Importance of Fish Age vs. Size one of Several New Whirling Disease Projects

by Annette Trinity-Stevens
MSU Research Editor

Whirling disease, discovered first in the Madison River in 1994, attacks the cartilage of young fish, causing them to swirl in a circular pattern and making them easy pickins= for predators. The disease, most prevalent among rainbow trout, is caused by a parasite.

But research shows that once a rainbow fry reaches the ripe old age of two months, it=s far less likely to die from the disease. Whether it=s the age that protects the fish or the size isn=t clear, but a new study at Montana State University may offer some answers as early as next spring.

AIt=s when we=re dealing with these little guys that things are really interesting,@ said Al Zale, acting leader of the Cooperative Fisheries Research Unit at MSU.

Zale and graduate student Eileen Ryce will start the age vs. size study this summer using about 2,000 rainbow fry. It=s one of seven new whirling disease studies funded in Montana this year by the National Partnership for the Management of Wild and Native Coldwater Fisheries.

If age is behind the greater resistance, that could mean the fish needs to be old enough to have a well-developed immune system, said Zale. If it=s size that counts, that may signal that larger fry have more bone. Since the parasite attacks cartilage, greater bone density would protect the fish.

ASo it might be that if the fish is younger [than two months], bigger and fatter might be better,@ he said. AOn the other hand, it may be two months no matter what.@

In another project, MSU fisheries professor Tom McMahon will continue monitoring the disease=s prevalence in a section of the Missouri River stretching from Holter Dam to Cascade. The study site includes three major spawning tributaries--the heavily infected Prickly Pear Creek, the lightly diseased Sheep Creek and the Dearborn River, which seems clear of the ailment.

So far the disease=s impact has been less severe in the Missouri River than in the Madison, where rainbow populations plummeted by 90 percent.

AThis study complements the Madison River [studies] because the trout behave differently,@ said McMahon.

Missouri River trout spawn in tributaries rather than in the main stem, so McMahon wants to know whether populations from healthy tributaries can keep the overall numbers from collapsing. Graduate student Andrew Munro is examining Aear stones,@ or chips of calcium carbonate in a fish=s ear, to link adult fish back to where they grew up. If the fish have whirling disease, then scientists can use the chemical make-up of the ear stones to pinpoint the stream or creek where they got sick.

AThe unique thing about this study is it began when whirling disease was first detected [in the Missouri],@ McMahon said. AWe=ve been able to follow how fast the disease has spread and have documented its impact as time has gone on.@

He estimated it will take up to five years to fully measure change because the oldest fish in the three-year study haven=t yet returned to their tributaries to spawn.

Other Montana projects funded this year include:

$ A study led by the U.S. Fish and Wildlife Service in Bozeman to create an accurate field method of collecting and counting the infective form of the parasite called a TAM;

$ A project based at the University of Montana focusing on whirling disease in the Rock Creek drainage;

$ Additional research on spawning and rearing sites in the Madison River aimed at minimizing the risk of infection among rainbow fry;

$ A project to uncover whether genetic differences among the aquatic worms that harbor the parasite have an impact on the severity of the disease in wild trout;

$ Development of a method to identify early infection in the aquatic worms that spread the disease.

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Send questions or comments to Annette Trinity-Stevens at annettet@montana.edu

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