Competition Within and Between Year Classes in Brown Trout; Implications of Habitat Complexity on Habitat Use and Fitness

Habitat use and fitness are restricted and altered by both intra‐ and interspecific competition over limited resources. As a result, population dynamics may be affected both by cohort structure, habitat complexity and habitat availability. This chapter summarizes the current knowledge of how competition between and within year classes in brown trout (Salmo trutta L.) interacts with habitat complexity and affect habitat utilization and fitness. Generally, newly emerged salmonids use shallow, low‐velocity habitats close to the spawning area whereas older cohorts occupy deeper, high‐velocity habitats. Data, based on both field experiments and laboratory experiments, will be provided suggesting that the observed pattern most likely is an effect of habitat exclusion from older cohorts rather than size‐dependent habitat preference. This highlights the importance of taking age‐class interactions into account when investigating density‐dependence and habitat use in brown trout. Thus, from a management point of view, marginal shallow stream habitats may be essential during the first months after emergence but not due to size‐dependent habitat preference for newly emerged fish. Instead, they can act as refuges from inter‐cohort competition, thus emphasizing the importance of maintaining and restoring these habitats.