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12 INFORMATION AND TECHNOLOGY REpORT--2003-0002 Such pulse-train waveforms were suggested by Haskell et a!. (1954) over 45 years ago. PDCs are often favored for electrofishing because they require much less-powerful generators or batteries than DC, and often AC, to create electric fields of comparable size and effectiveness. Through various manipulations of the current, DC and PDC have even been hybridized to produce a PDC on top of DC (Vincent, 1971; Fredenberg, 1992; Fig. 51). In such currents, the pulses drop only to a preset minimum voltage level when switched off rather than to zero volts. Strongly rippled DC (weakly filtered, rectifiedAC) could be considered a hybrid current. The various PDC waveforms generated by electrofishing control boxes are sometimes characterized by anomalies in the expected shape. For example, Fredenberg (1992) reported spikes at the leading or trail- ing ends of square-waveform pulses; Van lee et al. (1996) documented under test conditions the presence of a trail- ing voltage spike 50 to 60% higher than the rest of a square-waveform pulse followed immediately by a small exponential pulse of reverse polarity (magnitude 20% of unspiked pulse voltage); and Sharber and Carothers (1988) described small, rounded, secondary pulses immediately following pulses in a 60-Hz, exponential waveform. In the latter example, Sharber and Carothers (1988) suggested that the small secondary pulse was of sufficient voltage near the anode to produce essentially a 120-Hz, mixed waveform that enhanced the immobilization offish. 1esien and Hocutt (1990) noted that nominally square PDC waveforms (Fig. 5F) generated by their equipment changed shape as water conductivity increased. At con- ductivities of about 100 IlS/cm, the trailing edge was not perpendicular, and the voltage level was not constant across the top ofthe pulse. An exponential-like voltage spike became evident at 1,000 IlS/cm and was especially prominent at 10,000 JlS/cm. In contrast, they found that characteristics rof their pulsed AC waveforms remained constant with changes in water conductivity. Kolz (per- sonal communication) suggested that they may have used a faulty power source for their square-wave PDC. Because output waveforms are not always as ex- pected based on control box settings, it is important to periodically calibrate, verify, and document waveform in the output circuit with an oscilloscope. For example, an oscilloscope tracing illustrated by Van lee et a!. (1996) for square-wave PDC generated with control-box settings for 80 Hz and 50% duty cycle revealed an actual frequency of 73 Hz and duty cycle of 64%, as well as the trailing spike and negative secondary pulse described above. Review of the published literature and personal communications revealed that authors and biologists frequently fail to note the type of current and waveform used in electrofishing. Even when noted, some descriptions ofthe current are incomplete, misleading, or erroneous. PDC is often simply referred to as DC, reflecting its unipolar but not its pulsed nature. Also, referring to its typical origin via an AC generator, PDCs are sometimes incompletely called "rectifiedAC," which more specifically refers to either ofthe two half-sine PDC waveforms (Figs. 5D and E) or, when filtered or originating from 3-phase AC, rippled DC (Fig. 5C). Even the term "pulsedAC" has been improperly used for PDC. For example, Hill and Willis . (1994) used a current which they and an early manual for the Coffelt VVP-15 electro fishing control unit referred to as pulsed AC. Hill and Willis (1994) described it as the positive half of a sinusoidal AC waveform, and the manual illustrated it as quarter-sine PDC (Fig. 5G) but mislabeled it as pulsedAC (Van Zee et a!., 1996; the error has been corrected in more-recent versions ofthe Coffelt manual). Furthermore, an oscilloscope tracing of this waveform by Van lee et a!. (1996) closely approximated a square-wave PDC, possibly a slightly compressed quarter-sine waveform with the trailing margin squared off near the top. Field Intensity The responses of fish to electric fields in water are dependent, at least in part, on the field's strength or in- tensity. Field intensity can be described by any of three interrelated quantities: voltage gradient, current density, or power density. The relations between these descrip- tors of field intensity and water conductivity are illus- trated in Figs. 6, 7, and 8. Voltage gradient (E) is the average voltage differen- tial per unit distance along lines of current or flux be- tween two isopotential surfaces and is usually expressed as volts per centimeter, V /cm. Voltage is the amount of potential energy stored per unit of electrical charge, ex- pressed as volts (V, joules/coulomb). Lines of flux (or current) represent the net directions or paths of current in an electric field around and between electrodes of op- posite polarity. An isopotential surface lies perpendicu- lar to the lines of flux and is defined by a set of points having the same voltage differential from the surface of the electrode. Ifthe water is of uniform conductivity and unbounded for a sufficient distance in all directions (an unlikely condition), the electrode is spherical, and other electrodes are sufficiently distant, at least the isopotential surfaces near the electrode can be visualized as shells, all points of which are the same distance from the surface of the electrode. Voltage gradient can be physically measured in the water or approximated by calculation based on output voltage, the surface area, size, and shape of the elec- trodes, the distance between them, and proximity of bounding or surrounded surfaces or media of different conductivity. For practical purposes, the distribution of