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Diel rhythms of behavior in juvenile pink salmon (Oncorhynchus gorbuscha Walbaum) Godin, Jean-Guy Joseph


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 to 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 feeding rate and ration consumed positively. Fish fed continuously on live copepods under idealized laboratory conditions. During a 12-h session they rapidly (< 30 min) filled their stomachs with prey; thereafter, they maintained their stomachs full by feeding at a rate that balanced the rate of evacuation of prey from the stomach. Hence, I concluded that pink salmon have flexible feeding activity rhythms, which may permit opportunistic exploitation of prey, and feed at a relatively low hunger threshold. This feeding strategy may explain in part their relatively high growth rates in nature. During the periods corresponding to their juvenile coastal and pelagic ocean phases, the fish exhibited generally diurnal rhythms of swimming activity and of aggression, and nocturnal rhythms of vertical distribution in response to simulated seasonal photoperiodic and temperature changes. These rhythms were synchronized with the artificial light-dark (LD) cycle throughout most of the year. Some parameters of these rhythms varied on a seasonal basis, but not according to the Aschoff-Wever model. Mean swimming speed, the degree of diurnalism of the swimming activity rhythm, and the timing of the daily peak of the rhythms were affected by daylength. Hence, photoperiod might be an important proximate factor that pink salmon use to time their oceanic migration on a seasonal basis. Some data suggested the existence of an endogenous, circadian activity rhythm, and thus a daily "clock", in pink salmon. However, this remains uncertain. The free-running period of their activity rhythm was not related negatively to constant light intensity, as predicted by the Circadian Rule. The LD cycle affected directly swimming activity (masking), rather than entraining an endogenous circadian system. Since the activity rhythm of pink salmon does not possess a strong endogenous component, it is doubtful that the seasonal timing of its migrations results from interactions between a circadian clock and seasonal changes in environmental factors. However, the flexibility and inter-individual variability of their behavioral rhythms may be adaptive responses to the instability and heterogeneity of the marine environment.

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