Drift feeding by brown trout fry under downramping flow conditions: effects of substrate structure

In regulated rivers, hydropeaking may create abrupt sub-daily changes in discharge that can rapidly alter fish habitat. Although hydropeaking impacts on juvenile salmonids have been widely studied, short-term effects on foraging by early life stages remain poorly documented. We used a laboratory flume experiment to test how a downramping flow regime and substrate heterogeneity affect the foraging performance of brown trout (Salmo trutta) fry. We exposed fry to four treatments, combining two flow conditions (stable vs. downramping) and two substrates (gravel vs. gravel with cobbles). We quantified individual total prey intake, assessed from stomach contents, and drift-feeding activity from video analyses. Our results showed that the flow treatment had no detectable effect on total prey intake, indicating that both stable and rapid flow reductions did not disrupt short-term feeding under an adequate food supply. The number of drift-feeding attempts was similar across all treatments. In contrast, the presence of cobbles increased total prey intake, suggesting that structural complexity enhanced intake by providing hydraulic refuges and additional feeding opportunities. Prey intake showed a general size-dependent pattern, with larger fry consuming more prey. Our findings indicate that adding or maintaining fine-scale substrate complexity is a potent enhancer of fry feeding and should be considered as a restoration measure for habitat mitigation in hydropeaking rivers.