Abstract
The elemental composition (Ca, Na, Sr, Mg, Fe, Mn, Zn, P) of sagittae (otoliths) taken from several species of teleost fish living under different salinities was analysed using Inductivity Coupled Atomic Emission Spectrometry (ICP) after ashing with HNO3 and HClO4 (5:1) in a Teflon vessel (Experiment 1). Concentrations of Na, Sr and Mg in otoliths were dependent on the salinity. The sodium concentration and Na/Ca ratio in the otoliths of freshwater fish, except for rainbow trout, were lower than those of brackish water and sea water fish. The strontium concentration and Sr/Ca ratio in the otoliths of freshwater fish were lower than those of brackish water and sea water fish. The magnesium concentration and Mg/Ca ratio in the otoliths of carp, goldfish and gobid fish were higher than those of eel, rainbow trout and sea water fish.
The effect of environmental salinity on elemental composition (Ca, Na, Sr, Mg) in the otoliths of raibow trout and Japanese flounder reared for 3 months at 15±1°C and 20±1°C, respectively, was studied (Experiment 2). The element composition of the otolith portion formed during culturing was analysed using the method mentioned above. The otoliths of rainbow trout exposed to 100% sea water increased with regard to Sr concentration and Sr/Ca ratio, and decreased with regard to Mg concentration and Mg/Ca ratio compared to those for freshwater fish. In the otoliths of the Japanese flounder exposed to 50% sea water, lower values for Na, Sr, Na/Ca and Sr/Ca and higher values for Mg and Mg/Ca than those reared in 100% sea water were found.
These results indicate that not only Sr concentration and Sr/Ca concentration ratio, but also Na and Mg concentrations and the ratios of each of these elements to Ca in otoliths may be effective indices for determinating the salinity of a past environment of a fish. It is recommended assessing the history of environmental salinity in fish by means of utilizing synthetically the indices found in this study.
