Reducing Electrofishing-Induced Injury of Rainbow Trout

We conducted three sets of experiments to determine aspects of pulsed direct current responsible for high incidence of spinal injury in electrofished rainbow trout Oncorhynchus mykiss and to test the electrotaxis efficiency of a new complex pulse pattern (low-frequency bursts of high-frequency pulses). In the first set of experiments, we energized three types of anodes with current pulsed at 60 pulses per second (pps) to produce fields with relatively low, intermediate, or high voltage gradients near the anode. Results showed no direct relationship between voltage gradient and injury rate. In the second set of experiments, we tested the complex pulse pattern and four other pulse frequencies using a spherical anode. The results demonstrated a low injury rate with the complex pulse pattern and a curvilinear increase in spinal injuries with rising pulse frequency. Moderate to high pulse frequencies, therefore, rather than high voltage gradients, appear to be the primary cause ofspinal injury. We suggest that more fish injuries are seen at higher pulse frequencies because these injuries are caused by myoclonic jerks associated with shock-induced epileptic seizures, and such seizures develop more rapidly at higher frequencies than at lower frequencies. In the third set of experiments, we found the electrotaxis efficiency of the complex pulse pattern was similar to those of regular 60 pps and 30 pps patterns. The complex pattern, therefore, combined low incidence of injury with good electrotaxis and narcosis.