Feeding, growth, and hatch timing of juvenile chub and blue mackerels in a coastal nursery of western Japan

Early life history traits strongly influence survival and recruitment in small pelagic fishes, yet regional variation in these traits remains poorly understood for young-of-the-year (age-0) mackerels in Kuroshio-influenced coastal systems. We examined diet composition, onset of piscivory, otolith-based growth, and hatch-date composition of juvenile chub mackerel Scomber japonicus and blue mackerel Scomber australasicus collected from the Uwa Sea, western Japan, in 2021, 2022, and 2025. Diet composition varied with body size, year, and area, whereas no clear interspecific difference was detected. In both species, the probability of piscivory increased with body size, and the fork length (FL) at which 50% of individuals were predicted to have consumed fish prey was estimated to be 120–150 mm. Otolith daily increment analysis indicated rapid growth, with mean daily growth rates of 2.15 ± 0.25 mm d⁻ 1 in chub mackerel and 2.03 ± 0.18 mm d⁻ 1 in blue mackerel. Most juveniles were estimated to have hatched from late February to April, partly preceding the reported main spawning period around the study area of May–June. This temporal mismatch suggests that the juvenile assemblage may include individuals hatched earlier or originating outside the local area. These findings provide a regional baseline for understanding nursery use, feeding transitions, and early growth of commercially important mackerels in western Japan.