A repeatable pattern of rhythmic stomach fullness was established for Atlantic salmon, Salmo salar L. held in freshwater throughout the year. Seasonal differences in rhythmic stomach fullness were recorded and were observed to fluctuate during summer, autumn and spring with a peak of fullness during these seasons occurring at ISOOh. Stomach fullness in winter remained relatively constant, whilst food levels were minimal.
Gut fullness levels of postsmolts held in sea cages during the summer were monitored and a pattern similar to that in the summer in freshwater was observed. Tide and temperature had little influence whilst light levels showed a mirroring rhythmic pattern to that of gut fullness levels.

Under controlled environmental conditions stomach fullness was rhythmic whilst temperature and light changed. A correlation between temperature and foregut fullness was recorded. Simulated ambient photoperiod (i.e. the correct number of hours of daylight, but once on, unchanging) however, appeared to be insufficient to synchronise feeding peaks. Rhythmic stomach fullness was monitored under three different light regimes to establish the influence of fluctuating light levels. Only under simulated ambient light and photoperiod was a mirror pattern, of gut fullness and peaks in foregut fullness, to that of ambient light and photoperiod recorded. Under simulated ambient photoperiod a rhythm was observed but dissimilar to that found under ambient environmental conditions. Changing light levels as opposed to specific light intensities are important in synchronizing the timing of feeding peaks. Temperature was thought to be of secondary importance in the presence of fluctuating light levels.
Postsmolts held in sea cages were found to consume a larger meal size when fed a single meal in afternoon compared to the morning. When fed a meal both morning and afternoon the afternoon meal was again larger, thus indicating regardless of previous dietary history, the time of the day influences the amount of food taken. It is hoped that the establishment of rhythmic stomach fullness will be useful in designing feeding regimes for both research and commercial use.