Lack of effect of electroshock on the susceptibility of rainbow trout to enteric redmouth disease

In two experiments, rainbow trout Oncorhynchus mykiss 2.5 to 3.5 months old were electroshocked and then exposed to waterborne Yersinia ruckeri. the bacterial pathogen that causes enteric redmouth disease (ERM). The electroshock consisted of a 20-s exposure to a homogeneous 4-V/cm, 30-Hz square-pulsed (3-ms pulse) DC in water with an ambient conductivity of 70-100 muS/cm. The electroshock immobilized all fish. and after 1 h 12.9% of them (both experiments combined) had not resumed opercular movements and were considered dead. However, during the 18 d following the electroshock, no additional fish in the electroshocked treatment died without bacterial challenge. In the first experiment, the challenge mortality (the mortality attributed to the bacterial challenge and not including fish that were dead 1 h after the electroshock) of fish that were electroshocked and exposed to Y. ruckeri (58.4 +/- 8.9% [mean +/- SE]) was not significantly (P > 0.05) different from that of fish that were exposed to simulated electroshock and then to Y. ruckeri (55.8 +/- 8.2%). In the second experiment, in which handling was minimized and a lower concentration of bacteria used for the challenge, there was likewise no significant difference between the challenge mortality of fish subjected to electroshock (11.3 +/- 8.0%) and those subjected to simulated electroshock (2.1 +/- 1.4%). These results indicate that electrofishing is unlikely to alter the susceptibility of rainbow trout to ERM.