Culling Diseased Trout in Zero-Kill Streams?

Rainbow trout from Hot Creek. Does it have whirling disease or not? (Photo by Ray Found)

Question: We were fishing Hot Creek in Mono County last weekend, and my friend caught a rainbow trout that looked unhealthy. We thought it might have Whirling Disease (See photo above).

Based on the picture, is this a likely case of whirling disease? Have fish with this disease been found in Hot Creek before? Assuming this was a case of whirling disease, what should we have done? We never keep fish and Hot Creek has zero-kill regulations, but it would seem wise to remove a whirling-diseased fish from the stream to give to the Department of Fish and Game (DFG) for examination. Since we were unsure, we released it. In the future, what would be the best practice for maintaining the health of the fisheries in the watershed if we knew this was a diseased fish? Could we have collected this fish to turn over to the DFG for evaluation? (Ray F.)

Answer: This may be a case of whirling disease (WD), but it’s impossible to make that determination based on the photo alone. Whirling disease afflicts juvenile fish causing neurological damage and skeletal deformation. Afflicted fish may not be able to swim in a normal manner. When startled, they “whirl” rather than darting away as a normal fish would. Survival rates for infected fingerlings are low (~10 percent), and those that do survive have difficulty feeding and become easy prey for predators. Humans cannot be afflicted with the disease.

According to DFG Senior Fish Health Coordinator Dr. Mark A. Adkison, whirling disease has a tropism for the cranial cartilage (e.g., the cranium appears turned or twisted). The disease is carried by the aquatic oligochaete Tubifex tubifex (a segmented worm) wherein spores (actinospores) develop and are released into the environment. These spores infect fish through the skin. The parasite develops in the skin for a few days and then travels through the nerves and spinal cord, eventually emerging from the nerves into the cranial cartilage where it grows and develops into its final spore stage (myxospores).

As part of the development process in young fish, the parasite consumes and deforms the cartilage. This causes the cranial deformities such as a sloped head, crooked jaw and shortened operculum so commonly seen in WD-infected fish. Since the fish in the photo does not have the characteristic cranial deformities that typically accompany such severe spinal deformities, the deformities may be due to some other cause.

Other possibilities include nutritional deficiencies or coldwater disease (CWD) which can also cause spinal deformities like the ones seen in the fish in the photo. Flavobacterium psychrophilum is a bacteria present in most, if not all trout waters of the state and is the causative agent of CWD. This disease is not a problem in the wild. It is a disease of concern in our hatcheries and it’s fairly easy to control by reducing fish densities and antibiotic treatment. Mortalities are typically acute.

Whirling disease is probably present in Hot Creek since it flows into the upper Owens River, and the upper Owens River is positive for WD. Therefore, it is likely that Hot Creek is positive for WD. The only way to tell for sure if a fish has WD is to test the fish for the presence of the WD parasite (myxospores) itself. The test is a terminal one though and not something you could do visually or perform stream side.

DFG Associate Fish Pathologist Dr. Garry O. Kelley adds that once Myxobolus cerebralis (which causes WD) is established in a natural system, it’s there for good. There’s strong evidence that suggest WD prevalence in the wild may be reduced by eliminating susceptible or infected salmonids and by reducing habitats for the other host, the aquatic oligochaete.

Reducing WD prevalence will help recruitment efforts since the parasite prefers the young of the year. If the regulations allow bag limits, then removing any deformed fish would be welcomed. Just keep in mind that a fish that grossly appears WD-positive may actually be negative, even in WD-positive waters. Specifically, the deformities could be genetic, an injury or some other pathogen (e.g., cold water disease).

As far as what to do with a diseased fish, from a biological point of view, if the fish was infected with WD to the point where it had severe deformities, it would probably be good to remove it from the creek to decrease the WD spore load in the environment. However, from the enforcement side, if it’s a no kill zone then it’s up to enforcement as to whether they would cite the fisherman for not returning the fish to the stream.

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Carrie Wilson is a marine biologist with the California Department of Fish and Game. While she cannot personally answer everyone’s questions, she will select a few to answer each week. Please contact her at

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