Environmental Engineering Reference
In-Depth Information
In the spring of 2002, huge shoals of dead ish , primarily smallmouth bass, suddenly
floated to the surface of the South Branch of the Potomac River, in West Virginia, about
two hundred miles upstream from Washington, DC. Fishermen noticed that many of
the bass had painful-looking lesions along their sides. They alerted the West Virginia
Division of Natural Resources (DNR), which asked the US Geological Survey to invest-
igate.
That summer, a USGS fish pathologist named Dr. Vicki Blazer led a team of scientists
to collect fish in the river and to search for clues to what was happening to them. Out
on the South Branch of the Potomac, Blazer found that the best fish to study were
moribund bass—fish that were sick and on the verge of death but not yet dead. To
collect bass humanely, DNR scientists working with Blazer first stunned them with an
electro-shock in the river, which caused the fish to float to the surface, where they were
collected and put into buckets of water dosed with an anesthetic. Working quickly, the
biologists observed the outside of the fish, noting any lesions or other abnormalities.
They took blood samples, and samples of the mucus that coats the fish's skin and scales.
The mucus is the fish's first layer of defense against infection. If the flora in the mucus is
normal, it will inhibit pathogens; if the flora is abnormal, it is a sign that the fish's health
is compromised.
“Before big fish kills, we've seen a change in the mucous flora,” Blazer explained. “It
indicates contaminants in the water.”
The fish were dissected in the field. Biologists noted whether a fish's liver looked too
pale, the spleen was too big, or whether parasites were in the tissues.
Back at her lab in Kearneysville, West Virginia, Blazer and her colleagues examined
bass tissues in thin slices under a microscope. They discovered that some bass had bac-
terial lesions, others had fungal lesions, and some were afflicted by parasites. The lesions
had no single cause, suggesting that the fish were immunosuppressed, meaning their
immune systems weren't working properly.
Then came the shocker. When they sliced open the testes of the bass and examined
them under the microscope, Blazer and her colleagues discovered clusters of immature
eggs nestled where only sperm should be. This condition, not apparent to the naked eye,
is known as intersex, or TO, which stands for “testicular oocytes,” or testis-ova.
This abnormality had been reported in Western Europe (“intersexual” fish were first
reported in Europe in 1923 and in Britain in the 1940s), and it had cropped up sporad-
ically in heavily populated regions of the United States. But this was the first time inter-
sex fish had been identified in the Potomac, where they were found in unusually large
clusters in the sparsely populated region along the South Branch. “TO is a clear sign of
ecological damage in the bay's water,” Blazer said. “It's really strange.”
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