Effects of salinity and ontogenetic movements on strontium:calcium ratios in the otoliths of the Japanese eel, Anguilla japonica Temminck and Schlegel

To study the mechanism of Sr incorporation into otoliths of the Japanese eel, Anguilla japonica, a total of 100 elvers collected from an estuary were reared in the laboratory at salinities of 0, 20, 25 and 35 parts per thousand for approximately seven months. The elvers grew from 56 mm TL to 100-300 mm TL. Twenty elvers were randomly selected and the Ca and Sr concentrations of their otoliths were analyzed from the primordium to the edge, using an electron microprobe equipped with a four-channel wavelength-dispersive spectrometer. Sr/Ca ratios in the otoliths of eels reared in various salinities were much lower than the ratio of 15 x 10(-3) observed in elvers about one month before they arrived at the estuary. The irreversibility of Sr/Ca ratios at 35 parts per thousand salinity in this experiment indicated that the drastic change of the Sr/Ca ratios in otoliths of elvers was not due to the reduction of salinity in the coastal waters, but more likely to the development from leptocephalus to glass eel. The mean Sr/Ca ratios in the new increments of the otoliths of eels during the rearing period were highly correlated with salinity (S), [Sr/Ca] x 10(3) = 3.797 + 0.14S (n = 20, r = 0.77), which can be used to predict elver movements and habitat utilization rates.