-POWER ON 

 -POWER OFF 



60 70 BO 90 100 



120 130 ia0 150 160 170 iBO 190 200 210 220 230 



TOTAL FISH LENGTH (MM) 



Figure 6. — Alewife length-frequency distribution with 

 electrode arrays 1, 2, and 3 combined. Solid line, 

 power on (N = 387; mean = 165.7); dashed line, 

 power off (N = 364; mean = 163.1). Z test for 

 the difference between the two sample means at 0.05 

 level of probability: calculated Z = 1.54, tabular 

 Z = 1.96; not significant. 



POWER OFF 



195 205 215 225 235 245 255 265 275 285 295 305 315 325 

 TOTAL FISH LENGTH (MM) 



Figure 7. — Chub length-frequency distribution with elec- 

 trode array 1. Solid line, power on (N = 101 ; mean 

 = 250.0); dashed line, power off (N = 35; mean 

 = 234.1). Z test for the difference between the two 

 sample means at 0.05 level of probability: calculated 

 Z = 3.59, tabular Z = 1.96; significant. 



arrays 1 and 3. Length selectivity was not sig- 

 nificantly demonstrated with power on or off. 

 The Z-test was used to analyze the difference 

 between the sample mean lengths of fish caught 

 with each array system. The only species to 

 show selectivity of length toward pulsed direct 

 electrical current were chubs. They were all 

 significantly different except for the chubs taken 

 with array 3. 



SUMMARY AND RECOMMENDATIONS 



These tests have shown that the overall catch 

 rates (in kilograms of fish per drag) of a 12.5-m 

 (headrope) electrical trawl with power on are 

 increased by as much as 1.86 times or 86.9% 

 over the same trawl with power off. This in- 

 crease is much lower than that reported by other 

 researchers. The catch rates were influenced 

 by electrical lines of flux in the arrays tested. 

 In addition, we have shown that fish exhibit vis- 

 ually induced responses to hanging vertical elec- 

 trode arrays in the mouth area of a bottom trawl. 

 This aversion to vertical electrodes was great 

 enough to offset catch increases brought about 

 by energizing the electrical apparatus. 



Tests have also shown the overall catch rates 

 (in numbers of fish per drag) to be statistically 

 significant (P < 0.01) for the standard trawl 

 (no electrodes) and for the electrical trawl with 

 power on (P < 0.01). In some cases the catch 

 rates for individual species were not significantly 

 different with power on. 



With the exception of chubs, the dominance 

 patterns of the catches did not change signifi- 

 cantly for the different test situations. 



None of the test gear showed significant se- 

 lectivity for alewife or yellow perch. Size se- 

 lectivity was demonstrated for chubs when they 

 were exposed to arrays 1 and 2. 



It can be surmised from the results of these 

 studies that the commercial use of electricity 

 with a bottom trawl with array 2 is practical 

 since a projected 0.32 times or 32% increase 

 in income would more than offset the first year's 

 investment in electrical gear. 



Future application of electrode-array equipped 

 trawls should take the following into consider- 

 ation: 



1. Array designs which lessen the visual re- 

 action of fish to electrode elements. 



2. Increase in power to the electrode arrays 

 to overcome the loss of catch due to visual 

 stimuli. 



3. Investigate 0.6-m electrode element spacing 

 on the cathode to lessen the effects of elec- 

 trical lines of flux upon catch rates. 



