Electrofishing has been used widely as a standardized sampling technique for assessing density of fish. Although primarily used in fisheries and ecological studies, it could also play important roles in aquaculture practices such as for wild broodstock capture or the monitoring of stocked fish populations. However, the complex and dynamic impact of electricity on fish health and welfare still remain poorly understood.
In this laboratory-study, we investigated the effect of three electrofishing devices: LR-20 (Smith-Root), L-600 (Lug AB) and FA-50 (Terik Technology) on the acute stress response of rainbow trout (Onchorynchus mykiss) . Forty to forty-six fish with an initial weight of 220 g were distributed into four circular 400 L tanks . T hree tanks were exposed to the same protocol, 10 sec electrical pulse, followed by 5 sec of rest and then a second 10 sec period of electrical pulse . Ten fish from each treatment were sampled for blood and plasma at 0, 1, 6 and 24 h post exposure . A fourth tank served as undisturbed control, sampled at the same time points . After 6 h , fish showing difficulties swimming and balance issues were euthanized.
Our results show that the different electrofishing devices impacted the primary and secondary stress response differently. Overall , the LR-20 seemed to induce the lowest acute stress response . Two fish died and one fish was euthanized after 6 h in this group while six fish were euthanized with the L-600. No fish survived past 6 h with the FA-50, with 11 dead and four fish euthanized . The cortisol peaked between 1 and 6 h in all the experimental groups, with the LR-20 displaying the lowest levels (ca. 45 ng/mL) at both time points, while the FA-50 peaked at over 100 ng/mL. Both LR-20 and L-600 cortisol levels were back to basal levels after 24 h (Fig 1a) . Glucose levels showed a similar pattern (Fig 1b) , with an initial increase already after 1h for the L-600 and the FA-50, while the RT-20 was only significantly higher than the control after 6 h. There was no difference between the RT-20 and the L-600 for this parameter. Plasma osmolality, blood hemoglobin, hematocrit, and MCHC also indicated a mild transient stress effect, most pronounced in fish exposed to the FA-50. These more severe effects may be related to the type of electrical current used by the FA-50 (AC) known to cause more extensive neuromuscular disruption compared to pulsed DC used by the LR-20 and L-600.
Further studies are needed to assess potential long-term and indirect effects, including impacts on growth, skin integrity, and pigmentation. Additional field-based comparisons are also warranted in order to validate the results generated from different electrofishing gears.