A mysterious illness is attacking the carp and catfish in the tributaries of the upper Chesapeake Bay, killing some and leaving others sick with kidney damage and bulging eyes.
But so far, biologists with the Maryland Department of Natural Resources haven't figured out whether the cause of the sickness is some natural disease or toxic contamination.
Officials have been getting reports of problems for several years, but this winter and spring biologists began a more in-depth look for the cause and have collected samples of fish tissue as well as water and mud from the bottom of rivers.
"We are pursuing a wide-scale investigation with no indication of source or causation. We may have a great deal more information within a month or a month and a half," said Eric May, a biologist with the DNR's fish health and disease program.
Known as "trash fish" to finicky sports fishing enthusiasts, carp and catfish are not the most popular for catching or eating. However, at least several commercial fishermen make a good portion of their living from these bottom-feeding species, said W. Peter Jensen, director of fisheries for the natural resources agency.
The agency has not suggested that people stop eating carp, catfish or any other fish living in the Chesapeake or its brackish tributaries. "It is nothing we are alarmed about," said Mr. Jensen. The state simply wants to "understand it better."
It is not unusual to observe small or even sometimes large fish kills in pockets of the Chesapeake Bay and its tributaries in the spring and summer months when the water is depleted of oxygen because of pollution.
However, the fish kills being studied have occurred during the winter and in a wide area -- from the Bohemia River on the Eastern Shore south to Back River on the western shore. And the problems seem to be limited to the tributaries of the bay, not occurring in the bay itself.
The magnitude of the problem also is unknown, since the fish deaths are not occurring in a small area and therefore can't easily be counted.
Perhaps most mysterious is why the illness is hitting carp and catfish, two species believed to be among the most resistant to pollution. "They are generally regarded as tough fish that can survive in less than optimum conditions," Mr. Jensen said. However, fisheries biologists have not done much research on those fish.
"There are indications that the fish are undergoing something unusual," Mr. Jensen said. "It is intriguing from our point of view."
One of the first signs that something was wrong came when a commercial fisherman who ships live carp out of state told state officials he had caught a batch that looked and behaved abnormally, said John Goheen, a spokesman for the Maryland Department of the Environment, which is assisting the investigation.