Menhaden with lesions in the Pocomoke River,
Pfiesteria tests negative

July 19, 2000

This is the third biweekly report for 2000.

Fish and Fish Health
On July 11, 9.6% (180 out of 1877) of menhaden captured from the lower Pocomoke River/Sound showed ulcerative lesions (bar chart below). There were 179 of 1874 menhaden at one location in the Sound northeast of Williams Point and 1 of 3 menhaden at Williams Point with ulcers. An additional 851 fish of 16 other species were collected in the Pocomoke River/Sound system that day and all appeared healthy. Three water samples collected on July 11th at the sites of the fish with ulcers were tested on the morning of July 12th for the presence of Pfiesteria with the genetic probe by Dr. David Oldach’s lab at the University of Maryland Biotechnology Institute. No evidence of Pfiesteria was reported. The lower Pocomoke River and Sound were revisited for additional fish and water sampling on July 13. Menhaden with ulcers continued to show up in catches at 3 of the lower Pocomoke River and Sound seining locations (256 of 1068, 24%). All other species that were captured appeared healthy. Four additional water samples collected from the seine sites on July 13 have come back negative for the presence of Pfiesteria based on Dr. Oldach’s genetic probe. The river was again revisited on July 17th and 9.7% of the menhaden had ulcers (16 out of 165). Another 1564 fish of 6 species were collected and all appeared healthy. No Pfiesteria piscicida was found in the water samples from the river at these sites. Water samples were collected July 11, 13, and 17 and sent to additional laboratories for further Pfiesteria and water quality testing; results are pending. All fish appeared healthy in samples collected on the lower Pocomoke on June 13 and 27. Maryland DNR will continue to closely monitor fish health, water quality, and Pfiesteria on the lower Pocomoke River and Sound.

Figure 1

As of July 9, more than 125,000 fish have been examined during fish health monitoring throughout the Chesapeake and Coastal Bays. Menhaden have comprised over 53,000 of the fish in the total catch. Overall percentage of fish with anomalies has remained less than 1%. Local summer season sampling events having greater than 1% affected fish have been limited to the lower Pocomoke River and Pocomoke Sound this year.

In the spring of 2000, some very young menhaden (post larval stage) in the Pocomoke River/Sound again showed similar anomalies to fish examined in 1999. Tissue of affected fish is being examined microscopically by pathologists. The presence of Kudoa-like spores associated with a myxosporidian parasite have been observed in the affected muscle of some of these fish. Histopathological examinations are continuing on fish collected more recently and the affected fish are infected with an invasive plasmodia similar to what was observed in menhaden during 1999. Fish from other tributaries have not been observed with external anomalies, however, a sampling of fish will be selected from field collections on other systems and submitted for more extensive examination in the laboratory.

Experimental studies with menhaden are also underway to try and better understand how ulcers develop and progress in the fish. Live menhaden (some with anomalies) were collected from the Pocomoke River on May 11, 2000 and brought to Horn Point Lab. On May 29, 2000 the fish were separated according the lesion severity, counted and placed in separate tanks for observation. Samples for histological examination were taken on May 29th and June 30th. Gross observations indicated rather remarkably that all external lesions seen earlier had healed, with very little mortality. Histology results are pending. John Jacobs at the Horn Point Laboratory is also maintaining a captive population of young-of-the-year Atlantic menhaden (1000+), and age 1+ fish (500+) for future research.

A reminder - please report fish kills, sick fish or fish with lesions to the Maryland Fish Health Hotline at 1-888-584-3110.

Pfiesteria and other Algae.
Water samples have been collected since January and surveyed for the presence of Pfiesteria using a genetic probe developed by Dr. Oldach (University of Maryland). Rivers sampled from January to July 2000 have included: the Patuxent, Rhode, Potomac, and Middle on the western shore of the Bay; the Chicamacomico, Transquaking, Manokin, Big Annemessex, Choptank, Pocomoke, and Nanticoke Rivers as well as Tangier Sound on the Lower Eastern Shore; Trappe Creek and the St. Martin River in the coastal bays. Of the 327 samples tested between January 17 and July 18, six samples have tested positive for Pfiesteria piscicida (see map below). The genetic probe does not differentiate between toxic and non-toxic Pfiesteria but there has been no evidence of toxic Pfiesteria at any of these locations this year.

Figure 2

Weather, Water Flow and Water Quality
Streamflow conditions into the Bay this year remain near average according to a July 7 press release from the U.S. Geological Survey (USGS). Around the Bay, flow conditions for June were above normal and 3 to 4 times greater flows were recorded during June 2000 than during the drought in June 1999 at sites such as the Potomac River near Washington D.C. Eastern Shore flows were less than May according to USGS but still within the normal range for the region.

In the Pocomoke River, most physical water quality parameters are similar to previous years for this time of season with the exception of salinity. Salinity levels have tended to be lower this summer than 1999. Recent salinity measurements are the lowest on record for our intensive monitoring stations during the last three years in the river (see graphs below).

Figure 3 dissolved oxygen

Figure 3 Salinity

Figure 3 pH

Figure 3 Water Temperature

USGS monitoring station located on Nassawango Creek in the Pocomoke Basin in mid-July shows water levels 3-5 times greater than the 49 year historical median levels. Precipitation on the lower Maryland shore has been well above average recently (see map below)

figure 4

Precipitation data from

Continuous water quality monitoring stations are in place through the EMPACT program (Environmental Monitoring for Public Access and Community Teaching) at three sites on the Pocomoke River in the region of the 1997 fish kill; a fourth station will soon be running on the Chicamacomico River where fish health events have been recorded in previous years. River conditions for the Shelltown area during June and early July show water temperatures heating up to near 30 oC (about 86 Fo); dissolved oxygen levels in June fluctuated around 5-6 mg/L until the end of the month. The preliminary data set shows dissolved oxygen concentrations dropped below 2.5 mg/L on July 1 then quickly recovered to levels at or above 5 mg/L in the following days. Around the time of dissolved oxygen event, turbidity doubled on two occasions; once on June 29 and again on July 1, with temperature, pH and salinity also declining for a short period of time. Rain-water is naturally acidic and the pattern of events shown in the data are likely connected to rain and runoff in the region.

Search Maryland DNR


Restoration and Protection | Bay Grasses | Harmful Algae | Bay Monitoring
Bay Life Guide | Bay Education

Return to the Maryland DNR Home Page.
Your opinion counts! Take a