The Evolution of Migratory Behaviour among Juvenile Salmon of the Genus Oncorhynchus

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Abstract
The discussion is based on a detailed etiological comparison of four species of juvenile Oncorhynchus—coho, chum, sockeye and pink salmon. Their behaviour is described in terms of five fixed behaviour patterns—hiding under stones, occupying territories, schooling, feeding and escaping predators. These are performed in relation to five directive factors—light, temperature, current, salinity and objects in the environment. Behaviour patterns and directing factors are associated with characteristic appetitive behaviour. The internal motivation seems to have an endocrinological basis. The coho fry, because of its river habitat, territorial behaviour, low nocturnal activity and smolt transformation, is considered to show behaviour nearest to that of the parental type of the genus. The pink fry has the most highly specialized sea-going behaviour. Three major developments are evident in the evolution of obligatory pelagic and ocean dwelling species (a) early smolt transformation (b) increased nocturnal activity and (c) schooling. Some possible evolutionary sequences are considered.

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