二つの集魚灯の効力についての一考察
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概要
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In fishing aided with a lamp light there should be many factors determining an effect of the fishing light in use. However, if other factors are assumed the same between operations comparable one another, important variables may include the length of lighting hours, age of the moon, and conditions of the tide. As few works have been made available for these practical aspects of light fishing, the present paper deals with the relationship between these factors and effects of an electric lamp and a kerosene one. The data used for the study have been obtained from purse seine fisheries for the anchovy, Engraulis japonicus, operated in a part of the Seto Inland Sea during July and August 1953 (Table 1). 1) When the amount of catch was examined in relation to age of the moon, the relation was found to have nearly the same tendency between the electric light and the kerosene lamp fishing. Because between two succeeding periods of dark night, one from the dark to the first quarter of the moon and another from the last quarter to the dark, the catch by each type of fishing was greater in the former than in the latter (Table 1 and Fig. 1). 2) In the both kinds of lamp, efficiency of attracting fish seems to be accelerated in a parabolic curve with progress of the lighting hour (Fig. 2). It is interesting to note that a vertex on the parabola tells the time when a seining operation comes near to an end. 3) The fishing harvests under report may be classified into three categories: good fishing, poor fishing and no catch, where good fishing is represented by catch greater than 500 kans (1 kan=3.75kg) in a few hours, and poor fishing by catch at lower levels. In order to see whether fluctuations in the tide had an effect upon the harvests, these categories of catch have been arranged in relation between the mean fluctuation of tide (cm/hr) and the order of ebb and flow that occurred between two consecutive mornings. In Fig. 3 it is evident that the tide patterns for poor fishing and no catch resemble each other, but they are different from the pattern for good fishing. 4) The fishing hours may be sorted into N, M, and NM, where N stands for 6hours before midnight (2:00 a.m.), M for hours following N till daybreak, and NM for those of all-night operation, In regard to frequency of effective fishing when catch was more than 50 kans per hour, the electric light fishing showed little difference regardless whether it was operated in N, M or NM (Table 3). In the kerosene lamp fishing, however, ratio of the frequency of good catch based on the total number of that operation was 65% for NM, 50% for M and 25% for N. Although the numbers of operation made during N, M, and NM were different depending a kind of lamp, one lamp seems to have been used oftener than the other in one of the categorized hours when an efficient fishing was apparently expected from its use. 5) In regard to a question as to whether one type of lamp is ultimately more efficient than the other under report, further studies have to be made on a comparable base before drawing any conclusion.