On The Swimming Layer Of Engraulis Japonica Shoals In Bungo Channel And Its Adjacent Waters

Abstract

The depth of the swimming layer of Engraulis japniea shoals was continuously recorded by the echo-sounder during routine observations on the oceanographic and meteorological conditions in Dungo Channel and the western part of the Inland Sea (of Japan) from 1962 to 1965. During series of cruises, as many as 14,351 schools were detected. The examination of their behaviour by the echo-sounder trace revealed following tendencies in seasonal and daily rhythmical changes of the swimming depth of the schools and their variations according to either the weather or the wind force: (1) The swimming layer of the shoals was thick in the stagnant season (from spring to summer), and was thinner in the convection season (from autumn to winter) and tended to sink slightly from spring to summer. (2) The greatest depth of the swimming layer detected in these observations was 125 m (in spring) in the Bungo Channel and 70 m (in summer) in the Beppu Bay. (3) Two types of shoals were distinguished by different responses to the oceanographic and meteorological conditions-those swimming in the zone of 0 to 30 m depth and those in the zone below 30 m. The behaviour pattern of the latter group is affected by the water temperature only. (4) The analysis of the daily rhythmical changes of the swimming depth indicates that the shoals have a tendency to sink at dawn and dusk and to come up to the shallow layers in the daytime. (5) In rainy weather the shoals swim at deeper layers in the western part of the Inland sea (of Japan), but at shallower layers in the Bungo Channel. (6) When the wind force is over the grade 4 on the Beaufort scale, the shoals swim in the deeper zone, regardless of locality. (7) Results of the present study indicate that insufficient data on behaviour of fish shoals often leads to erroneous conclusions, for data at the neritic waters tend to be overemphasized. Furthermore results of previous investigations and the present study demonstrate that it is important in discussing this kind of problem to consider not only one factor but all possible factors. © 1966, The Japanese Society of Fisheries Science. All rights reserved.