Otolith Microstructure Analysis Reveals Different Growth Histories of Japanese Sardine (Sardinops melanostictus) in the Oyashio Waters

Abstract

After decades of low year classes, the stock of Japanese sardine (Sardinops melanostictus) has begun to recover since the mid-2000s. The hatch dates and otolith growth rates of age-0 juvenile sardine, which were collected in the subarctic Oyashio waters in autumn 2018, were determined from an otolith microstructure analysis. The sardines were hatched from late January to late April, while mostly in February and March. The otolith growth rate increased continuously up to 60 d after hatching and thereafter decreased. The revealed growth rate in a crucial growth period is faster than that reported for juvenile sardines collected in the 1990s, which is coincided with the recent recovery trend of the sardine stock. Two groups with different hatch dates, growth histories, and migration routes were identified using unsupervised random forest clustering analysis. They were considered inshore and offshore migration individuals in accordance with recent researches. In the offshore group, a high proportion of sardine juveniles hatched late and grew faster in the Kuroshio—Oyashio transitional waters, a finding consistent with the hypothesis of growth-rate-dependent recruitment. This finding on the population composition and growth rate of juvenile sardine in the Oyashio waters can be a basis for an improved prediction of their survival and provides us with valuable information on the recruitment processes of this stock during the period of stock recovery.