Experimental set-up for small fish. Picture by Dr. Jan Dean
Lab or tank experiments on fish have been around for decades, beginning with studies of fish behavior in electric fields. Presently, tank experiments are used for evaluating the effectiveness of candidate waveforms, estimating thresholds for various reactions that assist capture, guidance, and electrosedation, and determining probability of trauma. While insights gained by lab work, in combination with field trials, can and have improved fisheries sampling and provided insights for risk analysis, there are pitfalls that can sink the ship. A couple problems that often occur are the rationale for setting dose levels and the actual description of dose levels. These issues can lead to misinterpretations, inappropriate management decisions, and constrain application of experimental results. In fact, dose setting is becoming a big issue in electrofishing experimentation. I have seen studies lately that have used incredibly high doses, in fact extreme overdoses, preventing a connection from the lab to application in the field. I think the results of those studies are relatively meaningless. And, most of the disconnect is due to a poor understanding of electric fields generated by common sampling gears and typical exposure times while electrofishing.
The process of setting dose levels can be made more rigorous by making estimates of electric field exposure intensities. If the objective is to assess risk of electrofishing to non-target species, then determining likely exposure levels in a stream or lake is a solid approach. This means making in-water measurement of electric field intensities relative to the expected location of the animal in question, while applying voltage and current required for successful sampling of target species. This does not mean using applied voltage to the electrodes as the dose level. In other words, the dose is not 300 V on the control box. Rather, it is some voltage gradient and power density in the water. The in-water measurements are the meaningful variables; the control box voltage reading is irrelevant. So, when you then run the lab experiments, the dose rate is not the voltage applied (e.g., 250 V), it is the voltage gradient and power density adjusted via the power transfer model based on water conductivity.
Voltage gradient (V/cm) is the volts per centimeter at a location in the field.
Power density (µWatts/cm^3) is (voltage gradient)^2 x (ambient water conductivity).
Here’s the implication. You have two aquaria that you are using for the experiments. One aquarium is 50 cm long, the other aquarium is 100 cm long. Ambient water conductivity is 433 µS/cm. By placing electrodes on each aquarium end that cover the cross-section of the water, you set up a homogeneous electric field in each tank (i.e., constant voltage gradient and power density throughout the tank). On your control equipment, you apply 100 V to the smaller aquarium.
Voltage gradient is 100 V/50 cm = 2 V/cm and power density is 2^2 x 433 = 1732 µW/cm^3.
Now, you apply the same 100 V to the larger tank.
Voltage gradient is 100 V/100 cm = 1 V/cm and power density is 1^2 x 433 = 433 µW/cm^3.
Fish in the smaller tank are exposed to higher field intensity (in fact, 4x the power) than fish in the larger tank. Yet, the same level of voltage was applied to both. The voltage gradient and power density are the dose level, not the 100 V applied! And remember, reporting the experimental ambient water conductivity is critical since the voltage gradient and power density dose will vary with water conductivity.
Setting dose levels
Finally, another improvement to dose level setting has been the use of behavior categories of fish. For example, the most intense fish reaction is tetany (rigid immobility). That behavioral response can be used for capture in some cases or be a surrogate for the worst-case exposure level for injury studies. For electrosedation studies, the response sought is narcosis (slack immobility). The required dose to achieve narcosis of the target species size class is the voltage gradient and power density (within the experimental water conductivity).
Dose setting that reflects reality, contributes to standardized electrofishing, and provides insights on potential risks to aquatic species is no simple concept. It is not mimicing typical voltage settings used in the field. Dose setting instead requires an understanding of electric fields, sampling procedures, and associated behavioral responses.