Examination of Electrofishing Injury and Short-Term Mortality in Hatchery Rainbow Trout

Abstract

Four groups of hatchery-reared rainbow trout Oncorhynchus mykiss were exposed to different direct current voltages and pulse rates from a battery-powered backpack electrofisher. A control group was not exposed to electrofishing. The four experimental groups of 30–35 fish each and the control group of approximately 100 fish were monitored daily for 7 d following the initial electrofishing treatments to determine the extent of delayed short-term mortality. Samples were collected after 7 d with the same electrofisher settings as were initially used (a sample was seined from the control group) and were necropsied within 2 h of collection. Fish from each treatment group and the control group were examined for spinal injury by filleting the fish on both sides and visually determining the presence or absence of spinal column damage and associated hemorrhaging in surrounding musculature. In total, 114 fish (17–29 from each group) were necropsied. Fish from the control group showed no signs of injury. Injury among fish exposed to electric current ranged from 4% to 53%, with the higher pulse rates producing a higher incidence of injury. The consecutive electrofishing runs may have increased the injury rates. The short-term mortality due to the electrofishing conducted in this hatchery environment was negligible. The incidence of injury due to electrofishing was very high with some experimental settings, and electrofishing should be considered to affect sampled fish deleteriously until proven otherwise.