MULTISCALE APPROACH TO SPECIES-HABITAT RELATIONSHIPS - JUVENILE FISH IN A LARGE RIVER SECTION

1. Juvenile fish were sampled by electrofishing using the point abunda nce sampling method from August to November 1991 in the Lower Rhone Ri ver. Variations in fish abundance between point samples were partition ed according to three spatial scales of habitat heterogeneity. 2. The compartment scale, differentiating channel and 'dike fields' (an area marginal to the main channel partially enclosed by an artificial emban kment), was associated with variation in water current, temperature an d biological productivity. Every juvenile fish species studied was mor e abundant in the dike fields, using them as a nursery. 3. Within both compartments (i.e. main channel and dike fields), the same five bank types were represented, defining the intermediate scale of habitat het erogeneity. In the dike fields, beaches were selected by most of the s pecies, whereas steep silt banks were more often selected in the main channel. 4. After having removed the variation explained at the two la rger habitat scales, only depth and shelter were found to have a signi ficant effect on fish abundance at the microhabitat scale. 5. When com pared with a classical single-scale approach, the multi-scale approach was found to explain a greater amount of fish abundance variations an d to facilitate the interpretation of observed patterns by scaling the species' responses to habitat heterogeneity. The difference between t he amounts of variation explained was proposed as a measure of how muc h habitat selection is hierarchical.