Lethal effects of electricity on Northern pike embryos

U.S. Geological Survey

Northern Rocky Mountain Science Center

Background. - Northern pike (Esoxlucius) have been introduced to many areas of the U.S. and globally for recreational and commercial uses. In Southcentral Alaska, pike were introduced nearly 40 years ago and since have increased the range. Pike are various predators and are linked to salmonid declines in this region (Sepulveda et al. 2013). Managers are in need of effective control strategies to reduce their spread.

Pike spawn in slow moving, shallow waterbodies and after egg deposition eggs attach to substrate or to submerged macrophytes (Mecozzi 1989). Application of electricity in these critical spawning grounds could be an effective means of decreasing new generations of invasive pike. Electricity has been shown to effectively induce mortality ineggs ofseveral species of freshwater fish (Bohl et al. 2010; Nutile et al. 2013). This research demonstrates that electric field intensities (voltage), exposure duration, and embryo size can influence fish embryo susceptibility to electricity. However, there is no information on the susceptibility of Northern pike embryos to electricity.

Project goals. - The goal of the work was to (1)determine voltages that cause 100% mortality in pike embryos and (2) determine if varyingexposure duration (seconds)alters mortality rates in pike embryos.

Study location. - We conducted experiments in a laboratory facility at U.S. Geological SurveysNorthern Rocky Mountain Science Center (NOROCK) in Bozeman, MT.

Animal care & housing. - We acquired approximately 15,000 Northern pike embryos from Fort Peck State Fish Hatchery, Fort Peck, MT. Average embryo diameter was 2.81 mm (± 0.12; SD) were at the in the eyed-stage of the embryonic phase (Raat 1988). Embryos were housed in a 50-gallon holding tank with recirculating UV-sterilized water in the laboratory facility at NOROCK. Upon receiving the embryos, we slowly introduced water originating from the holding tank to the transport container (Hassler 1982). Water temperature was regulated in the holding tank using a chiller to achieve optimal temperatures (9-15°C; Hassler 1982). We monitoredwater quality conditions in the holding tank during and after experiments using YSI pro plus. Dissolved oxygen levels were on average 10.1 mg/L (± 1.0; SD), water temperature was 9.98 °C (± 0.23), and conductivity was 162.5 µS/cm (± 5.48).

Electrical system.- We generated homogenous electric fields with the Aqua Shock power control box designed by Aqua Shock Solutions (Pigeon Forge, TN, USA). Two parallel steel electrodes (14 cm wide x18 cm tall) were mounted in aglass aquaria(30 cm x 15 cm x 19 cm) with 2 cm water depth. Electrodes wereplaced 12 cm apart. We connected the anode and cathode cable alligator clips from theAqua Shocker power control box to either electrode. The control box provided straight direct current (DC). The power supply had 11 voltage settings. Once the appropriate voltage setting was selected we noted the corresponding output voltage (V) and amperage (mA). Before each exposure we measured voltage gradients using a voltmeter with positive and negative probes spaced 1-cm apart.

Exposure. - We conducted pilot experiments to determine appropriate output voltages and exposure durations. We used six voltage gradients (2.9, 5.4, 8.2, 12.7, 16.3, 20.6 V/cm) with five replicates and controls (n = 5) for a 20- and 30-second (sec) exposure duration. We followed the procedures of Nutile et al. (2013) for handling and exposing fish embryos to electricity. Embryos were exposed to electricity in PVC holding baskets. Holding baskets were5-cm tall2”diameter PVC with mesh screen lining the bottom. We drilled approximately 26 holes near the base of the PVC basket to allow electricity to flow into the basket. We lined the outside of the basket with nylon mesh to minimize embryos escaping. We placed approximately 30 embryos in each basket just prior to exposure. Baskets were exposed individually. Prior to exposure, approximately five embryos were removed from the basketand placed in a petri dish labeled and filled with water. These embryos were placed under a stereomicroscope(Nikon SMZ800) and measured to the nearest 0.001 mm. Immediately following exposure the basket was returned to the holding tankand mortality was assessed 24 hourspost exposure. We observed all embryos from each basket (≈ 25 individuals) under a microscope to determine whether each embryo were dead. Any individuals that look opaque or white in color will be considered dead (Bohl et al. 2010), but we also looked for movement within the yolk sac and used light external probing to look for a response to stimulus. We calculated the proportion of embryos from each basket that were dead after 24 hours.

Results. -Survival in control embryos ranged from 60 - 83%. We did not achieve 100% mortality at any voltage gradients at 20-sec exposure durations (Figure 2) but we did find 100% mortality at 30-sec exposure at 16.3 V/cm (Figure 3). Embryo mortality was related to voltage gradient (one-way ANOVA; F5, 16 = 5.67, P = 0.003) such that mortality increased with increasing voltage gradients when exposed to electricity for 20 seconds. Mortality was also higher with increasing voltage gradients for embryos exposed to electricity for 30 sec, however, mortality was higher at lower voltages (≥ 5.4 V/cm) compared to 20-s exposure (Figure 2).

References

Bohl, R.J., Henry, T.B. andStrange, R.J. 2010. Electroshock‐induced mortality in freshwater fish embryos increases with embryo diameter: a model based on results from 10 species.Journal of Fish Biology,76(4): 975-986.

Hassler, T.J. 1982. Effect of temperature on survival of Northern pike embryos and yolk-sac larvae. The Progressive Fish-Culturist, 44(4): 174-178.

Mecozzi, M. 1989. Northern pike (Esox lucius). Wisonconsin Department of Natural Resources, Bureau of Fisheries Management. Technical Report, p. 1-6.

Nutile, S., Amberg, J.J. and Goforth, R.R. 2013. Evaluating the effects of electricity on fish embryos as a potential strategy for controlling invasive cyprinids. Transactions of the American Fisheries Society, 142(1): 1-9.

Raat, A. J. P. 1988. Synopsis of biological data on the Northern pike Esox lucius Linnaeus, 1758.Food & Agriculture Organization of the United Nations (FAO), FAO Fisheries Synopsis No. 30 Rev. 2, Nieuwegein, Netherlands. ISBN 92-5-1023656-4.

Sepulveda, A.J., Rutz, D.S., Ivey, S.S., Dunker, K.J. and Gross, J.A. 2013. Introduced northern pike predation on salmonids in southcentral Alaska. Ecology of Freshwater Fish,22(2): 268-279.

Figure 1. Proportion pike embryos that were dead (24-hours after exposure) after exposure to straight DC at a specific voltage gradient (V/cm) for 20 seconds.

Figure 2. Proportion pike embryos that were dead (24-hours after exposure) after exposure to straight DC at a specific voltage gradient (V/cm) for 30 seconds.