Abstract

In late August and September of 1997 outbreaks of a toxic dinoflagellate, Pfiesteria piscicida, coincided with Atlantic menhaden Brevoortia tyrannus mortalities in the Pocomoke River, a tributary of Maryland's Chesapeake Bay. Subsequently other river systems were affected. In response, a suite of fish health investigations was undertaken in several river systems during 1998 by two units of the University of Maryland--Eastern Shore (UMES) and Horn Point Environmental Laboratory (HPEL); the Maryland Department of Natural Resources (DNR); and the Maryland Department of Agriculture--Animal Health Laboratory (AHL). One purpose of these studies was to monitor for the effects of Pfiesteria piscicida and other Pfiesteria-like organisms, both to observe impacts on fish populations and communities, and to alert the public should outbreaks of these toxic dinoflagellates occur. A second purpose was to gain better information about the causes, progression and outcomes of the high prevalence of fish abnormalities seen in the Pocomoke River in 1997.

While toxic P. piscicida outbreaks occurred in Chesapeake Bay tributaries in 1997 in association with fish abnormalities, questions remained about whether other factors were involved in the high incidence of fish anomalies.^1,3 It was clear that some of the less severe lesions seen in commercial fisheries catches were related to physical damage from confinement in traps (pound nets, fyke net and bank traps) and handling. Unfavorable environmental conditions, water quality problems, toxic discharges and epizootic fish diseases also were proposed to explain some or all of the abnormalities.⁷

This report includes results from: 1) onboard monitoring of commercial fishery catches in the Pocomoke River system; 2) directed Baywide fish health (lesion incidence) surveys; 3) experimental studies conducted to assess physical and physiological responses of caged and uncaged fish exposed to the Pocomoke River environment; and 4) pathology and microbiology of ulcerated Atlantic menhaden Brevoortia tyrannus collected from Chesapeake Bay tributaries.

Commercial Fishery Observations

Observers were placed on board commercial fisheries operations in the Pocomoke River and recorded numbers and types of external anomalies on all fish observed. Abnormalities in fish from commercial fisheries were uncommon in 1998, in contrast to observations from 1997. Of 34,942 fish observed from pound net and fyke net catches in 1998, including 44 species, only 0.306% were observed with some form of skin anomaly and only 0.192% possessed a severe skin anomaly (erosions or ulcers).⁵ No correlation between fish size (total length) and the occurrence of skin anomalies was observed for any species. Abrasions were the most commonly observed skin anomaly, followed by erosions and ulcers. A majority of the erosions and ulcers observed were on striped bass. A condition known as ulcerative dermatitis, caused by bacterial infection, has been seen widely in striped bass populations over the past two years, and is under separate investigation. Hemorrhages were present in some fish but were less common than other anomalies.

Fish Health (Lesion Incidence) Monitoring

Biweekly trawls, cast nets, and seines, were used to sample fish populations Baywide. Fixed fish monitoring sites and corresponded with water quality monitoring sites sampled by the Maryland Department of the Environment. The Maryland DNR and the University of Maryland sampled a total of 372,675 fish from all gear types and efforts in 1998.^5,8 The total number of menhaden was 46,789 or 12.6 % of the total catch. Of that number 976 menhaden or 2.1 % were recorded as having external anomalies. In the Pocomoke River 49,106 fish were sampled in 1998 with only 188 (0.4%) recorded as having anomalies.⁸

Experimental Studies

Seventeen in situ experiments were conducted from April-October 1998, to evaluate the formation and progression of skin anomalies in the lower Pocomoke River. The experimental fish (white perch Morone americanus and striped bass Morone saxitilis) experienced a consistent progression of common skin anomalies. Confinement of fish in the experimental cages consistently resulted in caudal fin hemorrhage (60-80% of fish). This high percentage of caudal hemorrhage also occurred in the uncaged reference fish, permitted to swim freely in 1000 gallon (3900 L) holding tanks.

The percentage of fish with abrasions was generally 20-60% at all sites, including both caged and uncaged reference experiments. The white perch were abraded by the wire holding cages and physical contact with fish holding tanks. Deterioration of the tail, often called red tail or tail rot, typically occurred after 48 hours and steadily increased throughout the experiments.^4,5 Tail rot was common in fish at all sites including caged and uncaged references. Skin abnormalities were least prevalent in the uncaged reference experiments, indicating that minor skin abnormalities, primarily abrasions and hemorrhage, resulted from caging white perch and striped bass. The occurrence of severe skin abnormalities such as erosions and ulcers was very low at all sites, consistent with observations from the nearby commercial catch.

