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The effect of weak electrical fields on troll success for spring (Oncorhynchus tschawytscha) and sockeye (Oncorhynchus nerka) salmon Nomura, Daniel Kenichi

Abstract

The purpose of this study was to determine whether or not weak electrical fields affect salmon troll success in target species Oncorhynchus tschawitscha Walbaum and Oncorhynchus nerka Walbaum. The study was conducted aboard a commercial salmon troller in two fishing areas of the Strait of Georgia, British Columbia. The two fishing areas were Canadian fishing statistical areas 29A and 17. There were two experimental fishing periods, 20 March to 22 May, 1978 and 2 September to 24 September, 1978 during which the target species were O. tschawytscha and O. nerka, respectively. The fishing experiments used a single-vessel split-gear design where the replicated sets of troll gear were used as a paired treatment and control condition. The control condition was a zero volts potential difference between the fishing gear and the vessel. Applied voltages of both polarities made up the treatment conditions. A preliminary field experiment investigated characteristic differences in troll success between port and starboard sets of gear. No significant differences were found and it was assumed that there was no difference between sides. The voltage tests were Test 1( + 0.5 vs 0.0), Test 2 (-0.5 vs 0.0), Test 3 (+1.0 vs 0.0) and Test 4 (+0.5 vs -0.5) for target species O. tschawytscha. The voltage tests for target species O. nerka were Test 1(-0.5 vs 0.0), Test 2 (+1.0 vs 0.0), Test 3 (-1.0 vs 0.0), Test 4 (-2.0 vs 0.0) and Test 5( + 0.5 vs 0.0). Differences in troll success between paired treatment and control voltage conditions were examined. Troll success was measured by both catch and catch rate data. Differences in troll success were found in all voltage tests conducted during spring salmon trolling except for Test 3( + 1.0 vs 0.0). Significant increases in troll success were found in Tests 1 and 4 whereas a significant decrease was found in Test 2. The effect of applied voltage conditions on sockeye salmon troll success was different than that on spring salmon troll success. Sockeye salmon troll success was not different between paired treatment and control conditions for the low voltages tested, positive 0.5 and negative 0.5 volts. However, a high positive voltage(+1.0) increased the troll success and and a high negative voltage(-1.0) decreased the troll success. Further analyses were done on the effect of voltage conditions on troll success for O. tschawitscha of different ocean ages. Prior to investigation, a functional ocean age-total length regression was developed using ocean age data based on scale readings. Each target spring salmon captured was recorded with information on total length in order that the target fish could be put into ocean age groups. Differences in troll success were apparent in the older spring salmon(combined .2+,.3+,.4+ ocean years old) but not in the younger spring salmon(combined . + ,.-1+ ocean years old). Investigation of size distributions for paired treatment and control catches of spring salmon showed no apparent differences in all voltage tests done. Ocean age analyses were not done on O. nerka because all troll-caught sockeye salmon were assumed to be four year old Adams River spawners. The effect of available fish density on the change in troll success caused by the treatment voltages sere examined for both O. tschawytscha and O. nerka. Differences in troll success in older spring salmon(combined .2+,.3+,.4+ ocean year olds) seem to be affected by the available fish density. In contrast, the younger spring salmon(combined +,.1+ ocean years old) as well as the sockeye salmon do not demonstrate this available fish density phenomenon.

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