Spinal injuries in Rainbow Trout and Mountain Whitefish captured in a wadeable river using backpack- and barge-mounted electrofishers

Objective We assessed whether spinal injury rates were lower when sampling salmonids in an intermediate-sized wadeable river using multiple backpack electrofishing units or a barge-mounted electrofishing unit. Methods The three backpack units emitted 60 Hz, 24% duty cycle, and 300 V, which produced ∼100 W of average (not peak) power output (average current = 0.25–0.4 A). An oscilloscope revealed that the electrical fields overlapped among the backpack units, effectively increasing pulse frequency at times to 180 Hz. The barge electrofisher settings were 60 Hz, 24% duty cycle, and 400 V, which produced 300–800 W of average power output (average current = 0.8–2.0 A). Electrical field overlap did not affect pulse frequency with the barge electrofisher because both anodes were synchronized to the same power source. Results In total, 395 Rainbow Trout Oncorhynchus mykiss and 128 Mountain Whitefish Prosopium williamsoni were captured with electrofishing gear. Size ranged from 180 to 504 mm for Rainbow Trout and 205 to 483 mm for Mountain Whitefish. The mean spinal injury rate was much higher for Rainbow Trout (44%) than for Mountain Whitefish (13%) but was similar between barge- and backpack-mounted electrofishing gear for both Rainbow Trout (46% and 41%, respectively) and Mountain Whitefish (17% and 10%, respectively). For Rainbow Trout, most spinal injuries were misalignments (62%), followed by compressions only (32%) and fractures (5%), whereas most injuries for Mountain Whitefish were compressions only (54%), followed by fractures (38%) and misalignments (8%). For both species combined, the fish with misalignments and fractures tended to have fewer compressed vertebrae than did those with compression injuries only. Many (21%) of the misalignments were merely slight curvatures of the spine, with no vertebrae noticeably misaligned with neighboring vertebrae. Conclusions In wadeable rivers that require multiple backpack electrofishers to effectively capture fish, using 60-Hz pulse frequency will likely result in more spinal injuries than has been demonstrated in prior research with one backpack unit. The present results indicated that spinal injury rates using three backpack units were equivalent to those that occur with more powerful barge-mounted electrofishing equipment that emits a much higher electrical output. Although at a population scale such elevated injuries and any resulting mortality are generally inconsequential, the use of gear and approaches that minimize injuries without needlessly sacrificing adequate capture efficiency is desirable.