Author(s): Daniel S. Hayes; Stefan Auer; Elora Fauchery; David Graf; Thomas Hasler; Daniel Mameri; Stefan Schmutz; Simon Fuhrer
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Abstract: Experimental studies constitute a foundational building block of holistic ecohydraulic study designs encompassing different spatial scales. Channel experiments largely have the advantage over in-situ studies in that biological processes linked to river flows and morphology can be more precisely quantified. However, near-natural experimental situations may pose unforeseen difficulties, particularly with small study organisms. This study aimed to quantify the stranding of nase larvae (Chondrostoma nasus) due to rapid drawdown scenarios. Since fish larvae were less than 2.4 cm in length (mean=1.9 cm), difficulties in handling, quantifying, and retrieving stocked fish after trials were expected. Hence, we developed mesocosms (2.25×2 m), providing semi-natural conditions. Here, we describe the feasibility of using such outdoor mesocosms to quantify the stranding of cyprinid larvae during the day- and nighttime by calculating post-trial retrieval rates and by comparing observed with calculated stranding rates, both for a homogeneous gravel bar and a structured riverbed setup (i.e., an island with a longitudinal depression ditch). Our results show that post-trial retrieval rates were on average 99.5%, with 100% being reached in around two-thirds of all cases; the lowest clearing rate was 96.0%. Calculated and observed stranding rates did not differ, the latter being only slightly lower than the first, and both can be considered to yield the same results. The setup of the gravel bar and time of day did, however, affect stranding: the highest stranding rate was found at the structured riverbed setup during the night (mean=24%), followed by daytime trials (mean=5%). The fewest fish stranded at the flat gravel bar during the day (mean=1%), and only slightly more during the night (mean=3%). Overall, our results underline the feasibility of using such outdoor mesocosm structures to quantify hydropeaking-induced stranding of small cyprinid larvae.