River habitat homogenisation enhances trophic competition and promotes individual specialisation among young of the year fish

In large rivers, the success of ontogenic development of fish is mainly influenced by resource availability and by the possibility of species to adapt their diet (i.e. trophic niche). Human have drastically modified freshwater habitats, notably for navigation purposes. Such modifications may drastically affect food availability for young of the year (YOY) fish and, consequently, influence their ability to reach the adult age. In the River Meuse, a decrease of fish abundance is thought to be linked to a drastic decrease of phytoplankton biomass. In this context of lowering phytoplankton biomass, we quantified the trophic niches of three cyprinid species (common bleak Alburnus alburnus, chub Squalius cephalus, and roach Rutilus rutilus) and one percid species (European perch Perca fluviatilis) at various stages of development, in order to estimate the influence of river channelization in the intra and interspecific competitions. It was done using stable isotope analysis in two reaches of the River Meuse differing by their degree of regulation. We hypothesized that habitat heterogeneity increases YOY abundance over time by offering more alternative resources which reduce food competition, notably during the early period of life. Our study provides evidence that the River Meuse flow and depth regulation significantly impacted the abundance and taxonomic diversity of YOY. In the context of low planktonic biomass, most YOY relied on benthic food sources. In the heavily channelized reach, between-stages competition and low resource diversity increased the diet partition between cyprinid larvae and forced a part of individuals to consume non-optimal energetic food sources such as aquatic vegetation. On the other hand, in the less channelized reach, larvae displayed a generalist feeding habit focusing on energetic prey such as different taxa of macroinvertebrates, suggesting that the diversity of habitat reduces the food competition within and between stages and the predation risk. Developing a sustainable and integrated river management may be useful for protecting biodiversity and restoring ecosystem function, in order to improve the ecological potential of regulated rivers.