INJURY TO FISH 



Output energies commonly used in electro- 

 fishing are capable of killing fish. Death can 

 occur with or without gross physical damage, 

 or by irreversible physiological damage. 



Mortalities caused by a.c. electrofishing 

 probably are higher than those caused by d.c. 

 or pulsed d.c. (Taylor et al,, 1957), and the 

 gross physical damage from a.c. can be severe 

 (Hauck, 1949). 



Harmful effects from pulsed d.c. are usually 

 a result of excessive exposure or intense 

 electrical fields. Pugh (1962) found that two or 

 more of his exper im.ental variables (i.e., high 

 or low voltage; high or low frequencies, with 

 pulse durations of equal time value; square or 

 triangular pulse shapes; and waters of low or 

 high resistivity--where the former was the 

 more severe) were required to produce signifi- 

 cant mortalities with pulsed d.c. His work 

 indicates that the quantity of power is a con- 

 trolling factor of mortalities in pulsed d.c. 

 electrofishing. 



The senior author has participated in much 

 electrofishing and has found fish mortalities 



attributable to pulsed d.c. to be rare. One 

 incident in which fish were electrocuted oc- 

 curred near the mouth of a stream entering 

 salt water; at this point the resistivity was 

 below 1,000 ohm cm. 3. Some of the coho 

 salmon fry subjected to the pulsed d.c. were 

 killed in the estuary, but those 200 yards up- 

 stream, in v/aters of 30,000 ohm cm. 3, were 

 not damaged. This mortality from pulsed d.c. 

 is in accord with Whitney and Pierce's (1957) 

 theory that a given electric potential places 

 more energy into a fish in the more con- 

 ductive waters and with Pugh's (1962) finding 

 that low resistivity is conducive to fish mor- 

 tality. 



Subjecting shocked fish to additional stress 

 commonly causes mortalities. Electric stimu- 

 lation interferes with or stops respiration in 

 fish for a period of time (Bodrova and 

 Krayukhin, 1958), producing a metabolic 

 deficit. The situation becomes precarious un- 

 less the fish are removed quickly from the 

 stimulating currents into water with optimum 

 temperature and dissolved-oxygen concentra- 

 tion. 



SUMMARY 



Personnel of the Bureau of Commercial 

 Fisheries Biological Laboratory in Seattle, 

 Wash., have developed a series of electro- 

 fishing shockers for sampling fish populations 

 in streams. One of the most effective and the 

 most light weight, dependable, and economical 

 is the Type IV shocker. This shocker is 

 described, and its electronic schematic is 

 shown. 



The output power capabilities of the Type IV 

 shocker are within the optimum range for 

 electrofishing. The optimum output depends on 

 a con-ibination of a complex of features. Pulsed 

 d.c. is the most effective type of basic power, 

 as unpulsed d.c. does not stimulate as ef- 

 fectively or attract over as great a range. 

 A.c. does not produce galvanotaxis in fish. 

 Pulse frequency and duration are of great 

 importance in electrofishing because they are 

 unaltered by water resistivity. The optimum 

 pulse frequency and duration range for electro- 

 fishing in waters less resistive than 30,000 

 ohm cm. 3 and the area of optimuin response 

 for water of higher resistivity are shown in 

 the figures. 



The "square- shaped" wave with a fast rise 

 is the most desirable because the voltage is 

 at its peak throughout the duration. The mini- 

 mum output voltage for electrofishing in con- 

 ductive waters is 150 v., but a maximum of 

 400 V. is more desirable for most usages. An 

 amperage rating of less than 0.2 a. is generally 

 not sufficient for relatively conductive waters. 



and general purpose shockers should have a 2 

 to 3 a. rating. 



The effectiveness in surrounding a fish in a 

 water-borne electrical field depends upon the 

 capabilities of the electrodes in transmitting 

 electrical energy into water and the methods 

 used to "fish" with the electrodes. The hand- 

 held anode, to which the fish are attracted, 

 should be small--about 40 cm. square. This 

 size permits ease in handling and causes high 

 voltage gradients near the anode. A large 

 cathode is needed to compensate the small 

 anode; and the more resistive the water, the 

 larger the cathode should be. A cathode with a 

 surface area of 2.3 m. square is sufficient for 

 waters up to 30,000 ohm cm. 3; the cathode 

 must be larger to be effective in waters of 

 higher resistance. 



Electrofishing can be carried on by wading 

 or floating. In waters suitable for wading, the 

 operators probe about with the anode. If the 

 stream is too deep or swift to wade, floating 

 is necessary. Larger fish are caught by float- 

 ing. Night fishing is not practical in streams. 



It is often difficult or impossible to explain 

 variability in electrofishing catches because 

 many factors are involved. Low concentration 

 of ionized salts in a body of water increases 

 resistance and reduces the electrical energy 

 that can be introduced; the stimulating current 

 which reaches a fish is lessened accordingly. 

 A maximal stimulating current of 300 to 400 v. 

 with high values of frequency and duration and 



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