Using Sediment Core Samples to Examine the Spatial Distribution of Myxobolus cerebralis Actinospore Production in windy Gap Reservoir, Colorado

—Studies of the whirling disease epizootic in the upper Colorado River drainage have suggested that Windy Gap Reservoir is a source of the fish-infective actinospore of Myxobolus cerebralis. We divided the reservoir into four quadrants (12 zones) and conducted a core-sampling study to determine the spatial dynamics of actinospore production within the reservoir from late June to early November 1998. Core samples of reservoir substrate containing aquatic oligochaetes were collected, held for 24 h in filtered lake water, and then examined microscopically for actinospore production. Actinospores were produced from core samples taken in every quadrant, but the samples from one quadrant gave the overall highest estimates of actinospore production. This quadrant was in a nearly direct line between the reservoir’s inlet and outlet and encompassed parts of the historic river channel. Of the four full months of the study (July–October), average actinospore production tended to be highest in July, but the differences among months were not statistically significant. We hypothesize that M. cerebralis myxospores are carried into the reservoir by runoff events in the Fraser and Colorado River drainages and that once they are in the reservoir they settle out and provide the basis for infection of the abundant Tubifex tubifex population. Throughout most of the 20th century, the 40km reach of the upper Colorado River in Middle Park, Colorado, supported a thriving wild trout fishery. From the 1940s to the early 1990s, wild rainbow trout Oncorhynchus mykiss were the dominant species. Brown trout Salmo trutta accounted for the remainder of the salmonid population. * Corresponding author: [email protected] 1 Present address: Colorado Division of Wildlife, 2300 South Townsend Avenue, Montrose, Colorado 81401, USA. 2 Cooperators are the U.S. Geological Survey, Colorado State University, and the Colorado Division of Wildlife. 3 Present address: 19238 East Gunnison Circle, Number 101, Aurora, Colorado 80017, USA. 4 Present address: 1308 East Fourth Street, Loveland, Colorado 80537, USA. Received June 20, 2001; accepted June 24 2002 Myxobolus cerebralis, the myxosporean parasite that can cause salmonid whirling disease, was detected during routine inspections of state and private fish hatcheries in Colorado in late 1987. In acute infections, whirling disease is a debilitating malady that can affect susceptible salmonids. Young rainbow trout are particularly vulnerable to the parasite (O’Grodnick 1979; Markiw 1991). In 1988, the Colorado Division of Wildlife (CDOW) implemented a statewide testing program to determine the extent of the dissemination of M. cerebralis among captive and free-ranging trout species. During this investigation, the parasite was first detected in the upper Colorado River drainage in Middle Park. Stocked rainbow trout obtained from private ponds at two separate locations upstream from Windy Gap Reservoir tested positive for the parasite (Walker and Nehring 1995). One collection came from the Willow Creek drainage 1 km upstream of Willow Creek Reservoir (lo377 SPATIAL DISTRIBUTION OF MYXOBOLUS CEREBRALIS FIGURE 1.—Map of the upper Colorado River basin study area. The area covered by the map is approximately 820 km2. The upstream boundaries of the study area, denoted by 1s, are the Colorado River at the base of Granby Dam and the Fraser River near the town of Tabernash, Colorado. The downstream boundary of the study area, denoted by 2, is approximately 2 km west of the confluence of Corral Creek and the Colorado River. Privately cultured stocked rainbow trout collected at location A in 1988 were found to be infected with the Myxobolus cerebralis parasite. This is the most probable point from which M. cerebralis became enzootic in Windy Gap Reservoir and subsequently throughout much of the Colorado and Fraser River basins. Location B is the reach of the Colorado River from which wild adult rainbow trout infected with M. cerebralis were first collected in April 1992. cation A in Figure 1); the other collection was from the Colorado River drainage upstream of Granby Reservoir. It is highly unlikely that the actinospores of the M. cerebralis parasite that were produced upstream of Granby Reservoir would survive entrapment in the lake and escape into the Colorado River. The reservoir is deep, oligotrophic, and more than 2,900 ha in area. Therefore, location A is the most likely point from which the parasite spread into the Colorado and Fraser River drainages. Adult wild rainbow trout in the river downstream of Windy Gap Dam have been tested annually since 1983 during annual spawning operations. The parasite was first detected in such fish in April 1992 at a point 15 km downstream of Windy Gap Dam (location B in Figure 1). Intensive research in the upper Colorado River drainage in 1994 implicated whirling disease as a major factor (and possibly the decisive factor) in the disappearance of three year-classes of wild rainbow trout beginning in 1991 (Walker and Nehring 1995). The downstream boundary of the study area was 2 km west of the confluence of Corral Creek and the Colorado River. The upstream boundaries were Granby Dam on the Colorado River and the Fraser River near Tabernash, Colorado (Figure 1). Systematic sampling of rainbow and brown trout fry populations in the river during 1994–1996 showed an increasing incidence and severity of whirling disease in samples taken closer to Windy Gap Dam (Schisler 1999). This strongly suggested a source of high infectivity near Windy Gap Reservoir. Water filtration studies throughout 1997 and 1998 revealed high levels of actinospores of M. cerebralis in the outflow of the reservoir (Thompson and Nehring 2000). Sentinel fish exposures also confirmed higher infectivity at exposure sites closer to Windy Gap Reservoir than at sites upstream and further downstream (Thompson et al. 2002). Zendt and Bergersen (2000) estimated the mean oligochaete density in Windy