Anadromous pink salmon undergo several migratory movements between different habitats during their life history. These migrations are accurately timed on a seasonal basis. Annual rhythms or seasonally-timed events may result from interactions between daily rhythms and annual changes in environmental factors. Therefore, knowledge of daily behavioral rhythms in pink salmon may improve our current poor understanding of the seasonal timing of its migrations. Hence, the objective of this study was

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... investigate, in a seasonal context and mainly under laboratory conditions, diel rhythms of ecologically-relevant behavior in juvenile pink salmon, and their timing mechanisms. Fry emergence from a simulated gravel redd in fresh water was mainly nocturnal below 13°C. Diel emergence timing was synchronized with the onset of night, but was affected by temperature in a non-linear manner. Temperature affected negatively the duration of the intra-gravel alevin stage and the rate of emergence. Nocturnal emergence was considered an anti-predator adaptation. Fry exhibited mainly nocturnal rhythms of swimming activity and of vertical distribution during the first week after emergence. However, a gradual shift from a nocturnal to a diurnal swimming activity rhythm occurred 7 to 13 days after emergence, when wild fish are residing in estuaries and adjacent coastal waters. Coincident with this shift was an increasing tendency of the fry to swimnnear the water surface during the day. This suggested a weakening of their negative phototactic response during this period. Thereafter, the fish usually displayed diurnal rhythms of swimming activity and nocturnal rhythms of vertical distribution. The ontogenetic shift in the phase of the activity rhythm and in photobehavior was considered adaptive for schooling and feeding during the day. Wild fry fed mainly during daylight hours in littoral areas of two marine bays. However, their feeding rhythms varied among study sites. Laboratory experiments showed that hunger level affected fish fe [...]