| Summary: | Diel activity, the partitioning of time between periods of activity and rest, gives insights into how organisms compete for resources in time. Some species show plasticity in the rate and timing of their activity, which enables to study associations with ecological factors. Streamdwelling fishes like salmonids are a textbook example of animals with variable activity patterns. During this Ph.D., I studied the diel activity of individually tagged juvenile Arctic charr in several field experiments and studies, focusing on critical factors for salmonids ecology. I found that Arctic charr increased activity (i) with rising temperature, (ii) when shelters are limited, (iii) in fast current, (iv) under relatively stable waterflow and (v) at high population density. The distribution of activity in time was also affected by ecological conditions in all studies. Flexible activity patterns coincided with modifications of other behaviors (aggregation, foraging mode, habitat selection). Arctic charr sometimes appear to maintain growth under suboptimal conditions by modifying their activity (e.g. limited shelters), whereas in other situations they increase activity under conditions that yield higher growth (high current velocity). In all but one experiment, more active fish grew faster. This relationship depended on the environment. It was stronger in faster currents, and under stable waterflow. These results have important implications for biological fields such as behavioral ecology, by estimating behavioral flexibility, salmonids ecology via food intake and growth under different ecological scenarii, and conservation biology by using behavior to assess the effect of future changes in the physical habitat of stream fishes. Dægursveiflur í virkni lýsa því hvernig dýr deila sólarhringnum á milli virkni og hvíldar, og hvernig þau keppa um auðlindir í tíma. Sumar tegundir sýna sveigjanleika í því hversu virk þau eru og hvenær, og eru hentug til rannsókna á áhrifum vistfræðilegra þátta á virkni. Fiskar í ám, þá sérstaklega ...
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