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in uS/ series, their Law, volt- elec- ! V/cm (i.e., n (r = 5, and ctrode nce to elec- tay be water d sub- or de- r den- Power Id can Thus, :trode, 4.997, i from hresh- ithout :rolled ng on (Kolz results )f Fish find- ,DFG, locked I mm) -), the kDFG elation ptable COMMENTS solution is found.) Furthermore, field experiments in net-pens demonstrated that only 0.5 V/cm or about 12 µW/cm3 [µW/cm3 = 50(0.5)21 were re- quired to narcotize these fish. Although the thresh- old level for spinal injury was not determined, it seemed that any rainbow trout that got within 0.5 m of the ADFG electrodes (? 1.0 V/cm or 50 AW/ cml) were likely to be injured; these power density levels were well under those near the Sharber- Carothers electrodes. Our explanation for the high injury incidence, then, is that large rainbow trout have low thresholds for various reactions (.e.g., narcosis, injury) to electrical energy, and that most electrofishing equipment is operated at levels well above those thresholds. Pulsed wave forms, especially AC, are damaging to fish when applied at power densities exceeding those required for narcosis, a state of muscle re- laxation. Excessive power densities cause tetany (muscle contraction) simultaneously on both sides of the fish, compacting the spine and associated nerves and blood vessels to the point of physical damage. We recommend that biologists who elec- trofish in waters containing large rainbow trout begin to evaluate continuous DC; this wave form is much less damaging to large fish and is better than pulsed DC for attracting fish to the anode (Vibert 1967). We commend Sharber and Carothers for pub- lishing their data on this troublesome issue and further recommend that biologists avoid using AC, use minimum voltage settings of pulsed DC to capture target fish, and strive to keep large fish, especially rainbow trout, away from the high-volt- age gradients and power densities that exist near all electrodes. JAMES B. REYNOLDS U.S. Fish and Wildlife Service Alaska Cooperative Fishery Research Unit 138 AHRB. University of Alaska Fairbanks Fairbanks, Alaska 99775. USA A. LAWRENCE KO U.S. Department of Agriculture Denver wildlife Research Center Federal Center, Building 16 Denver, Colorado 80225, USA We agree with Reynolds and Kolz on the i portance of power density in the water surround ing fish during the electrofishing process. Effectiv electrofishing requires that electric current flowi in the water adjacent to the fish must produce an 517 sustain a voltage difference between the head and tail sufficient to induce electrotaxis and eventually narcosis (Haskell 1954; Vibert 1967). There is no question that a fish is subjected to ever increasing levels of electrical energy as it approaches an elec- trode. Reynolds and Kolz suggest that if the high- est power level (near the electrode) is that which will induce narcosis, spinal injury can be avoided or substantially mitigated. The two experiments cited by Reynolds and Kolz do not provide data to adequately support this contention. In neither the experiment with gold- fish nor the net-pen experiment with rainbow trout were the fish subjected to pathological (autopsy or x-ray) examination (J. Reynolds and D. McBride, personal communications). Our previous work (Sharber and Carothers 1988) clearly demonstrat- ed that severely injured fish (massive hematomas and broken vertebrae) often elicited no behavioral responses that would indicate the severity of their injuries. We are presently conducting experiments de- signed to examine the relationship between the onset of narcosis and factors that cause compres- sion fractures in fish vertebrae. Our data support an alternative hypothesis to that of Reynolds and Kolz. Simply, we are finding that when the power level is high enough to cause narcosis in large trout, spinal injuries can and do occur. Reynolds and Kolz conclude that power density is the main parameter for describing reactions that lead to electrotaxis, narcosis, and injury. The more widely accepted view (Halsband 1967) is that the strength of the current, its duration, and the rate of change in its intensity all play a role in the stimulation of neuromuscular systems (Best and Taylor 1950; Lamarque 1967). This is true be- cause animal tissues form complex electrical cir- cuits composed of both resistive and capacitive elements that together define the impedance to electric current flow when the intensity of the cur- rent is constantly modulated, as in an AC or pulsed Lz DC system. In such circuits, the power density theory, based on a simple determination of the relative DC resistance of the water and fish, cannot predict the efficiency of energy penetration into tissues or characterize the nature or intensity of the reaction to the stimulus (Sternin et al. 1972; Fling 1981). m- Reynolds and Kolz are correct when they note - that we offered no explanation for the incidence e of injury found during our experiment-we do not ng have one at this time. Their explanation that most d electrofishing equipment is operated at levels well