Fungus affects New Fork Brown Trout
by Wyoming Game and Fish
November 12, 2014
The Wyoming Game and Fish Department has received a number of reports from anglers noticing sick or dead brown trout in the New Fork River south of Pinedale recently.
Affected fish have been reported in the section between the confluence with Pine Creek and Highway 351.
Pinedale fish managers believe it is due to a common fungal infection called Saprolegnia. Infected fish have patches of a cotton-like growth on their skin and often are quite lethargic.
Saprolegnia is a fungus that naturally occurs in aquatic systems. A similar outbreak has recently been noted in the Big Hole River in Montana, and also has been observed locally in Pine Creek and the Green River in past years.
Brown trout are susceptible to this malady in the fall, when they can become stressed by long movements and other activities associated with spawning. The unusually warm temperatures recorded this fall may be an added stressor that has allowed the fungus to infect more fish than normal.
While such fungal infections can be treated in fish hatcheries, there is no feasible way to treat fish populations in a stream or river. Fortunately, the spawning season for brown trout is nearly finished and water temperatures should begin to drop with the onset of cooler weather, so fish managers predict the fungal outbreak will likely begin to decline soon.
Fish managers report that the number of brown trout observed with this fungus is low compared to the overall density of brown trout in the New Fork River and some fish infected with the fungus may die. However, the New Fork River should still have a healthy brown trout population next spring.