By Doug Krieger
CPW Aquatic Biologist, Colorado Springs
We receive calls yearly about dead or dying brown trout along the Arkansas River (and elsewhere). Unless there is some special unreported circumstance (chemical spill, oil sheen present, etc), this fish loss is due to naturally occurring post-spawning mortality.
Brown trout spawn in the Arkansas from mid-October to mid-November. Typically fish will be found in deeper pools or other areas of slack waters where current deposits them.
Fish will be dead, or lethargic. Commonly they will have grayish, fuzzy looking patches on their skin, sometimes covering the majority of their bodies. Saprolegnia, an external fungus, causes this growth.
Brown trout are highly territorial, aggressive during spawning and sometimes make migration. These factors lead to physical aberrations that eliminate the usual occurring mucous and allow the fungus to become established. Further, the fish have been in the rigors of spawning for several weeks and have become weakened. The fungus is not the cause of the mortality (physical abuse & stress is), but is a secondary cause that further weakens the fish.
Anglers, if you have caught a fish with this fungus, there is no risk to human health by eating these fish. Although unsightly, we suggest you cut away affected areas before cooking.
Mortality on brown trout may be higher this year because of warmer temperatures we have experienced. his leads to more stress and more fungus. However, we normally see some level of mortality each year. Although it may look like the whole river is “dead,” we estimate about 10-20% of adult fish are affected or lost. The populations should do fine in spite of this loss.
The Arkansas River may not be the only river in Colorado affected by this malady. It may, however, be more noticeable along this stretch.