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18 INFORMATION AND TECHNOLOGY REpORT--2003-0002 could be shocked. Some of the first electrofishing systems in the United States were shore-based and used AC with one electrode or electrode array implanted in the ground along shore (Haskell, 1940, 1950, 1954); this practice is still used today, including DC and PDC systems with buried cathodes. Smith (1991) described an experimental electric shark barrier that also incorporated electrodes implanted onshore rather than directly in the water. Interactions between water and a bounding or sur- rounded medium or substrate of different conductivity apparently cause water conductivity near the interface to progressively increase or decrease toward that of the adjacent surface with corresponding changes in current density and voltage gradient (c = J / E). If the adjacent medium or substrate is more conductive than the water some distance away, the current in the water near its sur- face progressively concentrates (current density in- creases) as voltage gradient correspondingly declines (perhaps hypothetically, such that power density (D = JE) remains the same at each point as it would have been in the absence of the adjacent medium or substrate). Con- versely, current density is reduced and voltage gradient intensified immediately along or around less-conductive media, including air at the water surface (Zalewski and Cowx, 1990). As documented by Haskell (1954) and noted by many others since, fish themselves distort the field in their immediate vicinity if they are more or less conduc- tive than the water (Fig. 10). Except when used as the cathode, Riddle (1984) rec- ommended that metal boats not be used for electrofishing. He suggested that if a conductive vessel is positioned between the electrodes, it would interfere with the field (concentrate the current and thereby alter field size and shape) and might adversely affect electro fishing effi- ciency. According to Sharber (personal communication), when a metal boat is situated in an electric field and not used as the cathode, it has an intermediate electric charge, negative with respect to the anode and positive with re- spect to cathode. This concern seems to have been over- looked in much of the literature on boat electrofishing, although some, especially earlier, workers strongly dis- couraged use of metal boats for reasons of safety (Goodchild, 1990, 1991). Electrodes - Position, Size, Shape, and Other Matters According to Novotny (1990), the electrodes are the most crucial part of an electrofishing system. Their spac- ing, size, surface area, and shape, along with water con- ductivity, determine the electrical resistance ofthe system and, for a specified voltage output, the distribution of field intensity that determines the unconfined size and shape of the effective field. Electrode systems that are inappropriate for the power supply and waters to be sampled can result in poor electrofishing efficiency or unnecessary harm to fish. Novotny and Priegel (1974) listed the following desirable characteristics for an effec- tive electrode system: · establishment of the largest region of effective electric current distribution in the water to be sampled, · avoidance oflocal regions of unnecessarily large current densities, which waste power and are po- tentially harmful to fish, · adjustable to meet changes in water conductivity, · ability to negotiate weeds and obstructions, · ease of assembly and disassembly, and · avoidance of unnecessary disturbance to water to permit clear visual observation of fish. When electrodes are positioned sufficiently far apart (more than several radii in the case of spherical electrodes- Novotny, 1990; 10 to 20 radii for rings-Smith, 1989), the field around each electrode is effectively independent and has no significant interactive effect on electrode or system resistance. The water outside well-separated an- odic and cathodic fields is considered to be at "ambient potential" because its electrical potential does not vary significantly (its voltage gradient is nil-Cuinat, 1967). Fish that remain in water of ambient potential, even between the electrodes, are theoretically unaffected by the electrofishing operation. The level of ambient potential relative to the electrodes depends on voltage output, to- tal resistance (sum of anodic and cathodic resistances), and the ratio of anodic to cathodic resistances (Kolz, 1993). Novotny (1990) emphasized that "the most common electrode problem is that the electrodes are simply too small. . . .". At the same output voltage, larger electrodes have less electrical resistance in water and radiate larger electric fields but with lower maximum field intensity im- mediately around them. Larger electrodes thereby reduce the zone of tetany and extend the effective field for taxis (DC and PDC fields) and narcosis (see definitions and discussion later under "Major Intensity-Dependent Re- sponses"). Increasing the number of anodes or cathodes in a system has a cumulative effect similar to increasing the size of an individual electrode. Maximum size or num- ber of anodes or cathodes is dictated largely by practical considerations (e.g., maneuverability, transportability, interference with netting) and, especially in high conduc- tivity waters, by generator capacity. When water con- ductivity is high, the size of the electrodes must sometimes be reduced to prevent generator overload. To minimize cathodic effects on fish when using DC or PDC, cathodes should be as large as practical relative to anodes. This will also desirably maximize anodic field intensity and reduce the overall electrical resistance of