Temporal variations in daily patterns of swimming activity and vertical distribution in juvenile pink salmon (Oncorhynchus gorbuscha)

At times corresponding to their juvenile coastal and pelagic ocean phases, groups of juvenile pink salmon exhibited diel rhythms of generally diurnal swimming activity and nocturnal vertical movement in response to simulated seasonal variations in photoperiod and water temperature in the laboratory. Diurnal swimming activity was synchronized with the daily light cycle throughout most of the year, but not with ambient daily temperature cycles. On average, fish were increasingly more active during the day than at night with increasing duration of the photophase. Relative swimming speed also increased with increasing photophase duration, and was not affected by seasonal changes in temperature. The fish always showed a greater tendency to swim in the upper half of the water column during the night than during the day. This rhythm of vertical distribution was more strongly synchronized with the daily light cycle in winter than during any other season. Daily temperature cycles did not synchronize these distribution patterns. No significant seasonal trend was noted in mean levels or mean indices of vertical distribution. Results are related to the migratory behaviour of pink salmon, and it is concluded that photoperiod may be an important proximate factor in the timing of their seasonal migratory behaviour.