We used a quantitative genetics approach and estimated broad sense heritability (h2b) of myxospore count and the number of genes involved in myxospore formation to gain a better understanding of how resistance to Myxobolus cerebralis, the parasite responsible for whirling disease, is inherited in rainbow trout Oncorhynchus mykiss. An M. cerebralis-resistant strain of rainbow trout, the German Rainbow (GR), and a wild, susceptible strain of rainbow trout, the Colorado River Rainbow (CRR), were spawned to create 3 intermediate crossed populations (an F1 cross, F2 intercross, and a B2 backcross between the F1 and the CRR). Within each strain or cross, h2b was estimated from the between-family variance of myxospore counts using full-sibling families. Estimates of h2b and average myxospore counts were lowest in the GR strain, F1 cross, and F2 intercross (h2b = 0.34, 0.42, and 0.34; myxospores fish-1 = 275, 9566, and 45780, respectively), and highest in the B2 backcross and CRR strain (h2b = 0.93 and 0.89; myxospores fish-1 = 97865 and 187595, respectively). Comparison of means and a joint-scaling test suggest that resistance alleles arising from the GR strain are dominant to susceptible alleles from the CRR strain. Resistance was retained in the intermediate crosses but decreased as filial generation number increased (F2) or backcrossing occurred (B2). The estimated number of segregating loci responsible for differences in myxospore count in the parental strains was 9 ± 5. Our results indicate that resistance to M. cerebralis is a heritable trait within these populations and would respond to either artificial selection in hatcheries or natural selection in the wild.

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.

Portable radio frequency identification (RFID) PIT tag antenna systems are increasingly being used in studies examining aquatic animal movement, survival, and habitat use, and their design flexibility permits application in a wide variety of settings. We describe the construction, use, and performance of two portable floating RFID PIT tag antenna systems designed to detect fish that were unavailable for recapture using stationary antennas or electrofishing. A raft antenna system was designed to detect and locate PIT-tagged fish in relatively long (i.e., ≥10 km) river reaches, and consisted of two antennas: (1) a horizontal antenna (4 × 1.2 m) installed on the bottom of the raft and used to detect fish in shallower river reaches (<1 m), and (2) a vertical antenna (2.7 × 1.2 m) for detecting fish in deeper pools (≥1 m). Detection distances of the horizontal antenna were between 0.7 and 1.0 m, and detection probability was 0.32 ± 0.02 (mean ± SE) in a field test using rocks marked with 32-mm PIT tags. Detection probability of PIT-tagged fish in the Cache la Poudre River, Colorado, using the raft antenna system, which covered 21% of the wetted area, was 0.14 ± 0.14. A shore-deployed floating antenna (14.6 × 0.6 m), which covered 100% of the wetted area, was designed for use by two operators for detecting and locating PIT-tagged fish in shorter (i.e., <2 km) river reaches. Detection distances of the shore-deployed floating antenna were between 0.7 and 0.8 m, and detection probabilities during field deployment in the St. Vrain River exceeded 0.52. The shore-deployed floating antenna was also used to estimate abundance of PIT-tagged fish. Results suggest that the shore-deployed floating antenna could be used as an alternative to estimating abundance using traditional sampling methods such as electrofishing. Received December 19, 2013; accepted July 1, 2014

Stream habitat fragmentation caused by manmade structures is ubiquitous in Colorado, creating a need for passage solutions that accommodate multiple fish species. This study tested the effectiveness of a rock‐ramp fishway to pass nine fish species with a range of swimming abilities. Target species for fishway design included Brassy Minnow Hybognathus hankinsoni (weakest swimming), Longnose Dace Rhinichthys cataractae, Longnose Sucker Catostomus catostomus, and Brown Trout Salmo trutta (strongest swimming). Testing included a 46‐hour enclosure study and three‐month extended study, during which fish passage was evaluated using PIT tags. All species exhibited successful passage through the fishway during the enclosure study, but movement probabilities varied by species. Five species were not detected at the fishway during the extended study, possibly due to issues with attraction flows, entrance conditions, or motivation. Hydraulic conditions within the fishway were also evaluated. Roughness elements maintained a benthic, low‐velocity zone across a range of flows, even when surface and depth‐averaged velocities surpassed design criteria for the weakest‐swimming species. The methods from this study could be replicated at other locations to evaluate design criteria (e.g., slope, capacity, roughness, and configuration) and performance for a variety of fish species and fishway types.

Walleyes Sander vitreus support important recreational and commercial fisheries, and have been introduced into many systems throughout the United States and Canada. Use of sterile triploid fish is a valuable management strategy for protecting native fish species from naturally reproducing populations of introduced predators like Walleyes. Hydrostatic pressure is one of the more effective methods for inducing triploidy in Walleyes, and the technique is currently evolving. A 2.7‐L capacity electric pressure chamber (TRC‐APV Aqua Pressure Vessel; TRC Hydraulics) was used to produce triploid Walleyes using hydrostatic pressure with eggs collected from Pueblo Reservoir, Colorado. The TRC‐APV has many advantages over manually operated pressure chambers including its portability, large capacity, and safety. Here, the methods used to create triploid Walleyes using the TRC‐APV and the flow cytometric methods for estimating the efficacy of ploidy induction from pooled samples of fry are described. In addition, times of initiation (TIs), the times between fertilization and when eggs were subjected to pressure, were varied to determine the optimal time for maximizing induction and hatching success rates. Results suggest that triploidy in Walleyes is maximized with a TI of approximately 7.5 min, whereas hatching success is maximized with a TI of just over 8 min.