Juvenile roach were always more abundant in the less modified section (BPS) of the river than in the other sections (RES and TAIL). While the original primary river channel was directly impacted by the flash flood, no significant difference in otolith shape was observed before or after the flash flood. Our results suggest that this part of the reach provides refuge habitats used by fish during high flow events. K E Y W O R D SCyprinidae, flood event, Lower Rhône River, otolith morphometry | INTRODUCTIONThe channelisation and regulation of large rivers have severely impacted fluvial morphology and processes by changing natural hydrological and sedimentary regimes which contribute to the high ecological value of large floodplains (Bravard & Petts, 1996;Poff et al., 1997). The length and habitat heterogeneity of riverine ecotones have been reduced (Schiemer, Zalewski, & Thorpe, 1995;Tockner & Stanford, 2002) and the spatial and temporal patterns of hydrological connectivity have been highly altered (Pringle, 2003;Ward & Stanford, 1995). Populations of highly mobile organisms like fish, which use distinct habitats during their life cycle, have been seriously affected.Major changes in fish assemblages in large European rivers such as the Danube (Balon, Crawford, & Lelek, 1986; Holcik, 1988), the Rhine (Cazemier, 1988;Lelek, 1989) or the Rhône (Pattee, 1988;Persat, 1988) resulted from the 19th century river straightening, the channelisation and the construction of successive diversion dams (mainly during the 20th century) which have created new artificial environments. On the Rhône River, a typical hydropower scheme includes a reservoir (RES) created by the diversion dam built on the upper part of the river, a canal which redirects the main river flow to a power plant, and a bypassed old river that spans for several kilometres. Typically, the tailrace (TAIL) and the bypassed sections (BPS) converge in the downstream RES formed by next diversion dam (see Figure 1). Most of the time, the BPS receives a low minimum instream flow and overflows occur at the dam when river discharge exceeds the capacity of the hydropower plant. However, these released flows (discharge from the dam) are generally lower than the initial discharge prior to the

This assesses features of otoliths from laboratory‐reared embryos, larvae and juvenile European chub Squalius cephalus from hatching to 180 days post‐hatching (dph). We observed the development of the three pairs of otoliths (lapilli, sagittae and asterisci) and more precisely shape changes, as well as timing and deposition rate of increments of the lapilli. The lapilli and the sagittae were present at hatching, whereas the asterisci formed between 20 and 30 dph. The lapillus and sagitta shapes were round until 20 dph. From 60 dph the anterior and the posterior rostra of the sagittae were well developed, but very thin, making this otolith too fragile to manipulate for further studies of shape and validation of otolith increment deposition rate. The lapilli provided reliable age estimates for free embryos, larvae and juveniles up to 120 dph. However, caution should be taken when ageing fish older than 150 dph as an underestimation was noticeable. The regression of the number of otolith increments on age showed a slope and an intercept not significantly different from 1 and 0, respectively, which indicated that otolith growth increments were deposited on a daily basis, with the first microincrement occurring at hatching. Increment counts were consistent between three interpreters, indicating a consistent and reliable age estimate. This study validates that the otolith increment deposition rate can be used to assess hatching dates and daily growth of wild S. cephalus under 150 dph and in environments similar to the conditions used in this study.

Environmental DNA (eDNA) metabarcoding is revolutionizing the monitoring of aquatic biodiversity. The use of eDNA has the potential to enable non-invasive, cost-effective, time-efficient and high-sensitivity monitoring of fish assemblages. Although the capacity of eDNA metabarcoding to describe fish assemblages is recognised, research efforts are still needed to better assess the spatial and temporal variability of the eDNA signal and to ultimately design an optimal sampling strategy for eDNA monitoring. In this context, we sampled three different lakes (a dam reservoir, a shallow eutrophic lake and a deep oligotrophic lake) every 6 weeks for 1 year. We performed four types of sampling for each lake (integrative sampling of sub-surface water along transects on the left shore, the right shore and above the deepest zone, and point sampling in deeper layers near the lake bottom) to explore the spatial variability of the eDNA signal at the lake scale over a period of 1 year. A metabarcoding approach was applied to analyse the 92 eDNA samples in order to obtain fish species inventories which were compared with traditional fish monitoring methods (standardized gillnet samplings). Several species known to be present in these lakes were only detected by eDNA, confirming the higher sensitivity of this technique in comparison with gillnetting. The eDNA signal varied spatially, with shoreline samples being richer in species than the other samples. Furthermore, deep-water samplings appeared to be non-relevant for regularly mixed lakes, where the eDNA signal was homogeneously distributed. These results also demonstrate a clear temporal variability of the eDNA signal that seems to be related to species phenology, with most of the species detected in spring during the spawning period on shores, but also a peak of detection in winter for salmonid and coregonid species during their reproduction period. These results contribute to our understanding of the spatio-temporal distribution of eDNA in lakes and allow us to provide methodological recommendations regarding where and when to sample eDNA for fish monitoring in lakes.