Lab tests have ruled out toxic algae and bacteria as the primary cause of the carp die off at Lake Mohave, and instead confirmed that the main culprit is a disease called koi herpes virus. KHV is not a human health hazard, thank goodness, and it can’t spread to other fish. But it sure is bad news for carp. KHV is deadly, on the move, and here to stay.
Back in 1998, fish farms in Israel began experiencing mysterious die offs of ornamental carp (koi) and common carp being raised for food. Unregulated trade allowed the disease to subsequently spread to other areas of the world, including parts of East and Southeast Asia, Europe, and North America.
The culprit was a new DNA-based viral disease, carp interstitial nephritis and gill necrosis virus (CNGV). Outside of technical circles it is more commonly called koi herpes virus (KHV).
Whatever name it may go by, this disease is virulently infectious, very lethal, and not easily controlled. When KHV is present in ponds, nearly all the fish are infected and the mortality rate is 80 to 90 percent. The fish that survive are immunized, but still carry the disease as long as they live. That means that they can infect carp that lack protective antibodies. Unfortunately, newly infected carp tend to die in about a week, which is roughly two weeks short of the time needed to develop protective antibodies. As you might suspect, koi hobbyists have had a devil of a time dealing with KHV, and losses in the carp food industry have been quite severe.
It’s fairly easy to recognize KHV-infected carp. The major tell-tale symptom is gills that are bleeding, mottled, or patchy red and white in appearance. Infected fish also tend to have sunken eyes and skin with pale patches and/or blisters. Because KHV progressively destroys gill function, thereby cutting off the fish’s oxygen supply, carp dying of the disease are commonly seen gasping at the surface.
KHV has now invaded carp communities in the American southwest. The evidence is clearly to be seen – and smelled – on the Lake Mohave shoreline, where dead carp have been washing up by the hundreds. The disease has already spread to Lake Havasu, too, and we can expect die offs to be reported elsewhere in the region.
At Lake Mohave, as at other places with carp die offs, there is little that managers can do except educate recreationists about the situation, conduct cleanup operations where necessary, and wait for the disease to run through its current epidemic phase and subside. Meanwhile, marinas, boat rentals, and tour operations remain open
Since KHV cannot infect humans or other fish, Lake Mohave visitors need to know that the carp die off does not threaten them with anything worse than some aesthetic insults, some inconvenience, and a halt to carp fishing fun. The dead carp washing up on the shore in some places look bad and smell awful, that’s for sure, but visitors can seek out more suitable places. Even where dead fish are present, the water is safe for swimming. Parents should tell their kids to leave those yucky carcasses alone, but it’s very unlikely that handling a dead carp would be harmful. As a precaution, anglers should avoid catching carp and concentrate on other species for at least the time being.
Maintenance crews have been removing dead carp from the Lake Mohave shoreline in some developed areas, including Katherine Landing and Cottonwood Cove. Since the lake has about 200 miles of shoreline, there’s simply no way that maintenance crews could remove all of the dead carp from all of the beaches. Fortunately, fisheries biologists have seen some evidence suggesting that the carp die off at Lake Mohave may already be subsiding. Eventually, the situation should enter a near-dormant stage, with the great majority of the carp possessing enough protective antibodies against KHV.
Postscript: It seems doubtful that many will consider a carp die off as genuine wildlife tragedy. Carp can be fun to catch, especially the 40-pounders, but most Americans consider the bottom-feeder to be a trash fish and few find them good to eat. There is a folk recipe that goes like this: "Place medium-sized carp on a cedar board and bake for two hours at 315 degrees. Then throw away the carp and eat the board." In a more serious vein, I've tried various carp recipes and found that I can stomach the oily flesh if it's apple-smoked just right. I've also eaten carp stew (in Segetz, Hungary) that wasn't so bad.