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North American Journal of Fisheries Management 8:516-518, 1988
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<br />COMMENTS
<br />Electrofishing Injury to Large Rainbow Trout
<br />Sharber and Carothers (1988) recently evalu-
<br />ated the effect of three wave forms of pulsed DC
<br />on spinal injury to large (300-560 mm, total length)
<br />rainbow trout Oncorhynchus mykiss (formerly
<br />Sabno gairdneri) captured by electrofishing in the
<br />Colorado River. They reported an overall injury
<br />incidence of 50% and a significantly higher inci-
<br />dence in fish stunned with a quarter-sine wave
<br />(67%) than in those captured with either expo-
<br />nential or square waves (44% each). Although
<br />Sharber and Carothers focused their study on the
<br />comparison of the three wave forms, they recog-
<br />nized a more important conclusion from their re-
<br />sults: large rainbow trout are very susceptible to
<br />injury induced by any form of pulsed electroshock.
<br />Indeed, Hauck (1949) reported that 26% of 503
<br />large rainbow trout (he gave no size range for his
<br />sample, but mentioned weights of 0.7-2.3 kg) died
<br />within 2-5 d after they were exposed to AC; the
<br />incidence of injuries was not stated but probably
<br />was much higher. Other studies of electroshock
<br />injury to rainbow trout, cited by Sharber and Car-
<br />others, showed lower injury incidence; however,
<br />these studies were conducted on smaller fish or in
<br />less-conductive water, which could account for the
<br />lower incidence.
<br />Although Sharber and Carothers appropriately
<br />cautioned biologists about electroshocking large
<br />rainbow trout, they offered no explanation for the
<br />alarming incidence of injury or recommendations
<br />for addressing the problem. We believe an expla-
<br />nation and appropriate countermeasures exist, and
<br />herewith offer them for consideration.
<br />Given the electrodes (two spheres, 30 cm in di-
<br />ameter), peak voltage (260 V), and water conduc-
<br />tivity (about 700,uS/cm) described by Sharber and
<br />Carothers, one can calculate the intensity of the
<br />electrical field. Because the water conductivity of
<br />700,uS/cm is specific to 25°C (N. Sharber, personal
<br />communication), it must be recalculated to 10°C,
<br />the water temperature at capture, by assuming a
<br />2% conductivity change for every 1°C between 10
<br />and 25°C: 700/1.0211- 10 = 520 AS/cm. If one uses
<br />the equation given by Novotny and Priegel (1974),
<br />the resistance (R) of one electrode is R =1(y)/Ka'
<br />= 0.159/30(0.00052) = 10.2Q; fly) is a dimension
<br />constant for spheres, Kis the sphere diameter (cm),
<br />0) 1 t,3
<br />ti is 10-6v, and a is the water conductivity in µS/
<br />cm. There are two identical electrodes in series,
<br />so their combined resistance is the sum of their
<br />individual resistances or 20.452. By Ohm's Law,
<br />therefore, the current at peak voltage is I = volt-
<br />age/resistance = 260/20.4 = 12.7 A.
<br />Next, one calculates voltage gradient at the elec-
<br />trode surface (Novotny and Priegel 1974) as
<br />E = 11[4av'r2]
<br />= 12.7/[(4rr)0.00052(15)2]
<br />= 8.6 V/cm;
<br />r is the radius (cm) of the electrode. If 8.6 V/cm
<br />occurs at 0 cm from the electrode surface (i.e.,
<br />when r = 15 cm), E at 10, 50, and 100 cm (r =
<br />25, 65, and 115 cm, respectively) is 3.1, 0.5, and
<br />0.15 V/cm. Thus, E is highest at the electrode
<br />surface and decreases nonlinearly with distance to
<br />one-tenth of maximum within 0.5 in of the elec-
<br />trode. Our estimate of voltage gradient may be
<br />low because the electrodes are used in shallow water
<br />where the boundary layers (water surface and sub-
<br />strate) tend to intensify the electrical field.
<br />We believe that the critical parameter for de-
<br />scribing electrofishing effectiveness is power den-
<br />sity (Kolz and Reynolds, unpublished data). Power
<br />density at any point in an electrofishing field can
<br />be calculated by the equation µW/cm' = QE'-. Thus,
<br />at the surface of the Sharber-Carothers electrode,
<br />power density was 38,459 µW/cm3, and was 4,997,
<br />130, and 12 µW/cm' at 10, 50, and 100 cm from
<br />the electrode. For reference, the minimum (thresh-
<br />old) power density required to narcotize, without
<br />injury, goldfish Carassius auratus under controlled
<br />conditions was 100-180 µW/cros, depending on
<br />whether DC, pulsed DC, or AC was used (Kolz
<br />and Reynolds, unpublished data).
<br />More relevant to the issue at hand are the results
<br />of recent studies by the Alaska Department of Fish
<br />and Game (ADFG), which corroborated the find-
<br />ings of Sharber and Carothers (D. McBride, ADFG,
<br />personal communication). Among electroshocked
<br />rainbow trout (N = 22; fork length, >400 mm)
<br />from the Kenai River (a = 50 µS/cm at 7°C), the
<br />incidence of spinal injury was 50%. (The ADFG
<br />has suspended use of electrofishing for population
<br />studies of large rainbow trout until an acceptable
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