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I I I I I I I I I I I I I I I I I I I Impacts of Electrofishing on Fish relationship of these units). Siemens is the term approved for the Intemational System of Units and used in the remainder of this report. Conductivity in natural waters ranges from as low as 5 J.LS/cm in pure mountain streams (Gatz et al. 1986; Zalewski and Cowx 1990) to 53,000 J.LS/crn in seawater (Omega Engineering Inc. 1990). The upper limit for potable water is about 1,500 J.LS/cm (Wydoski 1980). Conductivity in a particular body of water, although generally more-or-less uniform on the same day, can vary considerably from one location to another depending on substrate composition and especially the inflow of tributaries or effluents of highly different conductivities. Conductivity will also vary directly with water temperature. As temperature rises, water viscosity decreases 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 oC for ultra pure waters to 1.5% per oC for acids, alkalis, and concentrated salt solutions (Omega Engineering Inc. 1990). For natural waters between 10 and 250C, the coefficient is approximately 2 to 2.3% per oC. To approximate water conductivities at various temperatures within this range, Reynolds et al. (1988) used the equation c2 = c/(1.0it1.t2~ and Stemin et al. (1972, 1976) c2 = e/(I + 0.023(tl-~)) where" e" is conductivity and "t" is temperature. Conductivity measures are often normalized to 250C; it is important to note whether reported conductivities are for the temperatures actually encountered (as is often, but sometimes incorrectly, assumed) or normalized data. In addition to the transfer of electrons, the process of electrolysis at the electrodes results in the generation of gases and, more importantly, the loss of metal ions from the anode to the water, deposition of metal ions from the water onto the cathode, and formation of metallic compounds such as oxides on the cathode (Sharber pers. commun.). Riddle (1984) suggested that it was not wise to buy aluminum punts (boats) second-hand from electro-fishermen because the gauge of the metal might be substantially reduced. According to Sharber (pers. commun.), this is not a problem when a metal boat is used as the cathode. But when a metal boat is situated in an electric field and not used as an electrode, it has an intermediate electric charge, negative with respect to the anode and positive with respect to cathode. In Review I Electric Fields in Water 9 this case, electrolytic reactions result in both the formation of non-conductive metallic compounds on the boat's surface and the loss of structural metal. Over time, the latter reaction can reduce the structural integrity of the boat. When a boat is used as a cathode, no metal is lost but the non-conductive metallic compounds that form on the boat's surface can decrease its electrical efficiency. This coating can be periodically scraped or sanded away to recover cathodic efficiency, but in doing so, some structural metal may be inadvertently lost. Some researchers switch electrodes periodically to reverse the buildup of metallic oxides (Sharber and Carothers 1988). The effectiveness of this procedure has not been reported. Field Strength The responses of fish to . electric fields are dependent on the field's strength or intensity (some responses in PDC and AC are also frequency and waveform dependent). Field strength can be described by any of three interrelated and conductivity-dependent terms: voltage gradient (E, yolts per unit distance, usually V/cm), current density (J, ~peres per unit area of an isopotential surface, usually J.LA.Icm2), or power density (P, watts per unit volume between isopotential surfaces, usually J.LW/cm3). [An isopotential surface lies perpendicular to the field or current lines and is defined by a set of points having the same voltage differential when measured from the surface of the electrode; if the water is of uniform conductivity and unbounded, the electrode is spherical, and other electrodes are sufficiently distant, each isopotential surface will form a shell the points of which are a uniform distance from the surface of the electrode.] Of these quantities, only voltage gradient can be measured directly. The other descriptors of field strength are functions of conductivity (c) and voltage gradient (J = eE and P = eE = JE ; Kolz, A. L., 1989). Figures 5 and 6 illustrate the relationship between these descriptors of field strength and conductivity. Note that in the middle graph of Figure 5, the curve for voltage gradient becomes asymptotic with the y-axis as conductivity approaches zero and with the x-axis as conductivity approaches infinity, whereas the reverse is true for current density. As a result, the curve for voltage gradient at a fixed power density is