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An experimental study of salinity preference and related migratory behaviour of juvenile Pacific salmon McInerney, John Edward

Abstract

The seasonal salinity preference of four species of Pacific salmon was examined. Each species showed a strong preference for hypertonic seawater during the normal period of migration. Pink fry (Oncorhynchus gorbuscha) and coho yearlings (O. kisutch) lost this preference during the summer in contrast to Chum fry (O. keta) and sockeye yearlings (O. nerka). Three other types of behaviour showed seasonal changes consistent with a transitory "migration disposition". A preference for hypertonic seawater was associated with high levels of activity, strong schooling tendencies and depressed aggressive behaviour. Subsequent seasonal changes showed a marked increase in aggressive behaviour accompanied by decreased levels of activity and group behaviour. A long daily photoperiod (16 hours) prolonged the behaviour complex associated with seaward migration. A short daily photoperiod (8 hours) delayed but did not totally inhibit the development of a hypertonic salinity preference and associated behaviour. The preference of chum salmon fry for a series of seawater concentration indicated an all-or-none type response. A consistently strong preference was shown for seawater hypertonic to plasma chloride levels as reported in the literature. No preference was shown for hypotonic seawater. A series of experiments in which the composition of an artificial seawater was altered indicated that under natural conditions the expression of a preference for salt water probably depends on the concentration of sodium chloride. The swiftness of the response (chum and sockeye) indicated stimulation of a peripheral salinity receptor. Coho underyearlings injected with mammalian somatatropin showed an increased although not statistically significant preference for hypertonic sea water. Both activity and aggressive behaviour were depressed in comparison to control fish.

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