Field Exposure of Seven Species or Subspecies of Salmonids toMyxobolus cerebralisin the Colorado River, Middle Park, Colorado

Abstract

Recent failures in recruitment of rainbow trout Oncorhynchus mykiss in the Colorado River in Middle Park, Colorado, USA, led to studies of the effect of the myxosporean parasite Myxobolus cerebralis, the causative agent of whirling disease, on the wild trout fishery in the river. conducted field exposures of sentinel fish to examine the vulnerability of seven species or subspecies of salmonids exposed to the parasite in the Colorado River. During 1995-1996, brook trout Salvelinus fontinalis and Colorado River cutthroat trout O. clarki pleuriticus experienced 85% or higher mortality within 4 months of exposure. Groups of fish introduced at greater mean weight or later in the summer tended to survive better than others, but mortality often was severe among these groups during the second summer. By April 1996, dead fish or sacrificed fish representing seven groups were shown by histology to contain mature M. cerebralis spores. During 1996-1997, three groups of cutthroat trout O. clarki spp. experienced poorer survival than brown trout Salmo trutta and rainbow trout introduced at similar weights. Snake River cutthroat trout O. c. bouvieri introduced in July survived well and developed fewer M. cerebralis spores (prevalence and mean concentration) than any group except brown trout. Rainbow trout parented by fish that recruited after M. cerebralis was established in the drainage developed significantly fewer myxospores than those parented by fish that recruited before establishment of M. cerebralis (P Յ 0.0001), but survival was not different between these groups (87% versus 89%, respectively). The poor performance of parasitized brook trout and cutthroat trout suggests that establishment of M. cerebralis in high-country streams may have serious consequences for these species.

Response of rainbow trout Oncorhynchus mykiss to exposure to Myxobolus cerebralis above and below a point source of infectivity in the upper Colorado River

“…Two locations were immediately downstream of Windy Gap Reservoir in the 'Spill Basin' and in the alternate outlet channel that we termed the 'North Outlet'. These were necessary choices to deter- (Thompson et al 1999). This was also where all fish were held after the initial exposure period at different locations, since it remains ice-free during the winter months.…”

“…A 'block' consisted of two 3.7 m livestock feed troughs, each modified into a flow-through floating tank with 4 quadrant cells (see Thompson et al 1999), and tethered adjacently in the river. One cell of each tank was randomly chosen to hold the test fish to obtain 2 replicates at each exposure location.…”

“…Halliday (1973Halliday ( , 1976 found acid-fast myxospores that he called mature in as little as 840 to 884 degree-days (120 d at 7°C or 52 d at 17°C). We previously observed myxospores by PTD at 1255 degree-days in rainbow trout Oncorhynchus mykiss, and in several subspecies of the cutthroat trout O. clarki (Thompson et al 1999, July 9, 1996 to April 8, 1997; however, degree-days are not detailed in the paper). El-Matbouli et al (1992) found that myxospores could be demonstrated at 1440-1530 degreedays (90 d at 16 to 17°C, or 120 d at 12 to 13°C).…”

A Review: Gaps in Our Knowledge on Myxozoan Parasites of Fishes

“…One explana tion of this inconsistent production of antibodies is that myxozoans may mimic antigens of host fish (Pauley, 1974). Acquired resistance to reinfection with PKD and whirling disease has been documented (Klontz et al, 1986;Foott and Hedrick, 1987;Hedrick et al, 1998) Introduction of the resistant strains The genetic resistance of certain salmonid fish spe cies or strains to C. shasta and M. cerebralis have been reported (Bartholomew, 1998;Hedrick et al, 1999aHedrick et al, , 1999Thompson et al, 1999). However, salmonid strains resistant to C. shasta were found to be suscep tible to M. cerebralis, suggesting that the mechanisms of resistance are different for the two myxozoans (Hedrick et al, 2001).…”

Accelerated deactivation of Myxobolus cerebralis myxospores by susceptible and non-susceptible Tubifex tubifex

“…During the 1990s, numerous investigations that included water filtration studies to estimate TAM production (Thompson & Nehring 2000, Nehring et al 2003, aquatic oligochaete studies (Zendt & Bergersen 2000) and sentinel fish exposure experiments (Thompson et al 1999 demonstrated that Windy Gap Reservoir (WGR) in Grand County, Colorado, was a major point source of M. cerebralis infectivity and a primary factor in the WD epizootic that decimated the wild rainbow trout population in the upper Colorado River downstream of the reservoir (Nehring & Thompson 2001, Nehring 2006. Between April 1997 and March 1998 and April 1998 and March 1999, total annual numbers of TAMs in the discharge from WGR were estimated to be 960 × 10 9 and 1.8 × 10 12 , respectively .…”