Microscale stable carbon and oxygen isotope measurement of individual otoliths of larvae and juveniles of Japanese anchovy and sardine

Abstract

The stable carbon and oxygen isotope ratios (δ13C and δ18O) of fish otoliths are useful proxies for habitat and physiological conditions affecting fish during their lifetime. To understand the ecology of the early life stage specimens of Japanese sardine Sardinops melanostictus and Japanese anchovy Engraulis japonicus collected from Toyama Bay, Japan, we used a submicrogram-scale stable isotopic analytical system called MICAL3c to investigate otolith δ13C and δ18O values of larvae and juveniles. The otolith δ13C and δ18O values of the two species showed a decreasing trend with ontogeny. Temperatures estimated from otolith δ18O values suggest that otolith δ18O values may record a decline in ambient salinity rather than seasonal increase in temperature during the transportation from the spawning to the fishing grounds, or an ontogenetic shift in the pH of the endolymph. Sea surface salinity in Toyama Bay was drastically decreased along the southern shore compared with elsewhere in the bay, whereas the sea surface temperature distribution was rather uniform. Considering that the main sardine and anchovy spawning ground is in the western part of Toyama Bay and that a counterclockwise current usually prevails in the bay, the salinity change associated with the transport of larvae from the spawning ground to the sampling site may explain the otolith δ18O change. The δ13C and δ18O values recorded in otoliths of two species are inconsistent with the observed temperatures, but this may be due to unmeasured variability of δ13C (dissolved inorganic carbon, DIC) and δ18O (H2O) in seawater and/or physiological effects on the calcification process.