Fish diseases and related fish kills in the Chesapeake Bay have received public attention with the discovery of toxic pfiesteria populations that appear to be linked with lesions and mortality in menhaden. A different type of skin lesion that has become prevalent on striped bass is believed to be caused by a bacterial infection.
While anecdotal information suggests an increase in fish disease problems worldwide, the vast majority of fish in the Chesapeake Bay are healthy. For example, recent surveys in Maryland this spring and summer show that less than 1 percent of the almost 300,000 fish collected appeared to have external health problems such as lesions.
Nevertheless, certain species at certain times and places can exhibit a much higher incidence of disease. In these situations, it is often relatively straightforward to identify disease organisms in the fish. What caused the disease outbreak, though, often involves a complex series of events that can be difficult to trace. Nutrient enrichment, for example, with its origins from the land and air, can produce various ecosystem imbalances that may be playing multiple roles in promoting fish diseases.
Fish disease initiation can be thought of as the intersection of three necessary conditions The first two conditions are the fish and the disease organism.Waters in areas like the Bay contain a rich brew of naturally occurring disease organisms including viruses, bacteria and fungi.
The third condition is a “stressor” that weakens the natural defenses of the fish to disease organisms, allowing them to enter the fish and gain a foothold. In this respect, fish disease has parallels to human disease.
Stressors to fish include a long list of environmental insults ranging from an abrasion from a fishing net or an encoun-ter with a predator, to toxic chemicals from either an industrial source or pfiesteria-like organisms, to ill health from lack of food or another disease, to poor or widely fluctuating habitat (dissolved oxygen, pH, salinity, temperature). Infestations by parasitic worms, crustaceans and protozoans could provide an entry opportunity for disease as well as act as a stressor.
These stressors can create create a physical breach in the protective skin or gastrointestinal wall of a fish, or they could weaken the immune response that normally keeps disease organisms at bay. Even sunlight, with its damaging ultraviolet radiation, could injure the skin sufficiently to allow entry by disease organisms.
Once the disease organism has gained a foothold in the fish, it may affect the internal organs as well as the more visible problems that occur when the skin is attacked. Often, the disease organism that is found in a damaged part of the fish is difficult to relate to the agent that initiated the disease. For example, exposure to the toxin released by pfiesteria is believed to cause a disruption of the skin which can subsequently be invaded by bacteria and fungi. These “secondary invaders” can further erode the skin and enlarge the lesion. Thus, pfiesteria is not the disease but rather the stressor that can lead to the invasion by disease organisms.
Again, there are parallels with humans — we are also more subject to disease when in a weakened condition. Declining health and resistance may lead to the development of additional diseases. By the time a diseased fish is caught, it may well be suffering from a number of problems that obscure the original cause.
The organism recently affecting striped bass in Chesapeake Bay is believed by a number of researchers to be a type of bacteria called a “mycobacterium,” giving the disease the name “mycobacteriosis.” The species of mycobacterium that affect fish are actually capable of causing a rash-like syndrome on the hands of people who handle large numbers of fish in their occupations. It is commonly known as “fish handlers disease.”
While there is no indication that infected fish in Chesapeake Bay represent a widespread health threat to humans, most health officials recommend the washing of hands after handling fish. This bacteria often manifests itself externally as a reddish rash on the fish skin but it may also cause other types of external lesions and affect internal organs.
As in most diseases, a stressor is suspected of causing a higher incidence of this disease over the past couple of years relative to what has been observed in prior years. One possible stress that has received attention recently is the poor “condition” of the fish as a result of the lack of adequate food. It has been hypothesized that the fish normally preyed upon by striped bass have grown less abundant because of low reproduction, a lower availability of their food sources, or heavy predation by large populations of striped bass and other predatory fishes.
Among the problems that plague those investigating fish disease issues is a lack of consistent long-term data on the incidence of various diseases. It is widely believed that the worldwide incidence of fish diseases is on the rise, but hard statistics are often absent.
Some of the increase in reports of fish disease may be due to the higher public visibility and vigilance by fisheries managers. Nevertheless, the long-term experience of the many individuals involved in the harvest, management and research of fisheries around the world would suggest that an overall increase in incidences of fish disease is likely.
Another issue that has more questions than answers is the impact that various fish diseases might have on the abundance of a fish population. Disease outbreaks could cause death directly if the disease is fatal, or indirectly by weakening the fish and thereby making it more vulnerable to predators. A weakened fish could also have lower rates of growth and reproduction. Many researchers are pondering this question, but few have made much headway because of the difficulties in measuring the various direct and indirect effects.
To explore the ways in which we could minimize the incidence of fish diseases in wild populations, we can refer back to the three conditions discussed earlier. Reducing the fish population might reduce disease incidence but would not necessarily be a desired management objective. Reducing the abundance and types of disease organisms would, on initial consideration, appear to be an almost intractable problem in natural waters. Yet some have hypothesized that disease organisms may be more prevalent in nutrient-enriched waters where the pathogen may be provided with a more favorable environment to survive. A study evaluating this hypothesis for the Chesapeake Bay will be initiated shortly by Andrew Kane of the University of Maryland Aquatic Pathobiology Center and his collaborators.
Perhaps the greatest opportunities to decrease disease outbreaks will come from a reduction in the number and severity of stressors. It is in this realm that the full Chesapeake Bay ecosystem comes into play. A few examples relevant to the Bay can serve to highlight the ecosystem interconnections that could lead to stress on a fish population.
In the pfiesteria scenario, increased loadings of nutrients could lead to expanded populations of non-toxic pfiesteria as a result of an increase in the organisms that serve as its prey (phytoplankton and bacteria) and the nutrients that nourish its growth. When sufficient populations of fish are present at the right time of year and in the right physical setting, pfiesteria populations can release toxins, either killing the fish or weakening them for invasion by bacterial and fungal disease organisms. In this case, reducing the prevalence of the stressor — pfiesteria toxin — could likely be achieved by reducing nutrient loading.
In a few areas, fish disease problems have been associated with high levels of man-made toxic compounds. In these cases, the stressor is more directly related to human activity and the reduction of these toxic compounds to levels that eliminate stress to fish could reduce the incidence of disease. The aforementioned mycobacteriosis afflicting striped bass, if brought on by the stress of a low food supply, may be even more complicated to track back to a solution that would be under management control. It must also be recognized that some stressors may be due to natural causes beyond our control.
The strong message that comes through from a careful look at fish diseases is that environmental stressors, which make fish more susceptible to disease, are often woven into the intricate fabric of the Bay ecosystem. As human influences in the watershed continue to expand, restoring the Bay’s fish populations may well depend upon a recognition of these stressors and their origins. If we are to make progress in understanding, and potentially managing fish diseases, a closer collaboration of those involved in the studies of fish diseases, fish population dynamics, habitat and water quality will be needed.
These following people contributed to this article: Andrew Kane, Renate Reimschuessel, Wolfgang Vogelbein, Ana Baya, Steve Jordan, and Peter Jensen and Harley Speir.