A hard days night: Limited shelters induce a shift to diurnal behavior in Arctic charr Salvelinus alpinus.
When resources are limited and not uniformly distributed, animals often partition space by establishing territories to monopolize these resources. Similarly, when resources are not uniformly distributed in time, some periods of the 24h cycle may be of more value than others, raising the question how individuals allocate their foraging and defensive efforts in time. The literature related to space use as a function of resource distribution and individual potential to defend these resources is plethoric, but much fewer studies report cases of partitioning of activity in time. In this study, we used an experimental approach in a natural stream, coupled with behavioral observations of individually tagged Arctic charr, to look at the effect of shelter availability on diel activity. Availability of shelters clearly affected individual activity patterns. Fish were nocturnal when provided with sufficient shelters, but diurnal when shelters were lacking. Many aspects of foraging behavior, such as mobility, aggregation or foraging rate were also influenced by shelter availability. The quantity of shelters did not influence growth, but when shelters were rare, more active fish grew faster. The results also suggest that bigger fish prefer feeding after midnight, whereas smaller individuals were more active and crepuscular. These results yield important insights on the influence of shelters on the behavior, individual fitness and population ecology of freshwater fish.