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12000 10000 (Water conductivity, c = 500 JJS/cm) Power density D, JJW/cm'3 8000 J = cE D = JE oE2 6000 4000 Lower graph / '" 5 2000 ~ ::J. "0 C Nctl 2000 E () i 1500 o o 2 3 4 (Water conductivity, c = 500 JJS/cm) Power density D, JJW/cm3 Lower left corner of upper graph 1000 500 Current density J, jJA/cm2 00 0.5 1 1.5 Voltage gradient, E, V/cm Fig. 8. Changes in current density and power density relative to voltage gradient in water with a conductivity of 500 ,uS/cm. (For other conductivities, adjust the values along the vertical axis in direct proportion to the change in conductivity-e.g., for half the conductivity, 250 ,uS/ cm, halve the values along the Y-axis.) voltage gradient in the middle graph of Fig. 7 becomes asymptotic with the Y-axis as conductivity approaches zero and asymptotic with the X-axis as conductivity approaches infinity, whereas the situation is reversed for current density. As a result, the curve for voltage gradient at a fixed power density is relatively flat over all but the lower end of the range for conductivity in fresh waters (bottom graph of Fig. 7) and practically horizontal for more saline waters. The relative stability of voltage gradient in medium- to high-conductivity fresh waters has important implications with respect to field-intensity response thresholds and standardization of electrofishing fields (discussed later in this review). Changes in current density and power density rela- tive to voltage gradient at a constant water conductivity of500 !is/cm are illustrated in Fig. 8 (the lower graph is an expansion of the lower left corner of the upper graph). Note that the values for current density, power density, SNYDER 15 5 and water conductivity are equal when voltage gradient is 1 V /cm (as in Fig. 6), and, as predicted by their defini- tions, current density increases linearly and power den- sity geometrically with voltage gradient. For values of voltage gradient less than I V /cm, power density is less than current density. For PDC and AC, in-water measures or calculations of peak field intensity (maximum voltage gradient, cur- rent density, or power density through one or more wave- form cycles) are substantially greater and probably more biologically significant (Kolz and Reynolds, 1989b) than corresponding values of mean PDC or rmsAC field inten- sity. For sinusoidal AC (Fig. 5A), peak voltage gradient and output voltage are approximately 41 % greater than corresponding rms values (V p = 1.41 xV rms)' Root-mean- square values are necessary for AC because mean volt- age would be zero. Concern that positive peak to negative peak voltage differential in AC might be even more bio- logically significant than peak voltage differential from zero or base level is unwarranted; the negative portion of the waveform represents a reversal in current direction rather than negative voltage per se (however, fish are polarity sensitive and accordingly some responses differ when subjected to alternating or unidirectional currents of comparable peak voltage). For square PDC waveforms (Figs. 5F and I), mean voltage varies directly with duty cycle (percentage of on time); for example, with a 25% duty cycle, peak voltage is four times greater than mean voltage. For other PDC waveforms (Figs. 5D, E, G, and H), mean voltage varies according to their shape as well as duty cycle. For smooth DC, peak and mean values for field intensity or electrical output are identical. To facilitate comparisons, researchers and authors must specify whether measures of field intensity or output (voltage, amperage, power) for PDC or AC are peak or mean (rms in AC) values. Meters on most eIectrofishing control boxes register mean output values for PDC or rms output values for AC (e.g., volt and ammeter on Coffelt's VVP-15 and ammeter on Smith-Root's GPP 5.0), whereas meters on very few units register peak output (e.g., ammeter on Coffelt's Mark XXII which generates CPS). Also, biologists should not rely on the accuracy of control box settings and meters without periodic calibration. Van Zee et al. (1996) revealed that voltmeters and ammeters included on some electrofishing control units (e.g., Coffelt's VVP-15) are meant to serve as references for relative or consistent settings rather than provide accurate measures of output. For example, using a boat- electrofishing system with a control box adjusted for an output of 230 V and 2 A for each of two currents, they reported oscilloscope measures of peak output to be 280 V and 1.7 A for a quarter-sine-wave PDC and 250 V and 1.5 A for square-wave PDC. The latter current also was set for 80 Hz with a 50% duty cycle, but the oscilloscope 2