Interdecadal variabilities in growth and temperature trajectories of Trachurus japonicus juveniles: 1960s-1970s versus 2000s-2010s

ABSTRACT: Changes in the distribution range and timing of life events in fish in response to warming oceans have been reported in several marine ecosystems. However, few studies have demonstrated the impact of increasing temperature on the individual growth and thermal trajectories of fish. We examined the growth and temperature trajectories of juvenile Japanese jack mackerel Trachurus japonicus in the western waters off Kyushu, Japan, based on growth increments and high-resolution isotopic analyses of archived otoliths between the 1960s-1970s and 2000s-2010s. T. japonicus juveniles in the 2000s-2010s hatched from February to April, whereas juvenile hatching in the 1960s-1970s ranged from March to May, corresponding to earlier spawning in recent decades. The otolith radius from the nucleus to specific dates represents the somatic growth trajectory of the fish: this distance was larger from April to August in the 2000s-2010s than that in the 1960s-1970s, indicating that earlier hatching in the 2000s-2010s resulted in a larger body size throughout life than in the 1960s-1970s. The trajectories of the oxygen stable isotope ratios in otoliths showed that the temperatures experienced by the fish from spring to summer were comparable between the study periods. Our results suggest that warming during late winter in recent decades has resulted in earlier spawning and occurrence of T. japonicus juveniles, resulting in the fish experiencing comparable thermal conditions between the 2 study periods. The phenological shift in spawning seemed to be recorded in individual growth and the temperature trajectories in the otoliths of T. japonicus.