Evaluating the Relative Abundance of Strains of Tubifex tubifex With Varying Vulnerability to Myxobolus cerebralis in Windy Gap Reservoir, Colorado Before and After a Dramatic Decline in Actinospore (TAM) Production
R. Barry Nehring and Kevin G. Thompson
Colorado Division of Wildlife
Myxobolus cerebralis (Mc), the parasite that can cause whirling disease (WD), was introduced into the upper Colorado River basin in the mid-1980s. Numerous studies have shown that:
This 103 acre reservoir provides an abundance of habitat for Tubifex tubifex (Tt), the aquatic worm host that produces and releases fish-infecting TAM spores into the water. Monthly water filtration efforts to quantify TAM production occurring in WGR began in 1997. Estimates of total annual TAM production emanating from WGR between 1997 and 2001 ranged between 557 billion and 2.17 trillion. This massive annual production of TAMs occurred even though WGR is not open to fishing, has never been stocked with trout, and gillnet surveys show that there are very few trout in the lake.
However, trends in relative abundance of TAMs detected in the outflow of WGR after 2001 have been dramatically lower than the levels seen for any previous comparable time period. This could not be explained by any management changes in the reservoir or differences in water flow and suggested that some other mechanism has played a major role in the decline in TAM production that began in 2001.
Previous collaborative studies conducted by the Colorado Division of Wildlife (CDOW) and researchers at the University of California-Davis (UCD) in the upper Colorado River basin and in WGR have demonstrated that four strains (I, III, V, and VI) of T. tubifex are present in the rivers and WGR. Laboratory studies have shown that strain III is highly susceptible to M. cerebralis, strains I and VI have a low susceptibility, and strain V is a dead-end host for the parasite and not susceptible. Those laboratory studies also demonstrated that the non-Mc susceptible strain V worms out-compete the Mc- susceptible strain III worms in mixed cultures in the laboratory. Studies by other investigators have repeatedly demonstrated that T. tubifex infected by M. cerebralis do not reproduce.
Taken together, the results of these investigations indicate that there has been a major shift in the relative abundance of T. tubifex strains in WGR away from the highly susceptible strain (III) towards those strains with lesser (I, VI) or no susceptibility (V) to M. cerebralis. In 1998, approximately 38% of the total worm population was comprised of lineage III worms compared to 2004 and 2005, when less than 15% of the total population was lineage III. This suggests that selection pressures operating on the tubificid worm strains that are susceptible to M. cerebralis have contributed to the decline in TAM release in the reservoir.