Abstract
Myxobolus cerebralis caused severe declines in rainbow trout populations across Colorado following its introduction in the 1980s. One promising approach for the recovery of Colorado’s rainbow trout populations has been the production of rainbow trout that are genetically resistant to the parasite. We introduced one of these resistant crosses, known as the GR×CRR (cross between the German Rainbow [GR] and Colorado River Rainbow [CRR] trout strains), to the upper Colorado River. The abundance, survival, and growth of the stocked GR×CRR population was examined to determine if GR×CRRs had contributed offspring to the age-0 population, and determine whether these offspring displayed increased resistance and survival characteristics compared to their wild CRR counterparts. Apparent survival of the introduced GR×CRR over the entire study period was estimated to be 0.007 (±0.001). Despite low survival of the GR×CRRs, age-0 progeny of the GR×CRR were encountered in years 2008 through 2011. Genetic assignments revealed a shift in the genetic composition of the rainbow trout fry population over time, with CRR fish comprising the entirety of the fry population in 2007, and GR-cross fish comprising nearly 80% of the fry population in 2011. A decrease in average infection severity (myxospores fish−1) was observed concurrent with the shift in the genetic composition of the rainbow trout fry population, decreasing from an average of 47,708 (±8,950) myxospores fish−1 in 2009 to 2,672 (±4,379) myxospores fish−1 in 2011. Results from this experiment suggest that the GR×CRR can survive and reproduce in rivers with a high prevalence of M. cerebralis. In addition, reduced myxospore burdens in age-0 fish indicated that stocking this cross may ultimately lead to an overall reduction in infection prevalence and severity in the salmonid populations of the upper Colorado River.
Highlights
Extirpations of wild salmonid populations have been caused by a variety of factors and have led to a focus on captive breeding to sustain or reintroduce populations [1,2,3,4]
Minimum and maximum discharge values, which were used as predictor variables affecting adult GR6CRR survival, were obtained from a USGS stream gauge located at the upstream end of the study section near the CR-57 bridge (Figure 1)
Detection probability differed with electrofishing method, with electrofishing method appearing in all six models with a DAICc,4.0 (Table 2), and was likely due to the amount of stream length covered by the two sampling methods and the season in which sampling occurred
Summary
Extirpations of wild salmonid populations have been caused by a variety of factors and have led to a focus on captive breeding (i.e., hatcheries) to sustain or reintroduce populations [1,2,3,4]. Successful reintroduction attempts using captive-reared salmonids usually involve mitigating or removing the factors responsible for the original extirpation [5]. In Colorado, introduction of Myxobolus cerebralis, the parasite responsible for salmonid whirling disease, caused the extirpation of wild rainbow trout (Oncorhynchus mykiss) populations from many of the state’s rivers. Unlike extirpations caused by factors that could potentially be mitigated or reversed, pathogens such as M. cerebralis cannot presently be removed once introduced into an ecosystem. To produce a suitable rainbow trout for reintroduction, management and research in Colorado have focused on using crosses between resistant, hatchery-derived rainbow trout and wild rainbow trout strains [9]
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