Trophic interactions, including “top‐down” predator‐prey interactions, are particularly important in influencing the structure of fish communities. While the varied impacts of piscivorous fish have been well investigated, the effects of fish‐eating birds on riverine fish behaviour and population dynamics still remain controversial, mainly because they are undervalued. Summer experiments were conducted in an experimental outdoor stream to evaluate the effects of avian predation threat, stream flow, and overhead cover on growth and behavioural tactics of wild juvenile chub (Leuciscus cephalus). Groups of fifteen chub maintained in riffle‐pool sequences were submitted to combinations of different conditions, namely absence or presence of a simulated fish‐eating bird, low or high flow, and absence or presence of medium or high cover. In the absence of predation threat, chub foraged in the riffles and maximized feeding opportunities. Under predation threat, they sheltered, foraged less and grew slowly and as expected, they increased their use of the riffles at high flow as water turbulence is an efficient shelter from birds but only in the absence of cover. In the presence of cover, fish sheltered exclusively under pool covers and were more prone to take risks at low flow because of higher costs in terms of lost feeding opportunities associated with these conditions. This result indicates that flow velocity altered cover use tactics through its impact on food supply, suggesting that it may affect the outcome of predator‐prey relationships. So, chub use cover in a flexible manner according to both the benefits in terms of predator avoidance and the costs in terms of lost feeding opportunities. A striking finding of the experiments is the drastic reduction in the range of growth variances amongst fish when they are maintained under predation threat, suggesting a homogenization of fitness between individuals. From all our results, we argue that in lowland streams, under summer field situations, fish‐eating birds may affect local prey population dynamics more through sub‐lethal effects on growth rates than directly through death rates.

SUMMARY 1. A study of microhabitat preferences was conducted on Zingel asper, an endangered endemic species from the Rhône catchment. A generalised linear model allowed us to test statistically the non‐random habitat selection and the effect of season and site on this habitat selection. 2. The analysis detected significant preferences for the three physical variables considered: water depth, water velocity and substratum size. 3. A seasonal shift in the substratum size preference was found: preference for stones increasing during the spawning season. Depth preference varied between sites, suggesting a possible plasticity in habitat selection. 4. These results suggest that the availability of suitable physical habitat plays a significant role in determining fish distribution in the River Beaume.

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This review attempts to assess the nature and the role of cover for riverine fish assemblages. Although early identified as a key factor for fish distribution, especially for salmonids, cover (i.e. woody debris, undercut banks, boulders, turbidity...) still remains the variable least considered in the studies of fish habitat relationships. This is mainly due to the diversity of ecological functions of cover structures in fish assemblages. Cover structures are structuring components of fish habitat and contribute to the biological productivity of streams. But, at the individual scale, cover fulfils three main functions: protection against predators, visual isolation reducing competition, and hydraulic shelter. In fact, the use of cover by fish results from a trade-off between the costs and the benefits associated with its use. Although the relationships between fish and cover appear extremely complex and context-specific, a growing body of evidence highlights the potential role of cover for management purposes.Key-words : cover, shelter, riverine fish, fish habitat. NATURE ET FONCTIONS DU COUVERT POUR LES POISSONS LOTIQUES. RÉSUMÉCet article décrit la typologie ainsi que les fonctions du couvert pour les poissons lotiques. Identifié très tôt comme un facteur explicatif de la distribution des poissons, principalement chez les salmonidés, le couvert (i.e. débris ligneux, sous-berges, blocs, turbidité...) demeure néanmoins la variable la moins considérée dans l'étude des relations habitat-poissons. Ceci s'explique notamment par les fonctions écologiques très diverses que le couvert remplit vis-à-vis des assemblages piscicoles. Les structures pourvoyeuses de couvert sont des agents structurants de l'habitat piscicole et contribuent à la productivité biologique des cours d'eau. Au niveau du microhabitat du poisson, le couvert remplit trois fonctions majeures : anti-prédation, isolation visuelle limitant la compétition, et abri hydraulique. En fait, l'utilisation du couvert par les poissons résulte d'un compromis entre les coûts et les bénéfices associés d'où l'extrême complexité de cette relation qui semble plutôt spécifique à un contexte donné. Malgré les difficultés d'extrapolation, de nombreux travaux mettent en évidence la signification écologique ainsi que l'utilisation potentielle du couvert pour une gestion optimale des ressources piscicoles.Mots-clés : couvert, abri, poissons lotiques, habitat piscicole.