General Risk Assessment

This is only slightly updated from my preliminary risk assessment, which has fared well even with much new data. This will continue to be general, so that the risk of any water body might be predicted by any person familiar with it. For more detailed risk assessments see the Tubifex summaries by river system as they develop.

Based on samples from many whirling disease positive sites, barely detectable levels of Tubifex are enough to cause major disease problems for the fish. It is important to note that even 50 worms per square meter would be hard to find in most samples, but these could add up to millions per mile. That should be enough to swamp the fish with parasites! The spores, worms and young fish are naturally concentrated in many streams.

Low Risk Areas

  • Wild and unimpacted Rocky Mountain streams and rivers- Streams and rivers that have their normal, highly diverse insect community intact make very poor Tubifex tubifex habitat. This includes many headwater streams, but fewer rivers. Note that an upstream tributary that has Tubifex and whirling disease, may cause problems in a stream that otherwise looks safe from whirling disease.

     

  • Warm trout waters- Tubifex appears to drop out of warm water before trout do, at least during the critical summer period when the young-of-the-year fish are most vulnerable to the parasite. In Montana these will generally be rivers rather than streams. For the entomologist, this transition occurs about when Claassenia outnumbers Hesperoperla. These waters will be fairly comfortable swimming nearly all day, during the summer. Ilyodrilus templetoni is often very abundant in these places. Again, an upstream tributary could be a TAM source and thus threaten these waters.

     

  • Lake outflows- Tubifex is conspicuously missing from the outflows of large, natural lakes. Even when the inflow to the lake is loaded with Tubifex, the outflow has Tubifex greatly reduced. Many of these streams have poor communities and it seems probable that temperature is important. Rhyacodrilus coccineus is often very abundant in these places. Lakes also seem to block the downstream movement of the free parasite.

High Risk Areas

  • Spring Creeks- Tubifex tubifex is normally present in almost all Montana spring creeks where trout live. It usually occurs in great abundance along with handful of other species that are also adapted to the unusual biological conditions found in springs. Persons making a "quick visit before dark" to a spring creek after "a day on the river" should be especially careful not introduce viable biological material of any kind.

     

  • Tailwater streams and rivers- No other single human-caused disturbance so quickly and clearly reduces the high biodiversity of Rocky Mountain streams and encourages Tubifex tubifex. The tailwaters which are not already known to be infected with the disease should be protected to the extend possible. In particular, the Big Horn River, the Madison River between Hebgen Dam and Quake Lake, and the West Fork of the Bitterroot River all deserve special care.

     

  • Impacted Streams and Rivers, and all canals- Almost any stream or river that is significantly impacted by urban, agricultural or foresty practices is likely to support Tubifex tubifex. Grazing with its sediment and manure loading, is the most frequent cause of Tubifex in Montana streams.

     

Lakes

The dynamics of whirling disease in lakes is not known. Most salmonids spawn and rear for some time in streams anyway. Tubifex is generally absent in the warm shallow water of lakes, but it is probably abundant in the deep parts of most Montana lakes. As with spring creeks, springs in lakes should support Tubifex. These springs may also be the only place salmonids reproduce in many lakes. These should be high risk for whirling disease. Very cold, high mountain lakes have worms that are probably Tubifex living even in shallow, shoreline areas (so far, I have seen only immature worms). Again, salmonids may spawn in these areas. Persons returning to a high mountain lake campsite with fish that were caught elsewhere should not clean those fish in or near the lake.

22 JUNE 1996, updated on 25 FEB 1997 D.L. Gustafson
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