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By Root 1940 0

’s state fish), Bull Trout, Arctic Grayling, and Whitefish. All of those species except Whitefish have now declined in Montana from a combination of causes whose relative impact varies among the species: less water in the mountain streams where they spawn and develop, because of water removal for irrigation; warmer temperatures and more sediment in those streams, because of logging; overfishing; competition from, and in some cases hybridization with, introduced Rainbow Trout, Brook Trout, and Brown Trout; predation by introduced Northern Pike and Lake Trout; and infection by an introduced parasite causing whirling disease. For example, Northern Pike, which are voracious fish-eaters, have been illegally introduced into some western Montana lakes and rivers by fishermen fond of catching pike, and have virtually eliminated from those lakes and rivers the populations of Bull Trout and Cutthroat on which they prey. Similarly, Flathead Lake’s formerly robust fishery based on several native fish species has been destroyed by introduced Lake Trout.

Whirling disease was accidentally introduced into the U.S from its native Europe in 1958 when a Pennsylvania fish hatchery imported some Danish fish that proved to be infected with the disease. It has now spread throughout most of the western U.S., partly through transport by birds, but especially as a result of people (including government agencies and private fish hatcheries) stocking lakes and rivers with infected fish. Once the parasite gets into a body of water, it is impossible to eradicate. By 1994 whirling disease had reduced the Rainbow Trout population of the Madison River, Montana’s most famous trout stream, by more than 90%.

At least whirling disease is not transmissible to humans; it is merely bad for fishing-based tourism. Another introduced disease, chronic wasting disease (CWD) of deer and elk, is of more concern because it might cause an incurably fatal human illness. CWD is the deer/elk equivalent of prion diseases in other animals, of which the most notorious are Creutzfeldt-Jakob disease in humans, mad cow disease or bovine spongiform encephalopathy (BSE) of cattle (transmissible to humans), and scrapie of sheep. These infections cause an untreatable degeneration of the nervous system; no human infected with Creutzfeldt-Jakob disease has ever recovered. CWD was first detected in western North American deer and elk in the 1970s, possibly (some people suggest) because deer housed for studies at a western university in a pen near scrapie-infected sheep were released into the wild after completion of the studies. (Today, such a release would be considered a criminal act.) Further spread from state to state was accelerated by transfers of exposed deer and elk from one commercial game farm to another. We do not know yet whether CWD can be transmitted from deer or elk to people, as can mad cow disease, but the recent deaths of some elk hunters from Creutzfeldt-Jakob disease have raised alarms in some quarters. The state of Wisconsin, concerned that fear of transmission could cripple the state’s one-billion-dollar-per-year deer hunting industry, is in the process of killing 25,000 deer (a desperate solution that sickens everybody involved) in an infected area in hopes of controlling the CWD epidemic there.

While CWD is potentially Montana’s most frightening problem caused by an introduced pest, introduced weeds are already Montana’s most expensive such problem. About 30 noxious weed species, mostly of Eurasian origin, have become established in Montana after arriving accidentally in hay or as wind-blown seeds, or in one case being introduced intentionally as an attractive ornamental plant whose dangers weren’t anticipated. They cause damage in several ways: they are inedible or poorly edible to livestock and wild animals, but they crowd out edible plant species, so they reduce the amount of livestock fodder by up to 90%; some of them are toxic to animals; and they may triple rates of erosion because their roots hold the soil less well than do roots of native grasses.

Economically,