Synthesis of U.S. Geological Survey Science for the Chesapeake Bay Ecosystem and Implications for Environmental Management

Chapter 13: Factors Affecting Fish Health
By Vicki S. Blazer, Christopher A. Ottinger, and Christine L. Densmore

The CBP has a restoration goal in the Chesapeake 2000 agreement to “restore, enhance, and protect finfish, shellfish, and other resources, their habitats and ecological relationships to sustain all fisheries and provide for a balanced ecosystem.” To address this restoration goal, the USGS had a science goal to “address the factors affecting the health of fish, wildlife, and their habitats.” This chapter summarizes USGS findings about fish health in the Bay and its watershed; the following chapter presents findings on waterbird populations. The USGS addressed four primary topics related to fish health including (1) menhaden and ulcerative lesions, (2) striped bass and mycobacteria, (3) tributary health assessments, and (4) intersex conditions in the Potomac. Multi-species management plans are being prepared by the National Oceanic and Atmospheric Administration (NOAA) and state partners for menhaden and striped bass as part of the CBP restoration goal for fisheries.

In 1997, USGS scientists were asked to assist in research directed toward understanding the causes of the high incidence of skin lesions and kills of Atlantic menhaden in a number of Chesapeake Bay tributaries. Menhaden are both ecologically critical and commercially valuable species. The lesions and fish kills were thought to be associated with the presence of Pfiesteria, which is believed to produce a toxin that affects fish as well as humans. However, the chronic nature of the lesions (fig. 13.1A) and the consistent presence of an invasive fungal pathogen (fig. 13.1B) raised many questions as to the actual cause of these lesions and the associated environ mental stressors (Blazer and others, 1999).

Figure 13.1. Photographs of (A) ulcerative mycosis of menhaden, and (B) microscopic appearance of ulcers illustrating the invasive fungal hyphae and chronic inflammatory response within muscle tissue, underlying skin. The USGS and collaborators determined lesions on menhaden were caused by a fungal pathogen Aphanomyces invadans. It is now recognized that A. invadans is a serious pathogen of both estuarine and freshwater fishes worldwide.

Figure 13.2. Photograph of mycobacteriosis lesions in striped bass (courtesy of Maryland Department of Natural Resources). The USGS and collaborators identified the cause of skin lesions in striped bass to be mycobacteriosis, which are species of bacteria that can impact both marine and freshwater fish. Improving environmental conditions in the Bay could improve the ability of striped bass to resist the impact of mycobacteriosis.

Research conducted by USGS scientists and collaborators resulted in the isolation and identification of the fungal pathogen Aphanomyces invadans, based on morphology, temperature and salinity growth characteristics, infectivity, and DNA sequence. Using the polymerase chain reaction (PCR) method, the same organism was found in menhaden with lesions from Delaware to South Carolina, and in similar lesions in selected freshwater fish species in Georgia and Louisiana (Blazer and others, 2002). Once isolated, the infectivity and relation of A. invadans to the skin ulcers of menhaden were investigated. Using injection of the infective zoospores, a dose-response in ulcer development and mortality was shown, with only about 10 spores needed to cause death in 50 percent of exposed menhaden (LD50). Injection of as few as one zoospore was sufficient to induce lesions in 31 percent of the fish (Kiryu and others, 2003). Experiments using bath exposure to the infective zoospores indicated that a low percentage of unstressed menhaden developed ulcers; however, stressed (net-handled) and traumatized menhaden had significantly higher mortality and incidence of ulcerative lesions (Kiryu and others, 2002, 2003). These findings suggest that a high incidence of menhaden with lesions in the wild may be a result of environmental factors that favor the proliferation of the pathogen, as well as damage the skin and/or cause immunosuppression.

Many factors, including water-quality parameters (temperature, dissolved oxygen, salinity, and nutrients), contaminants, toxins including algal toxins such as Pfiesteria, and other infectious agents may play a role in predisposing menhaden to A. invadans infections (Blazer and others, 1999; Reimschuessel and others, 2003). It is now recognized that A. invadans is serious pathogen of both estuarine and freshwater fishes worldwide. USGS scientists have worked with international colleagues to reexamine causal factors, describe a case definition, and attempt to standardize nomenclature for a number of syndromes associated with this pathogen (Baldock and others, 2005), as well as review existing knowledge (Blazer and others, 2005). The USGS findings imply that understanding the multiple factors that contribute to the occurrence of pathogens affecting fish will allow for more comprehensive multi-species ecosystem management plans to be developed to protect and restore fisheries in the Bay. The USGS findings also suggest that improving environmental conditions for menhaden, such as improved dissolved oxygen and lower contaminant concentrations, will make them less susceptible to A. invadans infections and other toxic algae.

