Effects of Electroshock on Embryos and Larvae of Rainbow Trout: Preliminary Tests for Eradication of Invasive Salmonids

Traditional electrofishing has been used for capture and removal of invasive fishes for many years. This method, however, only affects free-swimming individuals and is not inclusive of early life history stages such as embryos within spawning substrate. This study evaluates the susceptibility of embryonic and larval stage rainbow trout Oncorhynchus mykiss to continuous DC (CDC) in waters of varying conductivity to determine lethality for eradication efforts. Rainbow trout embryos were initially exposed to homogeneous electric fields (2 to 20 V/cm) for 5 s in water with a conductivity of 220 µS/cm. Embryo ages ranged from 1- to 15-days post fertilization or 27 to 405 temperature units (TU).  Mortality was assessed 24 h post exposure and the LV₅₀ (lethal voltage where 50% of embryos do not survive) at 220mS/cm was determined for each TU.  Embryos from six periods of development were then exposed to their respective LV₅₀ voltages in waters of varying conductivity waters (20-600mS/cm). Embryo susceptibility to a constant voltage decreased with development. Susceptibility to a constant voltage increased with increasing conductivity and was consistent throughout early development (81 to 292 TU). However, mortality in eyed embryos after 364 TU did not change with increased conductivity but stayed static with LV₅₀ values. When applied power was held constant, embryo susceptibility to CDC was inversely correlated with increasing conductivity.  These data suggest that a combination of low peak voltages of CDC and increased localized conductivity would be an effective means of eradicating invasive and nuisance salmonids prior to eyed embryonic stages.