Spinal injuries and catch per unit effort of riverine Rainbow Trout using backpack- and barge-mounted electrofishers
Kevin A Meyer, William J Lubenau, Susan E Frawley, Jennifer L VincentAbstract
Objective
We evaluated the frequency and severity of spinal injuries for Rainbow Trout Oncorhynchus mykiss (≥200 mm total length) captured in an intermediate-sized wadeable river (20-m wetted width) using three backpack electrofishing units or a barge-mounted unit at pulse frequencies of 20 and 30 Hz. Catch per unit effort (fish/min) was also determined for these settings as well as for 60 Hz to assess whether pulse frequency affected catch.
Methods
The three backpack units simultaneously emitted 25% duty cycle and 255 V, which produced approximately 100 W of average (not peak) power output (average current = 0.25–0.50 A). Barge electrofisher settings were 25% duty cycle and 450 V, which produced 350–800 W of average power output (average current = 0.8–1.8 A). A portable X-ray unit was used to assess spinal injuries.
Results
A total of 437 Rainbow Trout were captured with electrofishing gear and X-rayed. Size ranged from 195 to 482 mm. At 30 Hz, spinal injury rates were equivalent for backpack (23.6%) and barge (23.8%) electrofishing units, whereas at 20 Hz, injury rate was much lower for the barge unit (8.3%) than for the backpack units (20.0%). Spinal injury rates were highest for 300–350-mm fish and declined for smaller and larger fish. Nearly half of the fish exhibiting spinal misalignments or fractures had no associated vertebral compressions, and the average number of compressed vertebrae was twice as high for fish exhibiting only spinal compression injuries (X = 16.3) compared with fish that also exhibited misalignments or fractures (X = 7.5). When using the barge electrofisher, CPUE was equivalent (1.77–1.81 fish/min) across all pulse frequencies. For the backpack electrofishing units, CPUE at 60 Hz (1.81 fish/min) was equivalent to that of the barge unit, but CPUE declined to 1.45 fish/min at 30 Hz and 0.66 fish/min at 20 Hz.
Conclusions
Our results, combined with prior work, provide evidence that in wadeable rivers too wide for a single backpack electrofisher and too small for a boat, lowering pulse frequency below 60 Hz for multiple backpack units operated in tandem diminishes spinal injuries but also decreases CPUE, creating a data collection trade-off for biologists. This trade-off was not evident for a higher-powered, barge-mounted electrofishing unit because reducing the pulse frequency below 60 Hz lowered spinal injuries without reducing CPUE.