Table 5. --The relationship between total current, current 

 density, and electric field 



area of the column of water between the 

 electrodes, and L the length of the column. 

 These calculations gave for p a value of 18. 68 

 ohm-cnn. As would be expected, this is snnaller 

 than the value of the resistivity of sea water on 

 the west coast of the United States (about 29. 5 

 ohm-cm.), where both the temperature and 

 salinity are lower. 



The electric field established in the water 

 is related to the current density by E = p J, 

 where E is the electric field in volts /cm. , p the 

 resistivity in ohm-cm. , and J the current den- 

 sity in amperes/cm. 2. The values of the elec- 

 tric field corresponding to the various values of 

 current used in the experiment are listed in 

 table 5. 



The approxinnate head-to-tail voltages for 

 satisfactory response, calculated as the product 

 of electric field intensity and length of fish, 

 varied between 1 and 2 volts in these tests. 



EXPERIMENTS WITH TUNA AND 

 OTHER LARGE FISH 



It has been shown by Tester (1952) that tuna 

 may be successfully kept in captivity in a large 

 concrete tank (35 x 11 x4 feet) located at the 

 Hawaii Marine Laboratory, Coconut Island, Oahu. 

 Our problem was to design an apparatus which 

 would produce an electric field of sufficient 

 strength to induce electrotaxis in tuna in this 



relatively large volume of water (ca. 10,000 

 gals. ). 



Apparatus 



Preliminary experiments on aholehole in a 

 small tank, described in the preceding section, 

 showed that satisfactory electrotaxis could be 

 induced with a frequency of 10 c.p. s., with an 

 on-fractionof about 0.06 and with a peak current 

 density of 8.8 ma. /cm. ^ (for an 11 -cm. fish). 

 Although admittedly tenuous, the frequency was 

 assumed to be optimal for best directional 

 swimming and the on-fraction was assumed to 

 be optimal for minimum power requirement. 

 The combination of frequency and on-fraction 

 corresponds with a pulse duration of 6 

 milliseconds. The peak current corresponds 

 with an electric field of 0.16 volts /cm. (table 5). 

 If it is assumed that the electric field (E) for 

 satisfactory response varies with the length of 

 the fish (L) according to the rough relationship 

 E = k/L, k (a proportionality constant) nnay be 

 calculated at 1.8 volts and the minimum field 

 requirement is 



„ 1.8 volts/cm. 



With the further and perhaps questionable 

 assumption that k will be the same for aholehole 

 and tuna, the electric field necessary to induce 

 satisfactory electrotaxis in a 30-cm. tuna may 

 be calculated at E = 0.060 volts/cm. The 



