Spawning grounds of S. melanostictus expanded from the waters above the continental shelf along the Pacific coast of Japan to the oceanic waters around the Kuroshio current during a period of population increase in the first half of the 1980s. The annual average density of egg distribution over the entire spawning grounds increased from 1500 m - 2 in 1978 to 19 400 m - 2 in 1986. Percent survival in about the first 10 days of life increased through the 1980s. Increases in egg abundance and survival were related to the offshore expansion of the spawning grounds into the waters around the Kuroshio axis, which are considered to provide sardine eggs and early larvae with optimal rearing conditions.