The spawning amounts of the Pacific Population of the common mackerel, Scomber japonicus HOUTTUYN, were surveyed for 1968, 1969 and 1970, as had been done for the earlier years in the previous report (WATANABE1)). Based on the information thus obtained, recent trends in the stock size and fluctuation of this population were clarified. The results are as summarized below:
1. Egg abundance was remarkably low for the years 1969 and 1970 as had previously been predicted. This was caused by the depletion of population of the III-age fish, year after year, since the 1965-year-class which had provoked a rapid decrease of the whole parent fish stock.
2. Possibly, the decrease of stock sizes of the 1965- to 1967-year-classes was attributable to high mortality in the very early stages of life, such as from egg to young.
3. Immature fish populations showed a slight increase since the 1966-year-class which was the minimum, but this was still lower than those of the 1964-year-class or older ones. For the years 1968-1970, immature fish suffered from stronger fishing intensity than before, intensified by not only a decrease of population but also an increase of the number of fish caught.
4. The egg abundance for both 1969 and 1970 were slightly less than 200×1012. This suggests that the reproductive strength could still be expected to continue to some extent. However, judging from the recent growth of fishing intensity upon immature fish, the increase of parent fish will not occur for long.
5. In order to increase the stock size of parent fish to produce the eggs at optimum levels, say 400×1012 or so, the fishing of immature mackerel should be controlled.