Shape and size of Japanese sardine school in the waters off the southeastern Hokkaido on the basis of acoustic and aerial surveys.

Abstract

Acoustic and aerial survey was carried out during 1981-1983 in the area where a big catch of sardine Sardinops melanosticta has been taken. From the echo sounder survey, sardine schools keep at a depth between 10-20m (sometimes even deeper) during day time and the size of most sardine schools ranged between 10 and 20m. On the other hand, the longest sardine school dimensions ranged mainly from 20-40m and more than 100m was scarcely observed by the scanning sonar survey. During aerial survey, more than 293 schools were easily observed and identified because sardine swam in the shallower layer described above. On the contrary, the horizontal dimensions of school ranged mainly from 20-200m during the aerial survey. These values taken from the acoustic and aerial survey were different. The difference may be caused by the avoidance of the ship. The aerial survey technique is effective to study fish schools in nature when the fish commonly migrate to the near-surface. Although the efficiency of the aerial survey was seen, it is necessary to make synchronized air-and ship-borne observations in future.