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Locomotor performance and osmoregulation in juvenile anadromous salmonids following abrupt environmental salinity change Houston, Arthur Hillier

Abstract

The relationship between osmoregulatory stress and locomotor performance has been investigated in juvenile salmonids during their adjustment to sea water. Transfer from fresh water to sea water produced a statistically significant depression of the maximum swimming speed of chum salmon fry (Oncorhynchus keta). The effect of sea water was immediate and reached a maximum fourteen hours after transfer. From thirty-six to eighty hours (the duration of the experiment) relatively stable performance levels were recorded. Some recovery from the initial effects of sea water was apparent but the swimming speeds of "recovered" fish were significantly lower than those of fry in fresh water. Statistically significant correlations between swimming speed changes and changes in total body chloride and water concentrations have been demonstrated. Since chum fry were too small to allow separate sampling of plasma and tissue, the Steelhead trout (Salmo gairdneri gairdneri) was used to investigate the sequence of events in the osmoregulatory adjustment of salmonids to sea water. Transfer into sea water was accompanied by increases in plasma concentrations of chloride, sodium and potassium, but not of calcium. Cellular dehydration resulted from transfer of cellular fluids to the extracellular phase, and from loss of water to the environment. Cellular levels of calcium and sodium rose markedly. Smaller increases in cellular potassium and chloride were noted. Changes in cellular sodium and calcium were primarily due to ion uptake. Increases in cellular potassium were the result of cellular dehydration since tissue levels of this cation fell slightly in sea water. The data indicate that impairment of the efficiency of physiological processes sensitive to altered electrolyte concentrations, and the utilization of energy in the restoration of body fluid electrolyte concentrations to regulated levels may account, at least in part, for reduced locomotor ability in salmonid migrants during their adjustment to sea water. Body size was shown to influence the extent and duration of changes in plasma and tissue electrolyte concentrations and distributions. In chum fry the governing factor appeared to be size, larger animals adapting more rapidly, and undergoing less extensive changes than did smaller fish. In Steelhead trout efficiency of osmoregulation was highest at certain stages of growth, and suggested that adjustment to sea water was best achieved during a restricted phase of their life history. This stage appeared to be that of late parr-smolt transformation. The operation of plasma electrolyte homeostatic mechanisms was indicated in the Steelhead trout and the possible roles of the withdrawl of ions from the circulating fluids by complex formation and by active excretion have been considered.

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