Juvenile temperate seabass Lateolabrax japonicus were sampled along the Yura River estuary from April to July 2008 to determine their distribution and feeding habits during migration within a microtidal estuary. Throughout the sampling period, juvenile seabass were distributed not only in the surf zone, but also in the freshwater zone, and they were particularly abundant in areas with aquatic vegetation in the freshwater zone. This distribution pattern suggests that the early life history of the temperate seabass depends more intensively on the river (freshwater) than previously considered. Small juveniles in the freshwater zone fed on copepods and chironomid larvae and upon reaching a standard length (SL) of approximately 20 mm did they fed on mysids. In contrast, juveniles (approx. 17-80 mm SL) in the surf zone fed mainly on mysids.

We analyzed distribution and differences in body length, age, and maturation rate of Pacific saury Cololabis saira in the western North Pacific Ocean, during the main spawning season (winter), to locate its spawning ground. Sampling was conducted in the coastal area north of the Kuroshio axis (CA), the offshore area of the southern Japanese archipelago (SA), and the area east of the Honshu Island (EA) from 1996 to 2017. Higher densities of Pacific saury were widely observed on or just north (within 100–200 km) of the axis of the Kuroshio and the Kuroshio Extension currents. Body length, age composition, and sea‐surface temperature (SST) differed across the studied areas, with larger fish (>250 mm knob length, mainly age‐1) dominating at higher SST (16–21°C) in the SA and smaller fish (<250 mm knob length, age‐0) dominating at lower SST (14–16°C) in the EA. In the CA, both SST and body length values were in between those in the SA and EA. The area where matured Pacific saury were collected was broadly distributed in the study area, except for the CA and a part of the Transition zone where the SST was low and mean body length was short. This study suggests that the main spawning ground during winter, where the area of high density and high maturation rate overlap, is formed on or just north of the Kuroshio and the Kuroshio Extension axis.

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The migration pattern of juvenile temperate seabass Lateolabrax japonicus in the stratified estuary of the Yura River was examined using carbon stable-isotope ratios (δ(13)C). δ(13)C values of mysids, which are the most important prey items for this species, were consistently enriched in the lower estuary and surf zone (LES), while depleted in the freshwater zone (FW). δ(13)C values of juveniles in LES were enriched, while those in FW were depleted, consistent with the δ(13)C difference in prey items. The results of δ(13)C showed that many juveniles migrated upstream from April to June and most of them stayed in FW until at least July. Juveniles that stayed in FW and LES for a relatively longer period (> c. 20 days) showed higher condition factors than those that stayed in FW for a short period (several days). This indicates that residence in FW enabled juveniles to achieve as good a body condition as residence in LES.