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"- Fish were subjected to pulsating direct current ob- tained by conversion from an AC source. Fish were placed in stalls between the electrodes in the water- filled plastic aquariurn and the electrical system was energized. Irnmediately after the desired duration of exposure, the fish were placed in 94.6-liter aquariurns with continuous water flow and aeration. The aquariums were checked for dead fish at intervals of 1,3, 6, and 24 h after exposure and then daily for 7 days. Control fish were treated similarly, except that the elec- trical system was not energized. Ten fish per test were subjected to pulse frequencies of 1.6,8.8, and 16.0/s while other electrical characteristics remained constant. Each pulse frequency rate was tested on fish exposed for 5, 15,30,45,60, 120, and 180 s. All tests were replicated. Fish were exposed to an elec- trical field only once (a fish surviving one test was never used in another). Darters ranged in length frorn 2.5 to 7.5 crn and bluegills from 9.0 to 17.0 crn. Fish of similar size were tested together. No significant differences in rnortality between these size ranges were noted. Resul ts and Discussion Fishery biologists prefer to use direct rather than alternating curr~nt for electrofishing because of the electrotactic response. Pulsed direct current produces even greater anodic attraction than does continuous di- rect current (Collins et al. 1954; Miller 1962; Northrop 1967; Vincent 1971). Although optirnurn pulse rates for electrotaxis of fish have not been clearly defined, Nor- throp (1967) noted that low pulse rates (1 to lO/s) pro- duce poor electrotaxis. Higher pulse rates (above 50/s) tend to narcotize, damage, or kill fish and do not perrnit . electrotaxis (Northrop 1961; Collins et al. 1954). Mortality of the test fish increased with increases in the electrical characteristics examined. Duration of ex- posure appeared to be the factor most responsible for death offish. Mortality was low when fish were exposed for 15 s or less, but increased progressively with dura- tion of exposure. On the basis of these data, mortality of electroshocked fish should be negligible if fish are re- moved frorn the electric field within 15 s after initial . shock and if other electrical characteristics ernployed are within the ranges studied. Tirne required for fish to recover equilibriurn in- creased with duration of exposure. The loss of equilib- rium for even a short tirne could be of serious conse- quence in a strearn, where lethargic fish would be easy prey for predators. After the fish have been shocked, the fish rernoved frorn a stream should be held in well-aer- ated holding tanks until they recover equilibrium. The length of time that fish should be held for full physiolog- ical recovery is about 24 h (Schreck et al. 1976). How- ever, since this amount of holding tirne is unreasonable 162 for field. operations, releasing fish only after they have fully recovered equilibriurn should decrease loss of fish to predation. OUf data showed relatively high mortality of fantail darters and bluegills in the pulse frequency defined as gi ving good electrotactic response (Figs. 1,2). However, because the fish were held in alignrnent parallel to the direction of the field, they were subjected to rnaximurn voltage. At any other angle a fish would receive less than maxirnurn voltage, and would be less likely to be killed. In a stream, fish would be randomly located in an electrical field, prirnarily aligned parallel to water cur- rent and would be subjected to varying voltages. There- fore, the percentage of fish killed directly by the shock should be lower under stream conditions than under our experimental conditions. I , , 100 90 o - 1.6 pulses/s c - 8.8 pulses/s 6 - 16.0 pulses/s A 6 80 70 6 60 ~ .~ 50 OJ ~ 0 ~40 30 20 10 30 60 120 180 Duration of Exposure (s) Fig. 1. The relation of pulse frequency to the effect of duration of exposure on mortality offantail darters. Test run at 4.0 V/crn. . I Our data are not directly applicable to a field situation because of the enforced orientation of the fish parallel to the electric field, the rectilinearity of the field induced by the plate electrodes, and the confinement of the field within the plastic aquarium. However, our results offer guidance to biologists on the levels and patterns of pul- sating direct current applicable to field collection situa- tions. Preliminary sarnpling of a section of each stream L, THE PROGRESSIVE FISH-CULTURIST