Madison’s Good News

Whirling disease

News of whirling disease among trout in Montana’s upper Madison River isn’t all bad.

The number of yearling rainbow trout rose considerably last fall, a sign that some young fish are surviving the disease. But the number of larger fish continues to decline, meaning the river will have fewer breeding-size fish for a few years.

The disease is caused by a parasite that deforms the skulls and cartilage of young trout and makes them swim erratically. It has been detected in 42 bodies of water around the state, but has drawn the most attention on the Madison, where rainbow trout populations are down 80 percent.

The disease can infect most salmonid fish, but it does not appear to harm brown trout, which remain strong, even in infected streams.

Researchers are studying the disease this summer.