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An experimental study of the response of young Pacific salmon to sharp sea water gradients Houston, Arthur Hillier

Abstract

The responses of chum and pink salmon fry, and coho salmon fry and smolts (Oncorhynchus keta, O. gorbuscha, and O. kisutch) to isotonic and hypertonic seawater were studied in sharp-gradient tanks to determine whether or not salinity gradients can act as directive agencies in the seaward migration of juvenile Pacific salmon. Chum and pink fry responded positively to both concentrations of seawater. Coho fry responded positively to isotonic seawater but did not respond positively to hypertonic seawater. Coho smolts responded positively to hypertonic seawater. Acclimation of chum fry to seawater prior to observation of their responses resulted in a reduction of their initial response although there was no significant change in the levels of response finally obtained. Major differences in the concentration of the acclimatory solutions did not affect the responses of this species to hypertonic seawater. The activity of chum and pink fry, and of coho smolts generally decreased on first entry into seawater. Observations on acclimated chum fry suggest that activity is related to osmotic control. Decreased activity may arise from the interaction of absorbed electrolytes on muscle protein. This effect continues until the commencement of osmoregulation when excess absorbed ions are removed from the muscles. The relatively high levels of activity observed in pink fry may be the result of hyperfunction of the thyroid gland, a condition related to osmotic stress in fresh water. The influence of sea water on the seaward movements of juvenile Pacific salmon is probably two-fold. The effects of absorbed electrolytes on motor activity may decrease the intensity of rheotrophic responses which would tend to keep the migrants in river mouths, while at the same time increasing the probability of passive displacement out to sea by current action. Active positive responses to salinity gradients enhance the likelihood of movement into the ocean. Control experiments indicated the operation of some factor or factors resulting in preference reactions by chum and pink fry and coho smolts for the tanks in which they had been originally placed. The possible influence of olfactory and visual cues on this preference reaction has been discussed.

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