Does hydrology constrain the structure of fish assemblages in French streams? Regional scale analysis

With the increasing alteration of flow regimes worldwide, it has become a key challenge to estimate the general linkages between species and flow patterns. In fish, the young-of-the-year (YOY) cohort size is of fundamental importance to population dynamics, and can provide valuable insights into part of the ecological functioning of river systems. Here, using 65 French sites, each with at least 4 annual samples, I investigated the influence of the seasonal inter-annual flow variability on the YOY fish assemblage within sites. Assemblage structure was described by relative abundances of species, diversity measurements (richness and Shannon's index) and species traits. Flow statistics were derived so as to reflect average conditions, low and high flow conditions, floods and overall variability, and were computed for each biological season (reproduction, growth and overwinter periods). Relationships were explored and tested by within-site co-inertia analysis, and quantified by ANCOVA models. The flow levels during the reproduction and growth periods consistently explained differences among years in 4 species guilds, although average relationships were weak (i.e. max. ∼9%). Assemblage diversity slightly declined with increasing water level during the growth period. Species responses depended on their biological and functional abilities to ensure their reproductive success under adverse hydrological conditions (e. g. time of spawning, fecundity, protection of offspring), and clearly reflected two contrasting reproductive strategies. These results are in general agreement with 'habitat template'-based theories. They suggest that flow may be locally managed to sustain or restore taxonomic or functional assemblage features, keeping in mind that the strength of the expected effect is site-specific.