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Salinity preferences : an orientation mechanism in salmon migration. McInerney, John Edward

Abstract

The preferred salinities of five Pacific salmon species were studied. Each species was shown to undergo a temporal sequence of preference changes. The sequence began with a preference for fresh water then changed in the direction of increasing seawater concentration, the terminal pattern indicating a preference for water of open ocean concentration. This preference sequence was shown to parallel closely the horizontal salinity gradients typical of estuaries through which juvenile salmon pass on their seaward migration. On the basis of this evidence the following orientation mechanism was postulated: that juvenile salmon are able to use estuarine salinity gradients as one of the directive cues in their seaward migration. Further study of this orientation mechanism showed that the initial part of the preference sequence develops unaffected by seawater exposure. By contrast the latter part of the sequence (corresponding to seaward end of the estuary) was found to depend on a period of exposure to seawater otherwise a regression to a premigratory freshwater preference took place. The sensory stimuli leading to the salinity preference response were shown to depend on a complex interaction of naturally occuring sea salts. Experimentally the simplest salt mixture which would elicit a normal response consisted of two cations (Na⁺ and Ca⁺⁺) and one anion (Cl¯). It was shown further that taste or the common chemical sense was the primary sensory modality underlying the response and that juvenile salmon have an ability approaching absolute salinity discrimination. Speculation concerning the evolution of the salinity preference orientation mechanism was presented. Published evidence favours the view that migratory salmonids evolved from nonmigratory forms with limited osmoregulatory abilities. On this basis it was proposed that originally the ability to orient with respect to seawater concentration was of direct survival value to the stenohaline ancestral salmonid. Later, as diadromous movements expanded along with euryhalinity, salinity preference became integrated into a temporal sequence of changes and thereby an orientation device useful for migration.

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