Vertical habitat selection of sardine juveniles inferred from oxygen stable isotope of otolith

Abstract

Marine organisms can change their habitat depth based on feeding strategies and physiological requirements. Here, we investigated habitat depth selections of juvenile sardines in the western and eastern North Pacific, Sea of Japan, and East China Sea from summer to autumn, which have been poorly documented. Habitat depth was estimated by comparing stable oxygen isotope values at the edges of otoliths formed several weeks before capture with in situ temperature and salinity observations near the sampling sites. The estimated depths at the three stations in the Sea of Japan for which acoustic data were available were consistent with the maximum acoustic scattering layer. Habitat depths in the eastern North Pacific, Sea of Japan, and the East China Sea varied considerably between regions in the upper 70 m. Still, they were mostly ± 10 m from the seasonal thermocline. Average temperatures at the thermocline in the regions were 16.8−24.5°C, which included temperatures significantly higher than the presumed optimal temperatures. However, the habitat depths in the subarctic North Pacific, where the mean temperature at the thermocline was 8.4°C, were consistently in the surface mixed layer above the thermocline. These results suggested that juvenile sardines tend to reside in the thermocline to feed efficiently on aggregated plankton, provided the temperature is physiologically tolerable. Our results may be useful for increasing sampling efficiency, validating assumptions in behavioural models, and providing insight into how atmospheric forcing affects sardine growth and survival.