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Modifying Fish Hatcheries to Eliminate Whirling Disease Much has already been accomplished to eliminate WD from Colorado's fish hatcheries. Although more DOW fish culture facilities tested WD+ in 1996 than in 1988 or even in 1992, the level of infectivity and virulence in rainbow trout at the WD+ facilities has been dramatically reduced and in some cases controlled to the point that it is almost undetectable with standard techniques. However, controlling the spread of this fish pathogen within the state's hatchery system has proven to be a very intractable problem for two reasons. First, the parasite was widely distributed in the state's surface waters in 1987 and 1988. Once introduced into running water, this parasite is highly transmissible downstream and to a much lesser extent, upstream. Second, virtually all of the state's fish culture facilities rely on surface water supplies at some point of their fish culture operation and are, therefore, highly vulnerable to contamination. Prophylaxis and disinfection of equipment and facilities have been very effective in minimizing the spread of this pathogen from one fish culture facility to another. Most of the spread of the pathogen into the state's fish culture facilities has occurred as a result of the movement of the parasite onto the facility from contaminated surface water supplies. Fish held and reared in ponds, concrete raceways, and nurse basins using surface water have suffered the greatest exposure and infection rates. Since 1992, three hatch houses have also tested positive for the parasite as well. Clearly, more aggressive control and containment techniques are necessary. There are a number of measures shown by experience (in other states) to be highly effective in reducing or eliminating the pathogen. Some immediate solutions known to be effective are: 1. Enclose and secure hatchery water supplies in concrete. This eliminates a substrate for the tubificid worm, the obligate alternate host for the disease. Without the alternate host to produce the waterborne, fish-infective spore, the disease cannot be transmitted to the fish. 2. Eliminate all earthen ponds from fish culture facilities. According to the July 1995 Deloitte and Touche Hatchery System Analysis Final Report, five of eight WD+ facilities have earthen holding ponds used for rearing rainbow trout. Earthen ponds that become laden with organic nutrients are a prime breeding ground for the tubificid worm that transmits the disease to fish. Replacement of earthen ponds with concrete raceways that are properly maintained, cleaned, and disinfected annually should minimize the substrate for the worms. 43