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.
Fish and microhabitat data were collected at 542 prepositioned electrofishing sites (surface area of each site = 4 m2) in the Kootenai River, Idaho, during 2014 and 2015 to evaluate small‐scale habitat use by fishes, as it relates to large‐scale habitat rehabilitation efforts. Samples were collected from a 12‐km braided segment of river that had received localized habitat rehabilitation treatments since 2011. Fish and microhabitat data were collected to investigate habitat drivers related to the occurrence and relative abundance of fishes. Each sampling location was selected at random and characterized as “treated” (i.e., rehabilitated) or “untreated” based on proximity to habitat treatments. Fishes sampled from backwaters composed 71% of the overall catch and 84% of the catch from locally untreated areas of the river. Species‐specific regression models suggested that water depth and current velocity influenced the occurrence and abundance of fishes. In particular, shallow habitats with low current velocities were important for small‐bodied native fishes and likely serve as important rearing areas for juvenile fish. These habitat conditions typically characterize backwater and channel‐margin habitats that are vulnerable to anthropogenic perturbation. Prioritizing process‐based rehabilitation of these areas in large, regulated rivers would allow natural channel‐forming processes for the benefit of native fishes.
Catchable-sized hatchery trout (hereafter, catchables) have become a staple component of many fisheries management programs throughout North America. Due to their size, catchables create immediate fisheries once they are stocked, and fisheries managers have gradually shifted towards stocking fewer, larger trout. However, the cost of growing larger fish may reduce the efficiencies of catchable stocking programs overall. We grew catchable-sized Rainbow Trout Oncorhynchus mykiss to two target average sizes (254 and 305 mm total length) at a production scale, while tracking feed expenditures to examine the costs and benefits associated with increased size-at-stocking. Although larger catchables cost 31% more in feed expenditures than those reared to a smaller average size, catch (by anglers) of larger fish increased by 100% relative to smaller fish. Consequently, if target stocking size was changed from 254 to 305 mm and feed costs were held constant by reducing the total number of fish stocked, anglers would benefit by catching larger and more fish, despite the reduction in number of fish stocked. In lentic systems, larger catchables were reported by anglers more quickly than smaller fish, so managers must consider interactions between stocking size and residence time for lentic systems supported by catchables. In lotic systems, overall catch by anglers was much lower than catch at lentic waterbodies, and all catchables were either reported by anglers quickly or failed to be reported at all regardless of size-at-stocking. Producing larger catchables for hatchery-supported fisheries serves to benefit angling and would likely increase angler satisfaction while improving efficiencies associated with hatchery catchable stocking programs.
Kokanee Oncorhynchus nerka growth is often density dependent; thus, proper management of kokanee populations necessitates an understanding of population dynamics using age structure data. To date, no calcified structures have been validated for kokanee. We compared the accuracy (i.e., the percentage of reconciled age estimates that matched the known ages of fish) and precision (i.e., the percentage of fish for which complete agreement was achieved on age estimates among all readers) of aging estimates for scales, sectioned otoliths, and sectioned pectoral fin rays from 455 known‐age kokanee (ages 0–4) collected from five lentic waters in Idaho. Across all waters combined, mean weighted accuracy and precision were similar for scales (86% and 70%, respectively), fin rays (83% and 65%), and otoliths (82% and 65%), with no significant differences between structures. However, among water bodies, accuracy and precision of each calcified structure varied considerably. For example, scales were the most accurate and precise structure and otoliths were the least accurate and precise structure at one water body, while otoliths were the most accurate and precise structure and scales were the least accurate structure at two water bodies. Fin rays were the least precise structure at four of the five study waters, but were the least accurate structure for only one water body. Individual reader accuracy was most affected by fish age and water body, and older fish (age 3 and older) were consistently assigned incorrect ages regardless of the water body or the calcified structure. Taken collectively, all three structures produced satisfactory aging accuracy and precision for kokanee, but no structure was unequivocally best; at any individual water body, annual growth and local environmental conditions appeared to influence the readability of calcified structures.
Many pelagic fish species such as kokanee Oncorhynchus nerka undertake diel vertical migration in response to dynamic interactions between ambient light, foraging opportunity, and predation risk. Consequently, kokanee populations are almost universally sampled during the dark phase of the moon (i.e., the new moon), presumably to optimize capture efficiency. However, it is unclear if this sampling precaution is necessary to avoid bias in kokanee catch data related to the moon phase. We used experimental gill nets to sample kokanee populations in two thermally stratified reservoirs during three distinct moon phases (i.e., new, first quarter, and full) to understand the relative effects of moon phase and other ambient light variables on total catch and average size of captured kokanee. The total catch of kokanee differed significantly between populations but was not significantly affected by moon phase, secchi depth, or net depth. The average size of kokanee sampled from both populations increased significantly with moon illuminance and likely reflects behavior associated with predatorprey dynamics. Results from this case study suggest that the effect of moon phase and other ambient light variables on gill-net catch composition of kokanee is likely population-specific and is governed in part by population parameters such as abundance, growth rate, and size-structure. As such, investigators should be cognizant of-or perhaps standardize gill-net samples to-ambient light variables when indexing populations of kokanee and other pelagic fishes that undertake diel vertical migrations, especially when size indices are examined.
Hatchery fish exposed to exercise training often exhibit physiological and behavioral benefits compared with unexercised fish, but results from previous studies have been equivocal and have rarely examined postrelease performance of stocked fish. We evaluated various in-hatchery and postrelease consequences of rearing catchable-sized (~254-mm) Rainbow Trout Oncorhynchus mykiss in a raceway installed with baffles, the intent being to self-clean the raceway and exercise fish. Installing baffles increased water velocities experienced by fish, with some velocities exceeding 0.26 m/s (1.0 body length per second). In contrast, the maximum velocity experienced by fish in the control raceway was 0.07 m/s (0.27 body lengths per second). Prior to stocking, fin erosion (as measured by relative dorsal and pectoral fin lengths) did not differ between the baffled and unbaffled raceways, but surprisingly, survival was reduced for baffled fish. Catch by anglers and mean time to capture did not differ between raceways but did differ by water type (i.e., lentic, lotic, and community pond waters). While the augmented velocities along the bottom of the baffled raceway assisted with clearing some fish waste, they were not entirely effective and raceways still required some sweeping. Taken collectively, our results suggest that installing baffles in production-scale raceways rearing catchable-sized Rainbow Trout is not advantageous.
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