Diurnal variations in the diet of 0 to 3 years old Atlantic salmon Salmo salar L. under semiarctic summer conditions in the Alta River, Northern Norway
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Abstract
The stomach contents of 0 to 3 year old salmon were investigated throughout a single 20 hour period in each of July, August and September 1981. There was no marked diurnal variation in stomach content of fry, while that of parr indicated nocturnal maxima in all three months. In July and August there was a significant increase from evening to night. In September there was a significant increase from mid night to early morning (0400). The amount of food in the stomachs decreased from July to September during both day and night. There was only a weak correlation between diurnal drift and food content. Chironomidae and Ephemeroptera were the most important food items by volume in July, and Chironomidae in August. Increasing food content during the night in August was mainly caused by increasing content of Chironomidae larvae. Ephemeroptera was the most important food item by volume in September. Increasing food content during night in September was mainly caused by Ephemeroptera. The number of Chironomidae and Ephemeroptera ingested indicates greater feeding activity during the night in July - August by juvenile salmon in Alta River. The other food items showed only small diurnal variations (by number) in all three months.
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