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16 INFORMATION AND TECHNOLOGY RErORT--2003-0002 documented a pulse frequency of73 Hz and a 64% duty cycle. In addition to field intensity and output, biologists should document water conductivity and temperature and adequately describe or verify the waveform (shape, frequency, pulse width, duty cycle), electrodes (position, size, and shape), and operating procedure. Vo Itage gradients are best measured in water with an appropriate voltmeter or oscilloscope connected to insu- lated wires, the tips of which are exposed and set a fixed distance apart (Kolz, 1993; Kolz and Reynolds, 1990b; Kolz et aI., 1998). The maximum voltage differential per unit distance measured with this probe at any particular location is the voltage gradient and will be obtained when the exposed tips are oriented along the field's lines of flux (the principal direction of current flow in three dimen- sions around and between electrodes). When the probe tips are rotated horizontally and vertically precisely per- pendicular to the lines of flux (along an isopotential sur- face), there will be no voltage differential, and the voltmeter or oscilloscope will register zero volts. Voltage gradients can also be approximated as the difference be- tween voltages measured from the electrode to two suffi- ciently close points in the water (Kolz, 1993; Kolz and Reynolds, 1990b; Kolz et aI., 1998). Like voltage-gradient probes, fish are subject to the greatest voltage differen- tial when they are oriented along the lines offlux. This is often referred to as "head-to-tail voltage." Fish are sub- ject to the least voltage differential when oriented per- pendicular to flux lines. Voltmeters specifically designed to measure peak voltage (e.g., peak-voltage detectors; Jesien and Hocutt, 1990) or oscilloscopes should be used for accurate measurements of peak voltage gradient at specific reference points in PDC (or pulsed AC) fields. However, the presence of voltage spikes in a PDC waveform (discussed earlier) can affect r~adings in some peak- voltage detectors. Oscilloscopes, although more expensive, allow the user to observe voltage spikes and differentiate such from normal peak voltages or voltage gradients by ignoring any spikes, as well as to monitor other waveform characteristics (e.g., shape, pulse frequency, pulse width, and duty cycle). A typical voltmeter (or multimeter) can be used to measure the constant voltages or voltage gradients at specific reference points in smooth DC fields (peak = mean) and rms voltages or voltage gradi~nts in AC fields. But according to Jesien and Hocutt (1990) and Fredenberg (personal communication), such ~eters cannot accurately measure either peak or mean voltages in PDC or pulsed AC. For the latter, either an oscilloscope or special instrumentation (e.g., peak-voltage detector) is required (Kolz, 1993). However, if a smooth DC or AC field can be temporarily substituted using the same system and peak output, the DC voltage or voltage gradients measured or peak values calculated from AC rms measurements made with a standard voltmeter in that field should be identical to peak voltages or voltage gradients in the corresponding PDC or pulsed AC field. Heterogeneous and Homogeneous Fields Because the basins occupied by rivers, streams, lakes, reservoirs, and most other waters are irregular in shape, and their cross-sectional areas are much larger than the electrodes, electrofishing fields generated therein are heterogeneous. In such fields, lines of flux (current) can be visualized as radiating from and spreading widely around and between the electrodes (Fig. 9). Field inten- sity is greatest next to the electrodes and decreases to barely perceptible levels as distance from the electrodes increases, even in the area directly between anode and cathode when they are sufficiently separated. The actual field intensity encountered by a fish in a heterogeneous field depends on the fish's location in the field. Homogeneous fields are typically restricted to labo- ratory settings in raceways, troughs, or tanks with a con- stant cross-sectional profile and electrodes approximating that profile at each end of the desired field. In homoge- neous fields, the current flows parallel to the sides of the container directly from one electrode to the other. Except adjacent to bounding surfaces or substrates, this arrange- ment provides a constant voltage gradient, current den- sity, and power density regardless of location between the electrodes. Controlled experiments in homogeneous fields allow relatively precise control offield intensity and eliminate many of the electric-field variables that are encountered in natural waters. This greatly simplifies experimental con- ditions and facilitates determination of cause and effect, but results may be difficult to extrapolate to normal electrofishing operations. Bounding or Surrounded Media and Substrates Depending on their porosity and conductivity, the bounding media or substrates of a body of water can affect the distribution of electricity in that body of water (Sharber et aI., 1995). The conductivity of bottom substrates can vary considerably with location, even in the same water body. Haskell (1954) and Zalewski and Cowx (1990) reported that substrates offine particles and organic debris are more conductive than those of coarse gravel and rubble. Because of substrate and interstitial water conductivity, electric fields can extend well into the bottom substrate and even onshore. Riddle (1984) suggested that a person standing barefoot on a bank