Habitat exclusion and reduced growth: a field experiment on the effects of inter-cohort competition in young-of-the-year brown trout

Competition during the juvenile phase is a key process for regulating density in organisms with high fecundity. Juvenile density-dependent bottlenecks may become even more pronounced if several cohorts compete, but this has received relatively limited attention in previous literature. We performed a manipulation experiment in seven coastal streams to investigate the presence of inter-cohort competition, using habitat selection, body-size and density of newly emerged (age-0) brown trout (Salmo trutta) as response variables. The trout population (age ≥1 fish) was estimated using electro-fishing prior to the emergence of fry (April–May) and was either removed (manipulated sections) or maintained (control sections). Age-0 habitat selection was examined in June while density and body-size was evaluated in October (end of the growth season). We found that age-0 trout selected habitats that were located further from riffles (nursery habitats) in the absence of age ≥1 trout, suggesting a niche overlap between cohorts in the habitat dimension and, hence, that both inter-cohort competitive interactions and ontogenetic preference may influence habitat utilisation in the wild. Furthermore, we also found age-0 body-size to be significantly larger in manipulated sections and negatively related to its own density. We argue that competition from older cohorts influence the availability of age-0 feeding territories at the critical phase of emergence with secondary negative effects on age-0 growth. These results not only have implications for understanding the mechanisms of density dependence but can also provide valuable knowledge to the management of salmonid populations and their habitats in the wild.