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It is apparent that there is quite an oxygen demand put on fish as a result of shocking, and that ac current has the most effect, causing the largest consumption of oxygen and requiring a longer time to repay that oxygen debt. The physiological responses are dependent upon the severity and duration of the shock. The rapid increase in breathing amplitude after shocking is a method used by the fish to repay the oxygen debt suffered by the tissues. Shocked fish are not fully recovered simply because they have regained their equilibrium and are able to swim away. It takes a period of time fora stressed fish to fully recover to its normal preshock condition. While studying the effects of do and pulsating do currents on fish, Taylor et al. (1957) recorded the heartbeat of a 9-inch ,rainbow trout shocked with a do tetanizing current. The actions of the heart were recorded on a kymogram (Figure 7). The humped lines represent the heartbeats of the trout and the solid line beneath it indicates when the current was applied. After the current was first applied, the heartbeat increased a beat, and then skipped several beats before resuming its regular pace.. It resumed its regular pace while continuing to receive the shock. So, although the shock caused a drastic effect on the fish initially, the effect was not long lasting (a few seconds) and death .from shocking must be related to something other than cardiac arrest. Wood et al. (1983). also confirmed that cardiac failure was not the cause of death in severely exercised fish. o~ ~ of~ o~ Figure 7. Kymogram showing the heart action of a 9-inch rainbow trout during direct stimulation with a tetanizing current. The lower line indicates when the circuit is turned on and off (adapted from Taylor et al. 1957). Physical Injur to Fish Caused b Electrofishin Hauck (1949), using ac current, observed an increased respiratory action in all fish sub- jected to an electrical current. Schreck et al. (1976 )., observed breathing amplitude in- creases from 50-350; in fish immediately after shocking`: The rapid increase in breathing amplitude is an effort by fish to supply oxygen to the tissues, to pay off the oxygen debt created by the shock (Heath 1973). Hauck (1949) noticed that in some fish there was paralysis of the muscles on the side nearer the electrode, causing the fish to swim in an arc around the source of power. Fish also hemorrhaged from the gills or vent. He must have been shocking the fish severely because he also saw intestines protruding from the vent. Dark vertical bars or stripes (burn scars) were created on fish that touched "live" elec- trodes (Figure 8). This effect has also been reported for other electrofishing operations (Elson 1950, Horak-and Klein 1967, Sternin et al. 1976). Some fish receiving burn scars are unable to move any portion of their bodies behind the dark band. Burn marks, similar to those in Figure 8, can also occur on fish that have come into close contact with the electrodes without actually touching them. In these instances, the force of the electricity damages the autonomic nerve fibers which regulate melanophore constriction and the melanin granules are dispersed, darkening the affected area (Nilsson et al. 1983). Similar bands have also been produced physically by probing the vertebral area of fishes with a hypo- derm%c needle (J. Cech, Jr., University of California-Davis, pers. comm.). Hauck. (1949 also observed loss of locomotion, balance, and impaired circulation among fish several days after shocking. He performed autopsies on these injured .fish. and found numerous internal injuries. These included fractured vertebrae, broken ribs, curvature of the spine (S-shape), blood clots, and hemorrhaging and rupturing of the major arteries and veins. The secret of successful electrofishing is to use the minimum power necessary to collect fish, because the total energy absorbed 6y the fish determines whether that fish well be injured or not. CAL-NEYA WILDLIFE TRANSACTIONS 1984 61