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Researchers at the Virginia Institute of Marine Science continue to investigate last week's fish-kill event, in which observers reported hundreds of dead and dying adult menhaden in several Peninsula waterways. The researchers have so far discovered no conclusive evidence as to the cause of this relatively small event.
Analysis of water samples collected shortly after the fish-kill events shows that they contain cells of Pfiesteria shumwayae, one of two Pfiesteria species previously implicated in fish-kill events in East Coast estuaries. However, there is currently no direct evidence linking the Pfiesteria cells to the dead fish.
"All we have right now is a temporal correlation," says VIMS researcher Dr. Larry Haas.
As in all cases where Pfiesteria is detected, the researchers immediately reported their findings to the Virginia Harmful Algal Bloom Task Force, the group responsible for responding to harmful algal blooms in Virginia waters. The task force includes representatives from the Virginia Department of Health (VDH), the Department of Environmental Quality (DEQ), the Marine Resources Commission, VIMS, and Old Dominion University (ODU).
VIMS and ODU are responsible for analyzing fish, water, and sediment samples for the presence of Pfiesteria. The Virginia Department of Health is responsible for deciding whether a public health advisory or other action is warranted given current findings, and also determines the status of water-body restrictions. The Marine Resources Commission enforces any water-body restrictions required by the Health Department.
VIMS researcher Dr. Kimberly Reece detected the Pfiesteria cells by applying molecular probes to water samples that were collected in the Hampton River, the Back River, the Poquoson River, Chisman Creek, and Queens Creek shortly after the fish-kill events. Samples from the vicinity of the Poquoson River, the southwest branch of the Back River, and Queen's Creek showed low concentrations of less than two Pfiesteria cells per milliliter. Two water samples collected from the York River in the vicinity of Taskinas Creek on Friday showed relatively higher concentrations, at 177 and 505 Pfiesteria cells per milliliter.
Haas, who collected the York River water samples last Friday, also noticed discolored, reddish water in the vicinity. Discolored water is a typical sign of an algal bloom, or "red tide." Red tides are caused by dense blooms of photosynthetic algae that contain reddish pigments. These organisms are not the same as Pfiesteria, and are not implicated in the recent fish kills. During a return trip to the area on March 30, scientists from VIMS and DEQ observed no discoloration and saw only two distressed menhaden at the surface. Water samples taken during this trip are now being analyzed.
Most of the fish collected during last week's fish-kill event were already in an advanced state of decay and thus can provide no evidence as to the cause of death. VIMS fish pathologist Dr. Wolfgang Vogelbein is currently studying the four freshly dead specimens that researchers were able to collect. To date, his analyses are not informative as to cause of death, although one fish exhibited extensive inflammation of multiple tissues associated with bacterial infection.
A number of factors besides Pfiesteria can lead to the appearance of dead or distressed fish. These include insufficient oxygen, chemical contamination, harmful algal blooms (e.g., red-tide algae), and the dumping of by-catch in estuaries from gillnets. VIMS researchers continue to investigate these factors, which must be considered before ascribing Pfiesteria as an agent of mortality.