Effect of Population Density on Diel Activity and Growth in Stream-Dwelling Arctic Charr

Intraspecific competition plays a significant role in shaping how stream-dwelling fishes share and use habitats in time. Previous research is equivocal regarding the effect of increased population density on growth and behavior of salmonids, as responses are often context- or species-specific. We tested for the effect of population density on diel activity and growth in stream-dwelling Arctic charr (Salvelinus alpinus) in northern Iceland. Individually-tagged fish were stocked in semi-natural stream enclosures at high (6 fish/m²)  and low (2 fish/m²) density. Despite low activity overall, there were significantly higher rates of activity at high density (14.0%) than at low density (10.5%), indicating that fish spend more time foraging to satisfy their metabolic demands when competition increases. At both high and low density, fish were most active at night, though fish at high density showed significantly greater crepuscular activity than fish at low density. Individual growth rates were unaffected by population density. This study suggests that to obtain a more comprehensive understanding of the role of competition in fish populations, it is not only necessary to examine spatial patterns (e.g., territory size), but also how individuals may modify temporal aspects of their foraging activity to ensure access to resources.