A Note on the Effect of Electric Fields on Gadus virens
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To test electrotaxis earlier experiments have been performed in relatively small tanks. It was felt, therefore that the arrangement with a comparatively big tank would give greater allowance for a more varied behaviour of the animal. This varied beliaviour pattern, however, introduced an uncertainty as to which kind of change in behaviour should be termed electrotaxis. The data have been listed in Table I. As will be seen from the table the spread of data is considerable. A statistical test of the data was tried, therefore, by applying the analysis of variance. For changes in pulse duration there are significant differences (F = 7.72; P < 0.05). As the pulse duration is lengthened, lower voltage gradients are required to elicit the electrotactic responze. This is in accordance with earlier findings. (MORGAN, 1953; BARY, 1956; and others). For changes in frequency no significant differences could be found when the material was treated as a step by step rise of 5 c/s. The number of observations at each frequency is small, and if the material is grouped in higher (30-50 c/s.) and lower (10-25 c/s.) frequencies, a slightly significant difference can be found (F = 4.52; P < 0.05). The relation this time being that a rise in frequency leads to an increase in the voltage needed for electrotaxis. BARY (1956) showed that the voltage gradient required for electrotaxis does not change with frequencies above 5 c/s. It was observed during our experiments that a more cramped swimming would be encountered at the higher frequencies, making it difficult to distinguish an electrotactic behaviour. The apparent rise with frequency in the required voltage gradient found by grouping the data, might be caused by this difficulty. It seems difficult to suggest any reason for the differences in sensitivity between Gadus virens and Mugil auratus. For a corresponding size and pulse duration BARY (1956) found for Mugil auratus a potential difference along the fish of arounct 7 v., whereas the present investigation yields 2.5 v. for Gadus virens. Many suggestions have been made as to the physiological events underlying the electrotactic behaviour. For the future discussion on the topic, we wish to draw attention to the following observation: Even at voltages higher than the minimum for electrotaxis the effect on the animal was seldom so strong that it overshadowed the animals sensual perseption of the tank walls: The fish would usually turn before hitting the tank wall. Exceptions here are again during the cramped movements accompanying the higher frequencies.
SeriesFiskeridirektoratets skrifter, Serie Havundersøkelser
vol 12 no 4