OSMOREGULATORY ADAPTATION OF STEELHEAD TROUT (SALMO GAIRDNERI RICHARDSON) TO SEA WATER

Abstract

Steelhead trout transferred from fresh water into sea water exhibited two distinct phases in osmoregulatory adaptation. The first, or adjustive phase, was distinguished by marked departures from fresh-water levels and distributions of water and electrolytes. Increases in muscle chloride space indicated transfer of fluids from cells into the extracellular compartment. Plasma levels of water, chloride, sodium, and potassium rose, while that of calcium remained relatively constant. The ratio of plasma sodium to chloride fell sharply suggesting a condition of acidosis. Cellular calcium concentrations rose markedly, while smaller increases were seen in chloride and potassium levels. Changes in calcium appeared to be a function of both cellular dehydration and electrolyte uptake. Changes in cellular potassium concentrations were related primarily to dehydration since tissue levels of this ion fell slightly in sea water. The roles of active electrolyte excretory processes, and the passive withdrawal of ions from the circulating fluids by complex formation, have been considered as osmoregulatory mechanisms leading to the establishment of the second, or regulative, phase of adaptation to sea water.