Effects of stress on plasma homeostasis, endolymph chemistry, and check formation during otolith growth in rainbow trout (Oncorhynchus mykiss)

This is the first study in which the effects of an external stress were analysed at different levels: plasma ho meostasis, endolymph chemistry, and otolith growth. Stress was applied to rainbow trout (Oncorhynchus mykiss) by exposure to Cl-2 gas. In the plasma of Cl-2-stressed trout, Na+ and Cl- decreased (70 mmol.L-1) and K+ increased (2.0 mmol.L-1), whereas total Ca was unchanged. A slight hypercapny (+2.4 mmHg (1 mmHg = 133.322 Pa)) was observed related to a significant rise (40%) in total CO2 without pH variation. In the endolymph, Na+ and Cl- also decreased, whereas other parameters (K+, PO43-, Mg, and, peculiarly, total Ca) remained stable. The important effect provoked by Cl-2 stress in endolymph was an increase of proteins (factor of 2.6) and total CO2 (factor of 3.1) concentrations at the proximal side of the endolymph. The stress induced a decrease in otolith growth rate and produced a discontinuity (check) in the microstructure pattern of the otolith characterized by a large D zone. The variations in the endolymph composition are discussed and we propose that they result not only from changes in plasma concentrations, but also from changes (organic and crystallization) in otolith deposition.