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10 INFORMATION AND TECHNOLOGY REpORT--2003-0002 water is about 1,500 IlS/cm (Wydoski, 1980). Conductivity in a particular body of water, although generally quite uniform, can vary considerably from one location to another depending on substrate composition and especially the inflow of tributaries or effluents of highly different conductivities. Ambient water conductivity also varies with water temperature. As temperature rises, water viscosity de- creases and ionic mobility and solubility of most salts increase. Rates of change in conductivity depend on ionic content and vary from about 5.2% per degree C for ultra- pure waters to 1.5% per degree C for acids, alkalis, and concentrated salt solutions (Omega Engineering Inc., 1990). For natural waters between 10 and 250 C, the coef- ficient is approximately 2 to 2.3% per degree C. To ap- proximate water conductivities at various temperatures within this range, Reynolds et aI. (1988), Reynolds (1996), and Kolz et al. (1998) used the equation C2 = C J / (1.02(11-12)), and Stem in et al. (1972, 1976) used C2 = CJ / (1 + 0.023(tl - t2)), where C is conductivity and t is tem- perature. It is important to record whether measured or reported water conductivity is ambient (actual value for the temperature at which it was measured) or specific (value normalized to 250 C); if the latter, it needs to be recalculated for ambient (actual) temperature. Electrojishing Currents and Waveforms There are two principal types of electrical currents, but interrupted or pulsed variations of one are sufficiently different and important to be treated effectively as a third type. Bipolar or alternating current (AC) is characterized by continually reversing polarity and movement of elec- trons or ions of like charge (Fig. 5A). Unipolar or direct current (DC) is characterized by movement of electrons or ions oflike charge in one direction (Figs. 5B-J). How- ever, as used hereafter, DC specifically refers to a con- tinuous unipolar current of constant voltage (smooth or straight DC, Fig. 5B) or nearly constant voltage (rippled DC, Fig. 5C). When a unipolar current is periodically in- terrupted or pulsed, it is specifically referred to as the third type of current, pulsed DC (PDC; Figs. 5D-I). AC also can be pulsed, but pulsedAC (e.g., Jesien and Hocutt, 1990) is rarely used for electrofishing. For AC and POC, changes in voltage amplitude or differential (current intensity) over time define the shape (graphical form as displayed by an oscilloscope) and fre- quency (Hz-hertz = cycles, pulses, or pulse patterns per second) of their waveforms. Although other AC wave- form shapes and frequencies are possible, AC used for electro fishing usually consists of a sinusoidal waveform at a fixed frequency of 50 or 60 Hz (single-phase gene:ra- tor), 180 Hz (three-phase generator), or higher (e.g., 300 or 400 Hz) as a function of generator speed (Novotny and Priegel, 1974; Novotny, 1990). Depending on how they are produced, PDC waveforms used for electrofishing occur in a variety of shapes, most commonly square (rectangular), half-sine, quarter-sine, or exponential, and can be delivered over a wide range of frequencies, usually between 15 and 120 Hz, but at least experimentally from 1 to about 500 Hz. Pulse-frequency pattern can be either simple (uniform) or complex, the latter usually consisting of a high primary frequency interrupted secondarily at a much slower frequency to produce bursts, packets, or trains of the higher-frequency pulses (Fig. 51). PDC waveforms also are characterized by pulse width (time current flows during each pulse, usually expressed in ms, milliseconds) and duty cycle (percentage of time current actually flows from the beginning of one simple pulse or complex pulse-pattern to the next). For simple PDC, duty cycle is a function of pulse frequency and width. As frequency in a PDC is increased, a constant pulse width results in a greater duty cycle, whereas a constant duty cycle results in a proportionately shorter pulse width. In modem electrofishing, DC is usually produced by conditioning power from an AC generator, or a battery and inverter, with transformers, rectifiers, and filters (Novotny, 1990; Novotny and Priegel, 1971, 1974). DC produced by true DC generators is smooth (Fig. 5B), whereas that produced by filtering rectified current from an AC generator tends to be at least slightly rippled (Fig. 5C). However, DC generators are heavier, more ex- pensive, less flexible in voltage control, and less reliable than AC generators with comparable power ratings. DC produced by a three-phase AC generator is already rela- tively smooth and requires much less conditioning than that produced by a single-phase AC generator. In most cases, PDC waveforms also are produced from rectified AC. Rectified sinusoidal AC directly pro- duces half-sine PDCs at either the same or twice the AC frequency, depending on whether the current is half or full-wave rectified (Figs. 5D and E). Mechanical or elec- tronic choppers (pulsators) are used to generate quarter- sine and exponential or capacitor-discharge waveforms (Figs. 5G and H) from unfiltered rectified AC or square waveforms (Fig. 5F) from rectifiedAC that has been first filtered to produce DC. Square waveforms are perhaps the easiest to adjust in pulse width and frequency. Some very flexible electrofishing control units provideAC, DC, and PDC-the latter with variable pulse frequencies, widths, and sometimes shape. Some systems allow or incorporate secondary switching or interruption of PDC to produce complex pulse frequencies (e.g., University of Wisconsin Engineering and Technology Center's Quadrapulse, Smith-Root's P.O. w., and Coffelt's CPS).