and anglers have been observing adult smallmouth bass and sunfish with healed lesions later in the summer and fall. See Photos. Also the location and severity of these disease outbreaks are not uniform up and down these rivers. For that reason the fish population could be stronger in different sections of river. Overall, 2010 catch rates for smallmouth bass in these rivers should be similar to 2009.

Smallmouth bass spawning success in the SF Shenandoah and James River.

Electrofishing catch rate of different sizes of smallmouth bass from the SF Shenandoah River.

Electrofishing catch rate of different sizes of smallmouth bass from the upper James River.

Length frequency distribution of smallmouth bass from the SF Shenandoah River Fall 2009.

Largemouth Bass

Largemouth bass do not gain as much attention as there cousin the smallmouth bass, but the South Fork Shenandoah harbors a very good largemouth population (Figure 13). While biologists have observed a very small number of largemouth bass showing sings of disease, there appears to be no impact on the population. Good numbers of quality-size largemouths are available to anglers. Largemouth bass of up to seven pounds have been collected by biologists from the South Fork in recent years. Looking at a recent angler/creel survey conducted by the VDGIF, largemouth bass are being underutilized by anglers.

Length frequency distribution of largemouth bass in the SF Shenandoah River Fall 2009.


The South Fork Shenandoah is home to several sunfish species. Redbreast sunfish, bluegill, and green sunfish are the most common Rock bass can also be included in the sunfish group, but their numbers are quite low. Biologists and anglers have observed a drastic reduction of rock bass in the river over the past 10-15 years. DGIF has no explanation. Pumpkinseed sunfish are also present, but in very low numbers.

Redbreast sunfish are the most abundant sunfish species inhabiting the South Fork. They can be found in all types of habitat throughout the river. Usually where there is one many others will be in close proximity. Unlike the other sunfish species, redbreast will also occupy areas of the river with faster currents. Redbreast in the 6-7 inch range can make for some exciting fishing. Redbreast sunfish numbers have been on the increase the past two years (Figure 14) and the population appears to not have been significantly impacted by the recent disease and mortality episodes.

Electrofishing catch rate of redbreast sunfish in the SF Shenandoah River. Vertical bars indicate the variation in catch between sites each year.