Many fish initially had mild abrasions and caudal hemorrhage, which disappeared as they acclimated to the experimental holding conditions. Some abrasions and caudal hemorrhaging, however, progressed to fungal or bacterial infections. Abrasions induced by contact with experimental cages and holding tanks often progressed forming an erosion which eventually progressed to an ulcer.

One of the objectives of the experimental study was to determine whether either the Pocomoke River environment or confinement in cages caused significant physiological stress in fish. Blood samples from caged and uncaged white perch were analyzed for hematocrit (red blood cell concentration), leukocrit (white blood cell concentration), plasma osmolality (roughly equivalent to salt content), plasma chloride concentration and plasma glucose concentration. A pilot study also was conducted to measure acetylcholinesterase (an important enzyme that regulates neuromuscular activity) from the brains of caged and uncaged white perch.

None of the physiological indicators showed differences between caged and uncaged fish large enough to reflect severe stress. The largest differences from mean physiological conditions generally were measured in fish freshly removed from pound nets or fyke nets. These indicators tended to return to mean values after acclimation in the holding tanks.^6,9,10 It should be noted that no baseline data on these indicators was available for white perch prior to these studies. The values for the uncaged reference fish are the best available baseline physiology for this species.

Based on pilot work conducted in 1998, we believe that similar experiments can be done in 1999 using Atlantic menhaden, a more fragile species with direct relevance to Pfiesteria piscicida toxicity.

Pathology and Microbiology

Atlantic menhaden with skin abnormalities were collected from four Chesapeake Bay tributaries, the Wicomico (eastern), Annemessex, Chicamacomico, and Nanticoke Rivers. After tissue preparation, slides were examined microscopically. Twenty-four of 27 menhaden (89%) were diagnosed with mycotic granulomatosis, with evidence of specific immune responses by the fish. This diagnosis is equivalent to the ulcerative mycosis thought to be associated with toxic Pfiesteria piscicida activity.^2,7 While microbiological cultures failed to definitively identify the fungus found in histologic sections, bacterial cultures from did offer evidence of bacterial involvement. Aeromonas hydrophila and Pseudomonas fluorescens, ubiquitous aquatic species, were the most common isolates. Cultures from liver, kidney and spleen tissues from the experimental white perch also showed presence of A. hydrophila--the most common isolate. This species was most prevalent in fish that had been caged for over 96h.

Conclusions

The incidence of external anomalies on commercially harvested fish in the Pocomoke River and Sound was lower in 1998 than in 1997.

Atlantic menhaden with ulcers, collected from several Chesapeake tributaries, consistently showed evidence of invasive fungal disease along with bacterial involvement.

No evidence for acute toxic or pathogenic effects on fish was found either in field sampling or in the experimental deployments in the Pocomoke system.

White perch held in cages in situ in the Pocomoke system for more than 48h developed abrasions and bacterial infections, symptoms which became more severe with longer holding times. Mild hemorrhages and abrasions were common even with shorter holding times. Physiological indicators did not show symptoms of severe stress.

The high incidence of severe anomalies observed in Pocomoke system commercial fisheries during 1997 could not be attributed to effects of the pound nets, fyke nets and bank traps in which they were captured and confined, based on the results of 1998 caging studies and commercial fishery observations.

The experimental systems and procedures developed during these studies can be employed effectively to monitor and identify potential acute toxic or pathogenic events, and could be used with Atlantic menhaden if great care is taken in capture and transport of the fish.

Study plans for 1999 are under development and include possible tank studies with Atlantic menhaden to continue the effort started in 1998 with white perch and striped bass. It is hoped that these studies will assist the State of Maryland in Pfiesteria investigations and begin to answer fish health questions regarding the etiology of the skin lesions.

Acknowledgements

This work was supported by the Maryland Department of Natural Resources, the National Oceanic and Atmospheric Administration, and the Environmental Protection Agency. The authors wish to thank the agencies, Maryland fishermen, and Maryland biologists for their assistance. Many others contributed to this work and we are grateful for their assistance.

References

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