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IN-WATER ELECTRICAL MEASUREMENTS 21 what is being defined as horizontal, vertical, head- to-tail, dorsal-to-ventral, or some other general- ized description for voltage gradient. The directional characteristic of the gradient vector becomes particularly significant when meas- uring the thresholds of electroshock response. Fish are most sensitive when their spinal columns are parallel to the voltage gradient vector (L,arnarque 1990). This is not to imply that fish cannot be shocked in other orientations; in fact, fish can even be stunned in an electric field that effectively gen- erates no head-to-tail voltage. There is little infor- mation regarding these directional effects, but one possible explanation (an unproven hypothesis) can be developed by applying Fbyntmg's power vector (Remo and Whinnery 1953). Under this concept, when the direction of electrical current flow is aligned with the spinal column, the power vectors are directed radially inward along the entire longi- tudinal axis of the fish (Fig. 25a). This orientation is optimal for the transfer and dissipation of power into the axis of the spinal column. Conversely, the least favorable orientation for power transfer oc- curs when the direction of electrical current is perpendicular to the spinal column (Fig. 25b). When transverse, there is less opportunity to trans- fer power because the vectors only intersect the width of the spinal column (rather than its entire length), and there is an additional reduction in the power transfer caused by the angular relation be- tween the vectors and the spinal column. Discussion The type of electrode configuration used for any particular electrofishing operation is dependent on a variety of electrical and biological factors. The guidelines for selecting an electrode design must include a knowledge of the following factors: power capabilities of the electrical source, desired size and intensity of the electric field, estimates of the thresholds of electroshock response for the fish to be sampled, how power density can be adjusted for changes in water conductivity, and special considerations regarding the working habitat (aquatic plants, poor water clarity, fast water, feeding areas, hazards, maneuverability, etc.). I described a generalized approach with ap- propriate measurement techniques to allow re- searchers to compare, measure, and adapt elec- trode designs to best accomplish a particular need. Remember there is no such thing as the univer- sally perfect electrode. Power Vectors i ? 11 f /(O D1 Z"Of v Q119 *\\ Power Vectors i i Fig. 25. Diagrams illustrate how power vectors intersect and transmit energy into the spinal column of a fish: (a) Power vectors intersect the entire length of the spinal column for a maximum transfer of energy when the spine is parallel to the direction of the electrical current. (b) Power vectors intersect only the width of the spinal column for a minimal transfer of energy when the spine is perpendicular to the direction of the electrical current. The determination of an electrode's resistance and the development of the in-water voltage profiles