Electroimmobilization as an Alternative Method to Chemical Immobilization of Larval Fish

Abstract

Electroimmobilization is commonly used in field and hatchery settings for juvenile to adult fish; however, its use in laboratory studies is limited, specifically with larval fish. Here, we describe the use of electroimmobilization for temporary immobilization of larval fish, with Zebrafish Danio rerio as the species of interest. We quantified immobilization duration and survival of larvae for 72 h after exposure to a range of electrical settings and electric waveforms. We also compared effects of electroimmobilization between two wild-type strains and between larvae 4 days postfertilization (dpf) and 7 dpf. Our results indicate that an electrical power density level of 10,406 μW/cm3 (corresponding to a 10V setting with our experimental parameters) was ineffective at immobilizing fish, regardless of waveform; however, mean immobilization duration increased with an increase in applied power density, which was longest (mean ± SE = 100 ± 11 s) when using a pulsed direct current at a power density of 860,000 μW/cm3. Significant differences in mean immobilization duration and survival between strains were not detected for larvae exposed to the same setting. However, 7-dpf larvae exposed to the same setting as 4-dpf larvae had significantly longer immobilization durations across all treatment groups. Survival of 4-dpf larvae within any given electroimmobilization treatment ranged from 80% to 100%, which was comparable to chemical immobilization with tricaine methanesulfonate or to age-matched controls. Survival of 7-dpf larvae ranged from 48% to 90% depending on electroimmobilization treatment, but survival of age-matched controls was 65%; thus, lower survival of 7-dpf larvae in treatment groups cannot be attributed specifically to electroimmobilization. These findings indicate that electroimmobilization can be a valuable tool for rapidly immobilizing larval fish in laboratory research, but we caution that it should only be used for short-duration procedures.