Effects of Healing Time and Pulsed‐DC Waveform on Injury Detection and Incidence in Electroshocked Juvenile Chinook Salmon

Abstract

AbstractElectrofishing is widely used to assess juvenile Pacific salmonids in freshwaters, but electroshock‐induced injury may result. The small size of juvenile fish makes detection of internal injuries, particularly spinal injuries, more difficult than in adult fish. To test the idea that growth and healing of vertebral injuries over time would render injuries more detectable, we exposed age‐1 Chinook salmon Oncorhynchus tshawytscha to conventional 60‐Hz pulsed direct current (PDC) and dual‐frequency PDC (DFPDC; 568 and 15 Hz) in a tank study. To simulate field conditions, capture‐prone behaviors (immobilization and forced swimming) were achieved by applying 60‐Hz PDC at a voltage gradient of 1.1 V/cm and DFPDC at 5.0 V/cm. Because electroshock treatments occurred only on day 0, we assumed that all injuries occurred on that day. Evaluation of injuries on days 1, 7, 14, 28, and 56 of a 56‐d experiment revealed decreasingly detectable hemorrhages (as a result of healing) but increasingly detectable spinal injuries. Incidences of spinal injury induced by 60‐Hz PDC and 568‐Hz and 15‐Hz DFPDC were both high (40–80% range) and not significantly different. However, fish responses to electroshock (behavior, bruising, mortality, and growth) followed a trend of greater severity for PDC than for DFPDC. Fewer age‐0 fish were available; on day 0, they were exposed only to PDC and evaluated only on days 1 and 56. The most marked increase in detection of spinal injuries was observed in age‐0 fish: the injury rate increased from 0% on day 1 to 53% on day 56. Immobilization of Chinook salmon (and presumably other juvenile salmonids) by 60‐Hz PDC will cause significant trauma. Use of dual‐frequency PDC, even at the higher voltage levels sufficient to immobilize these fish, will reduce these effects. Estimates of spinal injury rates in electroshocked juvenile salmonids may be biased downward unless sufficient time has passed for healing and growth to occur, thereby increasing the probability of radiographic detection.