MEPS 231:237-245 (2002)  -  doi:10.3354/meps231237

ABSTRACT: This is the first report on the aragonite saturation state of the endolymph in a single fish species, the rainbow trout Oncorhynchus mykiss, based on the direct quantification of electrolyte concentrations in the saccular endolymph. The Ca²⁺ level, CO₂ partial pressure and pH of the saccular endolymph in 1+ and 2+ yr old trout were simultaneously determined using an automatic pH/blood gas/electrolyte analyzer. From the values of CO₂ partial pressure and pH, HCO₃^- and CO₃^2- levels were obtained using the Henderson-Hasselbalch equation. In addition, Na, K, Cl, Mg and inorganic P levels were measured in order to determine ionic strength of the endolymph. The aragonite supersaturation rate (S_a) was calculated from the Ca²⁺ and CO₃^2- concentrations and the ionic strength. In both age groups, Ca²⁺ and CO₃^2- concentrations were around 0.75 and 0.68 mmol l^-1, respectively. Small differences in Na, P, and HCO₃^- concentrations were observed between the 2 age groups, but endolymph ionic strength was similar. The S_a ratio was 2.885 to 3.507 in 1+ yr old fish and 2.027 to 4.303 in 2+ yr old fish. Therefore, the endolymph is supersaturated with respect to aragonite. S_a was significantly correlated with CO₃^2- levels, which were largely determined by pH. As a consequence, S_a was strongly dependent on pH, indicating that endolymph pH-regulation is important in the aragonite crystallization of the otolith.

KEY WORDS: Otolith · Endolymph · Chemical composition · Aragonite saturation state · Teleost