Striped bass are a highly prized sport and commercial fish in the Chesapeake Bay and along the eastern coast of the United States. They are also one of the five targeted species for which the CBP partners are developing Ecosystem-Based Fisheries Management Plans. The striped bass population has increased in the Bay after a moratorium helped provide relief from overfishing. In the late 1990s, however, fishermen and field biologists began to report a high incidence of emaciated striped bass, many with skin lesions (fig. 13.2). The USGS and collaborators identified the cause of the skin lesions to be mycobacteriosis, which are species of bacteria that can impact both marine and freshwater fish.

USGS investigator processing fish for health evaluations. Fish are bled, organs cultured for bacteria and viruses, and pieces of tissue removed and fixed for microscopic evaluation. (Photograph by U.S. Geological Survey.)

A variety of previously described species of mycobacteria have been isolated from diseased Chesapeake Bay striped bass (Rhodes and others, 2004, 2005); some of these are potential human pathogens (Ottinger and others, 2005). The bacteria affects relatively high numbers of striped bass caught in the Chesapeake Bay with external lesions observed in up to 28 percent of bass caught and internal lesions in more than 62 percent (Ottinger and others, 2005). The multiple factors that promote the presence of mycobacteria and lower the resistance of striped bass to the bacteria are still not well understood. In 2006, the USGS co-hosted a workshop with NOAA to summarize the state of the knowledge and prioritize next steps to address the issue (Ottinger and Jacobs, 2006). The USGS and NOAA findings imply that the resistance of striped bass populations to disease appears to have been lowered due to multiple environmental conditions including low dissolved oxygen, contaminant concentrations, and improper diet. Improving these environmental conditions in the Bay could improve the ability of striped bass to resist the impact of mycobacteria.

Given the problems with lesions in key fish species of the Chesapeake Bay, the USGS conducted tributary health assessments from 1998 to 2003 to better understand fish health in the Bay and its tributaries. The assessments included developing new methods to document fish health and to use the information to compare the “health” of various tributaries. White perch were selected as a sentinel species because they are less migratory than menhaden or striped bass. Several methods to assess fish health were enhanced (Blazer, 2000; Smith andothers, 2002), while new methods were developed (including cellular and subcellular assays) to better identify immunosuppression (Gauthier and others, 2003; Harms and others 2000; Iwanowicz and others, 2004). Findings from the assessments showed that the suppression of the white perch’s immune system occurred in several tributaries and increased from the spring to the summer. The immunosuppression that occurred in the summer coincided with the finding of lesioned menhaden in the same tributaries (Harms, Ottinger, and Kennedy-Stoskopf, 2000). The new techniques that USGS developed for fish-health assessments could be adopted by resource management agencies to provide a more thorough understanding of the health of fisheries in the Bay. The National Ocean Service (NOS) of NOAA is implementing these methods in a program to monitor fish health in Chesapeake Bay tributaries.

Since 2002, USGS has been involved with numerous cooperators in examining potential causes for skin lesions and kills of various fish species in the watershed, particularly smallmouth bass and redbreast sunfish. The presence of various pathogens, including multiple bacteria, fungi, and parasites, indicated these fishes suffer from immunosuppression. During more comprehensive fish-health assessments, the presence of testicular oocytes, a form of intersex, was noted in the male bass. As previously stated, the CBP has a restoration goal “to achieve and maintain the water quality necessary to support the aquatic living resources of the Bay and its tributaries and to protect human health.” The toxic reduction strategy requires information on (1) the sources and occurrences of contaminants, and (2) the potential for contaminants to adversely impact aquatic-dependent wildlife. Reproductive abnormalities in fishes have been strongly linked with a variety of contaminants that have endocrine-modulating activity. Intersex, specifically testicular oocytes, has been linked to exposure to estrogenic compounds, which also have immunomodulatory activity. A preliminary assessment of the occurrence of testicular oocytes in smallmouth bass indicates that (1) it is widespread within the Potomac drainage, and (2) the prevalence and severity may increase as human population and agricultural intensity increase. Further research is underway to assess causes of intersex and fish kills in the watershed, document the spatial distribution, and compare species and life stages to determine the population effects.

Fish kill in Choptank River, suspected to be from toxic algal bloom entrapped in floating seagrass mat. Photograph by Adrian Jones, IAN Image Library (www.ian.umces.edu/imagelibrary/).

References

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