7784 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I Impacts of Electrofishing on Fish relatively flat over all but the lower end of the range for freshwaters and practically horizontal for more saline waters (bottom graph, Figure 5). Peak values of field-strength (e.g., voltage gradients) are probably more biologically significant than mean values in PDC or AC (Kolz and Reynolds 1989; mean and peak values are the same in DC). It is important that researchers and authors document whether field-strength measures (as well as output voltage, amperage, and power) are peak or mean values; the difference can be substantial. Voltage gradients can be measured with a voltmeter or oscilloscope connected to insulated wires, the tips of which are exposed in the water a fixed distance apart. The maximum voltage gradient or voltage differential measured with this probe will be obtained when the line between the exposed tips is oriented along the field's lines of current. When the probe tips are rotated perpendicular to the lines of current there should be no effective voltage differential and the reading on the voltmeter or oscilloscope should be 0 V. Like voltage-gradient probes, fish are subject to the greatest voltage or potential differential when they are oriented along the lines of current. This is often referred to as "head- to-tail voltage." They are subject to the least voltage difference when oriented perpendicular to the field lines. Voltmeters specifically designed to yield peak voltage (e.g., peak voltage detectors-Jesien and Hocutt 1990) or oscilloscopes should be used for accurate measurements in PDC (or pulsed AC). The presence of voltage spikes in PDC waveforms may affect readings in some peak voltage detectors. Oscilloscopes, although much more expensive, allow the user to not only document voltage spikes, peak voltages (ignoring spikes), and voltage gradients, but monitor waveform and frequency of the current. The typical voltmeter (or multimeter) works well for measuring peak-voltages in DC and mean-voltages (rms, root mean square) in AC fields, but according to Jesien and Hocutt (1990), such meters cannot accurately measure voltage in PDC or pulsed AC. Fredenberg (pers. commun.) also found that PDC voltage-gradient readings with a standard voltmeter were neither accurate nor consistent. Some voltmeters can be modified to provide accurate mean voltages or voltage gradients for specific PDC waveforms. Such is the case for at Review I Electric Fields in Water 13 least one Commercial instrument and probe designed specifically for measuring voltage gradients in electrofishing fields-the combined field-strength and conductivity meter, FS/C-III, built and sold by Micro-Technologies of Idaho (M.T.I., Pocatello). Although this meter provides only mean voltage- gradient readings for only one PDC waveform (50% duty cycle and an unspecified waveform, probably half-sine), peak values for this waveform and either peak or mean voltage-gradient measures for other currents, waveforms, and duty cycles can be calculated by applying appropriate correction factors. For example, in DC fields the manufacturer notes that the meter reads 1.33 times greater than the actual peak value. M.T.I. will recalibrate their meters for specific alternative waveforms and duty cycles. Provisions for peak voltage gradients and user selection of settings for a variety of waveforms and current parameters would make this and other field-strength meters much more flexible and useful. Heterogeneous and Homogeneous Fields Because the basins are irregular in shape and electrodes are much smaller than their cross;..sectional areas, electrofishing fields generated in rivers, streams, lakes, reservoirs, and most other waters are heterogeneous. In such fields, lines of current can be visualized as radiating from and spreading widely around and between the electrodes (Figure 7). Field strength is greatest next to the electrodes and decreases to barely perceptible levels as distance from the electrodes increases, even in the area between anode and cathode when sufficiently separated. The actual field strength encountered by a fish in a heterogeneous field depends on the fish's location relative to the electrodes. Homogeneous fields are typically restricted to laboratory settings in troughs with a constant cross- sectional profile and electrodes approximating that profile squared at each end of the desired field. In homogeneous fields, the current flows parallel to the sides of the trough directly from one electrode to the other. This arrangement provides a constant voltage gradient, current density, and power density between the electrodes. Controlled experiments in homogeneous fields eliminate many of the electric-field variables that are encountered in natural waters. This greatly