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554 TRANS. AMER. FISH. SOC., 1972, NO.3 I I, I, I " :) I' :~ ! I waters of differing conductivity. This has been illustrated by Denzer (1956). Lamarque (1967) and Halsband (1967) agree that fish reactions depend essentially on the voltage drop per unit length of fish. They show that the threshold voltage drop varies with the difference existing between water conductivity and fish conductivity. In salt water, which is more conductive than the fish, the body voltage drop relative to the voltage drop in the me- dium is greater than in a situation where fish and water have the same conductivity. This makes the fish slightly easier to shock (in terms of voltage required, not power output) and the reaction threshold occurs at a lower voltage drop between electrodes. In water more resistant than fish, the body voltage drop is lowe'r than in a medium where fish and water have the same conductivity, and high voltage is required to shock the fish. I 'j DETERMINING THE RELATIONSHIP BETWEEN ENERGY DENSITY AND RECOVERY TIME To study the effects of electrical energy on the recovery time, the amount of electrical energy going through the fish has to be known. A simple method of determining the amount of energy going through the fish is to adjust the conductivity of the water until it has the same conductivity as fish flesh. This was done in a! unique way,. A small polyethylene tube, 125 mm in length and 14 mm inside diameter, slightly larger and longer than the fish was filled with water and plugged at each end with corks covered with aluminum foil. An ohmmeter was attached to the aluminum foil, and the resistance was measured and recorded. Next a fish was placed in the tube and the excess water was discarded. A second resistance meaf~rement was made and compared with the first. !his pJ;"ocedure was repeated with increasing :;;1 '"", , ;i) :::j ... amounts of salt in solution until the two read- ings were the same. Several fish were com- pared with less than a 10% variation in the resistance readings. The most important requirement for this determination is that the amount of water displaced by the fish be a large fraction of the total water in the vial. In our experimental setup this was approximately one-half that held by the vial. After this was done, the amount of energy going through the fish per unit volume was assumed to he the same as the energy going through the water per unit volume. The con- ductivity of the common shiners used in these experiments was determined to be approxi- mately the same as a .02?% solution of sodium chloride in deionized water. In comparison, sea water is approximately 140 times saltier. Any effect the salt may have on the recovery time, other than the obvious effect of increas- ing the conductivity of the water, was con- sidered to be negligible. Experiments were designed (Table 2) to study the effects of energy density on the re- covery time of common shiners in a .025% "solution of sodium chloride. Twenty shiners were used in each experiment. The total en- ergy through the medium was constant in each experiment, but energy density vaned with the quantity of water used. As expected, no corre- , lation existed between the total energy through the medium and the recovery time. Energy density was the more important factor, since it measured the energy per unit volume through the fish as well as the medium. Results show that the recovery time can be directly related to energy density (Fig. i 2) . DISCUSSION Previous studies by Halsband (1967) ,La- marque (1967L and Vibert (1963) ~ave TABLE2.-Average recovery time of common shiners subje~ted to different levels of electrical energy density. The total energy applied to the medium in each experiment was 169 joules Energy density Voltage Current Time Volume of water (millijoules/ cubip Average recovery ( volts ) ( milliamps ) ( seconds ) (cubic centimeters) centimeter) time (seconds) 130 260 5 3750 45 6.30 65 86.6 30 ~320 73.5 18.40 130 130 10 1750 96.5 18.70 I 130 97.5 13.3 1350 125 49.60 130 65 20 900 188 68.50