Bacteria Implicated in Perch Kill Puzzle
Recent fish deaths blamed on
Vibrio anguillarum
Chris Luckett's phone started ringing off the hook on
May 16. In the week to follow, the Maryland Department of the
Environment (MDE) fish kill expert would log over 60 calls from anglers
and boaters in multiple tributaries of the Chesapeake Bay, on both the
Eastern and Western shore. These calls reported large numbers of dead
white perch in the water, many with severe hemorrhaging around the eyes
and fins. Now scientists have found the culprit - a bacterium called
Vibrio anguillarum.
V. anguillarum is not a
stranger to the Chesapeake Bay ecosystem, although some suspect that it
may have originally been introduced through ballast water dumped into
the Bay, perhaps many decades ago. Currently, the microbe is widespread
and often detectable in multiple fish species and oysters. Rarely,
however, has this bacteria been linked to a large-scale fish kill.
"I have been working on the Bay for 15 years and this is the first fish kill I have seen caused by
V. anguillarum,"
says fish microbiologist Ana Baya at the Maryland Department of
Agriculture Animal Health Laboratory in College Park, who diagnosed the
responsible organism. Other Vibrio species have been implicated in fish
mortality events, but not this one, she says.
Scientists suspect that May's unusually warm temperatures are in
part to blame for inducing a relatively common marine bacterium to
cause a widespread mortality event.
V. anguillarum
becomes active in a very narrow temperature range, and it dies off at
surface temperatures greater than 24 degrees Celsius, explains MDE's Luckett. A rapid
increase in water temperature could cause a sudden increase in the
activity of bacterial populations, at the same time stressing the fish.
"It is also important to remember that even naturally occurring
acute thermal stress can act as a kind of 'pollution' in the marine
environment," says Maryland Sea Grant water quality specialist Dan
Terlizzi at the Center of Marine Biotechnology in Baltimore.
"Temperature causes key changes in the bacterial and planktonic
communities."
And since close to 99 percent of the fish that succumbed to
V.
anguillarum infections were white perch, Luckett suggests that fish
abundance might be a contributing factor, given that high densities can
increase the rate of disease transmission between fish. Other than
specific symptoms related to the disease, the perch looked quite
healthy, he says, "but there certainly are a lot of white perch in the
Bay right now."
This white perch mortality event seems to have run its course and,
at least for now, Luckett's phone has finally quieted down. No doubt he
will receive calls about many more fish kills before the summer ends,
but he thinks this one will be the season's most talked about. "Disease
events are not usually this widespread or species specific. It
certainly generated a lot of phone calls," he says.
- Erica Goldman
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