OCCURRENCE OF MACKEREL EGGS IN ISHIKARI BAY, HOKKAIDO, IN JUNE, 1954, AS INVESTIGATED BY UNDERWAY PLANKTON CATCHER *

Mackerel fishery is commonly carried on from June to July in the Ishikari Bay and the adjacent area off the west coast of Hokkaido when mackerels (5comber japonicus HOUTTUYN) are concentrated there to spawn. In order to figure the distribution of mackerel eggs in this region the present brief rese3.l'ch was undertaken in late Ju.'1e, 1954. The underway plankton catcher, model 1I, (Motoda, 1954) was towed from fishing boat while underway at about 9 knots for convenient duration ranging from 13 minutes to one hour along the course betwee'l each pair of hydrographic stations. It is proved from this brief trial tow that this method is useful for deterinining the distribution of the pelagic eggs in the surface water. Counting the number of mackerel eggs was done by classifying them into two groups, viz., the eggs in blastodisc stage and those in more advanced stage (Motoda, 1955). Table 1 and figures 1 and 2 give the occun:-ence of those two stages of mackerel eggs in the colle~tions. In the table and figures the number of occurrences was in all cases converted to the number in 100 cubic metres of water. The underway catcher used can filter 100 cubic metres of water during a tow of three hours and a half, assuming tha