Low incidence of Pfiesteria this season

Maryland's Pfiesteria Monitoring Activities:
July 9, 1999 Summary:

In 1997, the occurrence of fish kills, fish lesion events and associated human health impacts were linked with the effects of a toxic dinoflagellate, Pfiesteria piscicida, on the Lower Eastern Shore of Maryland. The State of Maryland, through the interagency Pfiesteria Study Team and other agency and academic partners, is conducting an intensive monitoring effort to search for any signs of a toxic outbreak and to continue the evaluation of key factors that could contribute to outbreaks in the future.

The three rivers affected in 1997, the Chicamacomico River, Kings Creek (Manokin River) and the Pocomoke River, are being monitored as well as five other systems (Nanticoke River, Wicomico River-Eastern Shore and Big Annemessex in the Chesapeake Bay and Trappe Creek and St. Martin River in the Coastal Bays) that may be susceptible to outbreaks. Water quality and fish populations are also being evaluated Bay-wide through ongoing fisheries surveys and the DNR's Chesapeake Bay Monitoring Program. The following summary updates monitoring and environmental conditions for the spring and early summer 1999 sampling season.

Weather patterns:
Despite the June rains in the State, drought conditions continue in the Chesapeake Bay region. USGS reports that flow into the Bay in June was below the minimum flow recorded for the month of June since 1951 for the estimated total streamflow entering Chesapeake Bay. Flow in Nassawango Creek, a tributary to the Pocomoke River, was also very low during the week of July1-7, and well below the 49-year median daily flow recent streamflow information).

Water Quality:
Salinity levels at Lower Eastern Shore monitoring stations continue to be above average or have reached record high levels connected with the continuing low river flows from the drought. At previous fish kill locations on the Pocomoke and Chicamacomico Rivers, salinity levels were also well above 1998 levels for the same time of year. Water temperatures at long term monitoring stations and historical fish kill sites were also above average for June. Daytime measures of dissolved oxygen were below the long term average at these stations in June but remained at levels acceptable for Bay fish in this region. Water clarity was generally low but near normal for this time of year. The latest water quality information for selected sites can be found at Eyes on the Bay.

More than 50,000 fish were examined during sampling from June 19-July 2, and low rates of anomalies (<1.0%) were being observed overall. Menhaden abundance continues to be high in the Lower Eastern Shore rivers compared with 1998. Sampling of young-of-the-year menhaden in the Pocomoke River on June 24, June 29 and July 2 showed low levels of lesions (June 24: 0.3%, 25 of 9867, June 29 and July 2: 1.5%, 33 of 2162 fish sampled).

Preliminary results are now available on possible causes for the small percentages of lesioned menhaden observed in the Lower Eastern Shore Rivers this year. Menhaden examined to date were collected in May and early June from the following tributaries: Nanticoke River, Fishing Bay, Wicomico River, and Pocomoke River. The samples taken on the Pocomoke on June 9 were collected when 66 out of 5065 (1.3%) menhaden showed some form of a lesion. The fish samples have been processed and are awaiting final diagnosis. Many of the menhaden that have been examined by pathologists to date had a sporozoan (parasite) infection in their muscles. Frequently, these parasites were present in the muscles around the head, gills and eyes. This infection ranged from quite mild with only a few muscle cells containing parasites, to severe with parasites causing distension and disruption of the cells. In the most severe cases, these parasites caused a raised lesion on the surface of the skin or a space-occupying lesion in the gills. Positive identification of this parasite is pending and will be based on histological (microscopic) appearance. Transmission of these parasites is not completely understood, but a possible mode of infection is direct entry in the mouth area or indirect entry via an intermediate host. High concentrations of parasites may occur in poorly flushed waterways.

Two tributaries of the Chesapeake Bay are currently experiencing major sustained fish kills as a result of severely depleted dissolved oxygen levels. Thousands of fish continue to die in the shallow, sub-tributaries of Magothy and Patapsco Rivers; over 200,000 fish have succumbed thus far. Drought conditions are causing low dissolved oxygen conditions, common in bottom water layers in summer, to occur in surface layer waters as well; several natural and human induced factors have acted together to intensify this effect. Dissolved oxygen readings have been recorded near zero in many locations. The die-off is expected to continue as long as summer drought conditions prevail in the region.

Fish kills involving yellow perch were also reported on Middle River on July 7. Maryland Department of the Environment believes these to be a result of high water temperatures (29 to 32 C, or 84 to 90 F) and increased salinity. Yellow perch are extremely temperature sensitive and were the dominant species affected in this kill. Other species, such as white perch, did not seem to be affected. A reminder - please report fish kills, sick fish or fish with lesions to the Maryland Fish Health Hotline at 1-888-584-3110.

Tracking of algal blooms is an important component of the monitoring program since most of the Pfiesteria outbreaks have been associated with elevated levels of algae. In the most recent Pocomoke sampling on June 28, algal levels were highest in the region from Shelltown (station POK0014) downriver into the Sound (around station XAJ7164), continuing the pattern seen in May and June. In the Chicamacomico/Transquaking system, algal levels are highest in the Chicamacomico and rise sharply from the mouth to the station at Drawbridge (CCM0069). This difference is more pronounced in the most recent sampling on July 1.

Dr. David Oldach (University of Maryland Biotechnology Institute) has been examining water samples with an experimental genetic technique designed to rapidly identify the presence of Pfiesteria piscicida. As of July 7, results had been reported for 139 water samples collected on 17 different tributaries of Chesapeake Bay and Maryland's coastal bays. Only one sample, collected June 24 in the Shelltown region of the Pocomoke River, was positive for Pfiesteria piscicida. This technique does not differentiate between the toxic and non-toxic forms of the organism, and the lack of distressed fish during this time strongly suggests that the test detected the presence of Pfiesteria in one of its non-toxic forms. These results are not a reason for great concern, as previous experience has already demonstrated that non-toxic Pfiesteria exists in the lower portions of the Pocomoke River. Additional sampling of the lower Pocomoke for Pfiesteria and associated algal species will be conducted during the week of July 12.

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