Habitat use by larval and 0+ year juvenile fishes in European rivers

The habitats used by larval and 0+ year juvenile fishes can significantly influence individual growth rates, nutritional condition and survival, and thus recruitment success and population sizes. The quality, quantity and accessibility of potential nursery habitats are therefore among the most important factors driving the structure and dynamics of fish communities. In European rivers, the majority of species favour relatively shallow and slow-flowing areas in the early larval period. Such areas minimise the risk of predation by larger fish, are frequently warmer than deeper and faster-flowing water, maximise the amount of energy that can be allocated to somatic growth and generally support the greatest quantities of planktonic prey. However, improvements in swimming ability and predator and prey detection during development can facilitate species-specific habitat selection. Furthermore, distinct shifts in habitat use may coincide with key intervals in ontogeny, particularly the transition from the larval to juvenile period. Many species also exhibit seasonal shifts in habitat use, typically moving to deeper, slower-flowing and/or more structurally complex areas in the winter. In addition, species-specific diel variations in habitat use may occur, probably as a behavioural response to changes in predation risk, which may change during ontogeny or on a seasonal basis. Environmental management invariably has detrimental impacts on habitat quality or integrity and, thus, larval and juvenile fishes, with the systematic rehabilitation of degraded and fragmented ecosystems therefore essential for the recovery of affected populations.