Evaluation of Three Electrical Outputs in an Electro-Immobilization System to Reduce Induction Time during Routine Handling of Fish at a Hatchery

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Abstract Tools designed to electrically induce fish immobilization are increasingly being implemented by practitioners for safe and effective fish handling. Currently used methods for electro-immobilizing batches of fish incorporate a period of hyperactivity that serves to reduce spinal injuries in fish. However, like the hyperactive period that is seen in other fish immobilization techniques, it provides a visually unappealing effect and introduces questions surrounding humane animal treatment. In this study, three different electro-immobilization treatments were evaluated for rates and severity of injury as well as time to immobilization to determine ways of reducing the hyperactive period while also maintaining low rates of injury. Tests were conducted in a Smith-Root EA-1000B System on adult summer steelhead Oncorhynchus mykiss at the Cowlitz Salmon Hatchery (Salkum, Washington). Treatments included an electrical output setting commonly used at the facility, two experimental electrical outputs, and a chemical anesthetic (MS-222) treatment that served as a control. Efficacy, indicated as time to immobilization, was evaluated among electrical output treatments; humane fish treatment, measured by hemorrhage, spinal injuries, and mortality, was quantified and compared among all treatments. Analysis of variance indicated a significant difference in induction times among electrical treatment groups, which ranged from 14.2 ± 4.9 to 53.4 ± 2.6 s (mean ± 95% CI). Some spinal and hemorrhage injuries were observed in all treatment groups; however, only one electrical treatment had significantly higher (P = 0.003) incidence and severity of injuries compared to the control. Results indicate that hatchery managers can significantly reduce the time needed to immobilize fish while maintaining minimal fish injury rates when using a commercial electro-immobilization system.

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