Important food sources and feeding selectivity of larval Japanese sardine Sardinops melanostictus and anchovy Engraulis japonica with the notochord length (NL) 3.1–16.0 mm were determined by gut content analysis of larvae captured simultaneously around the Noto Peninsula in the southern Japan Sea in May, during the main spawning period between 2011 and 2012. First-feeding larval sardine and anchovy fed exclusively on copepod eggs and cyclopoid nauplii (Oithona spp.). From the index of relative importance and Chesson’s α index of selectivity of food organisms, nauplii of Oithona atlantica type (Oithona atlantica, O. plumifera and O. longispina) were the most important food items for larval sardine against Oithona similis, even though the latter was the most abundant prey taxon in the study area, while nauplii of Oithona similis was the most important food item for larval anchovy. Food sizes (length and width) increased as the larvae grew. The NL of larval sardine feeding mainly on the copepod eggs and nauplii was larger than that of larval anchovy. Larval anchovy more often selected the copepodids and copepod adults in the earlier growth stage than larval sardine, although the empty stomach rates of larval anchovy with the NL 5.0–10.0 mm were higher than those of larval sardine. It was suggested that sardine larvae have a feeding strategy of selecting the smaller-sized (energy-inefficient), higher-density (leading to easy feeding) prey items in their nursery ground, while anchovy larvae have a feeding strategy of selecting the larger-sized (energy-efficient), lower-density (leading to difficulty feeding) ones in the same nursery ground.

Somatic growth of the Pacific stock of Japanese sardine deteriorated during the 2010s corresponding to its stock recovery. The previous study detected stronger density-dependent growth in the 2010s than in the 1970s and 1980s and suggested that lower mesozooplankton biomass during the 2010s in the summer feeding grounds (FG) controlled the strength of density-dependence, however, the effect of FG area has not been examined. After the 2014/15 oceanographic regime shift, the Oyashio area diminished, the sea surface temperature of FG increased principally during winters, and net primary productivity (NPP) increased in the northern FG around the Oyashio, whereas NPP decreased in the southern FG around the Kuroshio-Oyashio Transition Zone. To explore potential causes of variability in annual increments of body weight by age of this stock during 2003–2020, we performed a generalized linear model (GLM) analysis using biomass and recruitment of this stock, the Oyashio area, and NPP of northern and southern FGs as candidates of explanatory variables. The GLM results suggested that the biomass of this stock and the Oyashio area was responsible for the recent growth decline of age-0 fish, and age-1–3 fish, respectively.