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Q & A
Dead Carp
Question
The last couple of days we're seeing a large number (several dozen) dead carp floating past our place at Long Level on the Susquehanna River. Why are these fish dying? Is it a pollution? Should we be worried about going out on the river with this fish kill going on?
Answer
The fish you are seeing most likely died as a result of a bacterial infection. Pollution can most likely be ruled out based on the fact that the dead fish are almost exclusively carp. If this were a pollution case, a wide variety of fish species would be involved.

Your observations are not uncommon at this time of year (May). The Commission annually receives reports of similar kills for the Susquehanna and some of its tributaries as well as on the Delaware River and some of its tributary streams. As temperatures warm rapidly and carp spawning is occurring, some carp become stressed. Carp, weakened by the stress, are then susceptible to infection. The last two weeks of May represent times when most anglers and biologists report carp exhibiting these symptoms.

This type of fish kill is not unusual. In fact, they're more common than many people realize. Fish, like humans, battle disease and infection all the time. Often, because relatively small numbers of fish succumb, people don't even know a kill has occurred. But just as humans sometimes endure "bad flu seasons," the naturally occurring microorganisms that affect fish (or the agents like parasites that help transmit them) are periodically more prevalent or widespread than normal. This can be a result of environmental conditions such as water flow and temperature in a given year. Carp can sometimes be more susceptible than other fish species to certain infections. Because they are aggressive spawners, carp frequently scrape themselves in the rocky shallows. As we know from our own bodies, open wounds are often the pathway for an infection to take hold.

Other than the unsightliness and smell of decaying carcasses, there's most likely no threat to humans. Much as common human illnesses like a cold usually can't be transmitted to the family dog, fish diseases rarely affect people. And while a fish kill like the one you describe can be aesthetically displeasing to us, don't forget there are scavengers such as crayfish, birds, catfish - even raccoons - that greatly benefit from the plentiful food these remains provide. There's an old saying: "Mother Nature wastes nothing."

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In 2000, the Commission received more reports of dead carp than usual, below are updates from that year.

(7/11/00) Similar carp die-offs have now been reported in the Harrisburg area, as well as the Upper Delaware River. In addition, kills have been noted in New Jersey, New York, Ohio and Ontario Canada. In working with our colleagues in other states/provinces, there has been discussion that the culprit could be viral rather than bacterial. There is a newly recognized virus, koi herpesvirus (KHV), that has been associated with mass mortality of juvenile and adult koi - a close relative of the common carp.

(7/12/00) Illinois and Michigan have also reported carp die-offs this summer. Aeromonas hydrophila (which in the past has caused carp die-offs in Wisconsin, Arkansas and the United Kingdom) and KHV are the leading suggested pathogens by biologists monitoring the kills. A flexibacter (bacteria) has been identified in the Delaware River kill. It is important to note that while it is interesting to observe widespread reports of carp kills this summer, it is also premature to assume a direct connection or single causative agent.

(7/13/00) Biologists monitoring the carp die-offs concur that given the large geographic region in which mortality has been noted, a broad reaching phenomenon (such as weather influence) seems plausible. It is unlikely any singular infectious organism or other such agent would spread that quickly across such a large area, yet leave many populations of carp unaffected. The fact that the dead carp are almost exclusively of adult size lends credence to the theory that spawning stress accentuated by fluctuation water levels and temperatures (heavy rains, winds, etc.) were the main contributing factors leading to die-offs. Fish may die due to thermal stress (large shifts in temperature after a rain) or oxygen stress (associated with runoff). Spawning fish are already under stress (including diminished immune capacity). Either a sudden or gradual shift in temperature or oxygen can further weaken a fish which may either directly kill the fish or increase the likelihood of secondary infections.

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