The application of electric fields (i.e., electroshocking) may be an option in the eradication of invasive freshwater fishes. Electric fields can be used to facilitate capture or to kill vulnerable life history stages of fish. The objective of this study was to evaluate the relations between water conductivity and the electrical variables of voltage gradient and power density on the mortality of Rainbow Trout Oncorhynchus mykiss embryos at different developmental stages. Embryos in age-groups ranging from 27 to 405 daily temperature units (TU) were exposed for 5 s to constant (nonpulsed) DC electric fields (2-20 V/cm) at a water conductivity of 220 mS/cm to determine (1) the stage of embryonic development most susceptible to electroshock-induced mortality, and (2) the lethal voltage gradient (V/cm) predicted to kill 50% (LV50) of embryos in each age-group. Mortality was assessed 24 h postexposure. Using LV50 voltages determined for each age-group, the relation between water conductivity (20-640 mS/cm) and electroshock-induced mortality from exposure to LV50 was determined for six different stages of embryonic development . Results show that the mortality of embryos exposed to electroshock decreased with the stage of embryonic development. The mortality of embryos exposed to the LV50 for their respective age-groups increased with water conductivity and was consistent throughout early development (81-292 TU), but the effects of increased conductivity were not observed in eyed embryos after 364 TU, as mortality remained constant with the determined LV50 values. Voltage was determined to be the most influential factor of embryo mortality. As conductivity increased, maintaining a constant voltage (LV50) elicited greater mortality than maintaining a constant power. These data suggest that a combination of DC electric field and increased localized conductivity could be effective at eradicating nuisance salmonids prior to eyed embryonic stages.
Aquaculture and hatchery industries are in need of effective control methods to reduce the risk of spreading aquatic invasive species, such as the Asian clam Corbicula fluminea, through aquaculture and hatchery activities. The planktonic nature of Asian clam veligers enables this life stage to enter water-based infrastructure undetected, including hatchery trucks used to stock fish. Once in hatchery trucks, veligers can disperse overland and establish in previously uninvaded habitats. As a result, there is a need to develop techniques that result in veliger mortality but do not harm fish. In September 2012, we conducted laboratory trials to determine if a molluscicide (750 mg/L potassium chloride and 25 mg/L formalin) commonly used to kill zebra mussel (Dreissena polymorpha) veligers in hatchery trucks can also effectively kill Asian clam veligers. We exposed Asian clam veligers to this molluscicide for 1, 3, and 5 h in each of two water types: deionized water and filtered lake water. We found ,20% mortality at the 1-h exposure period and 100% mortality at both the 3-h and 5-h exposure periods, regardless of water type. This laboratory study represents an important step toward reducing the spread of Asian clams by aquaculture facilities.
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