The Colorado Division of Wildlife recently revealed that it is breeding and stocking rainbow trout that appear to be resistant to the parasitic spore that causes whirling disease, a fish-borne ailment that has severely impacted the species in some of the state’s most popular streams.

According to the DOW, the disease-resistant fish are the result of experimental breeding that crossed Colorado River rainbow trout with a resistant strain of trout from Germany. Stocking is expected to begin next year.

Whirling disease is caused by a parasite that penetrates the head and spinal cartilage of fingerling trout. The parasites multiply and affect equilibrium, causing the fish to swim erratically. It also affects feeding and the ability to avoid predators.

In experiments beginning in 2003, crossbred fish were exposed to high doses of the parasite, and those that showed the greatest resistance were kept to breed the next generation. About 20,000 of those disease-resistant fish have been stocked in the Gunnison River, and the results have been encouraging to biologists.