Establishment of Myxobolus cerebralis (Mc) resulted in declines of wild Rainbow Trout Oncorhynchus mykiss populations in streams across Colorado during the 1990s. However, the risk for establishment and spread of this parasite into high‐elevation habitats occupied by native Cutthroat Trout O. clarkii was unknown. Beginning in 2003, tubificid worms were collected from all major drainages where Cutthroat Trout were endemic and were assayed by quantitative PCR to determine the occurrence and distribution of the various lineages of Tubifex tubifex (Tt) oligochaetes. Over a 5‐year period, 40 groups of Tt oligochaetes collected from 27 streams, 3 natural lakes, 2 private ponds, and a reservoir were evaluated for their relative susceptibility to Mc. Exposure groups were drawn from populations of pure lineage III Tt, mixed‐lineage populations where one or more of the highly resistant (lineage I) or nonsusceptible lineages (V or VI) were the dominant oligochaete and susceptible lineage III worms were the subdominant worm, or pure lineage VI Tt. Experimental replicates of 250 oligochaetes were exposed to 50 Mc myxospores per worm. The parasite amplification ratio (total triactinomyxons [TAMs] produced / total myxospore exposure) was very high among all pure lineage III Colorado exposure groups, averaging 363 compared with 8.24 among the mixed‐lineage exposure groups. Lineage III oligochaetes from Mt. Whitney Hatchery in California, which served as the laboratory standard for comparative purposes, had an average parasite amplification ratio of 933 among 10 exposed replicates over a 5‐year period. Lineage I oligochaetes were highly resistant to infection and did not produce any TAMs. Lineages V and VI Tt did not become infected and did not produce any TAMs. These results suggest that the risk of establishment of Mc is high for aquatic habitats in Colorado where Cutthroat Trout and lineage III Tt are sympatric. Received April 19, 2013; accepted November 4, 2013

Electrofishing studies in the 1990s established that higher frequencies of pulsed DC (e.g., 60 Hz) generally result in more spinal injury to fish in comparison with lower pulse frequencies (e.g., 30 Hz). In response to those findings, some agencies adopted low pulse frequency standards to minimize fish injury. However, those earlier studies did not assess whether capture efficiency (CE) was also influenced by pulse frequency. We sampled small trout streams (1–6‐m average width; SE = 0.14 m) by backpack electrofishing with settings of 30 and 60 Hz to evaluate the effect of pulse frequency on both CE and spinal injury rates for Cutthroat Trout Oncorhynchus clarkii, Rainbow Trout O. mykiss, and Brook Trout Salvelinus fontinalis. Duty cycle was held constant at 24% and average power output was held at approximately 100 W. Using a four‐pass removal protocol, cumulative CE (all four passes) averaged 0.84 for 30‐Hz reaches and 0.94 for 60‐Hz reaches. Capture efficiency in pass 1 averaged 0.59 for 30‐Hz reaches and 0.75 for 60‐Hz reaches and declined with successive passes using both pulse frequencies. X‐ray images revealed vertebral compressions and misalignments for 4% of fish captured with 30 Hz (n = 230) and 4% of those captured with 60 Hz (n = 222); no fractured vertebrae were observed. No spinal injuries were observed in control fish that were captured via angling (n = 92). Our results indicate that in small streams where trout are generally less than 300 mm TL, backpack electrofishing with 60 Hz will result in greater CE, improved trout occupancy and abundance estimates, and no increase in spinal injury.

Salmon in the Klamath River of northern California contend with water temperatures that reach stressful and sometimes lethal levels during summer, forcing them to seek thermal refuge at coolwater tributary junctions. During migration, these fish also encounter a range of pathogens that affect their survival. A significant myxozoan parasite, Ceratonova shasta, causes enteronecrosis in salmon, and this disease increases in severity as temperature and parasite dose increase. In complementary laboratory and field studies, we examined how the use of a thermal refuge (an area at least 2°C colder than the main stem) affects progression of enteronecrosis in juvenile Chinook Salmon Oncorhynchus tshawytscha and Coho Salmon O. kisutch. We compared fish use, water temperature, and C. shasta concentration in a refuge at the Beaver Creek–Klamath River confluence during the summer in 2008 and 2010. Salmonid numbers ranged from 190 to 2,125, and temperatures were 2–8°C cooler than in the main stem. In June and July of 2008, parasite levels in the refuge were lower than in the main stem, where they exceeded 100 spores/L. In 2010, main‐stem parasite levels did not exceed 10 spores/L, and levels in the refuge were lower in June. In the laboratory, we compared the effect of fluctuating and constant temperature treatments on mortality rates of Chinook Salmon and Coho Salmon exposed to C. shasta. Under most experimental conditions, fluctuating temperature, within the range experienced by fish using thermal refuges (15.5–21°C), had no significant effect on disease progression compared with a constant midrange temperature (18.5°C) with equivalent degree‐day accumulation. We propose that in the Klamath River thermal refuges can function as disease refuges from enteronecrosis by (1) providing areas of decreased C. shasta exposure and/or (2) alleviating disease effects as a result of relatively lower water temperatures. The trend of increasing water temperatures suggests that juvenile salmon will rely even more on these critical habitats in the future.Received October 9, 2015; accepted February 25, 2016 Published online June 22, 2016