Author(s): Susana Amaral; Paulo Branco; Filipe Romao; Maria Teresa Ferreira; Antonio Pinheiro; Jose Maria Santos

Linked Author(s): Paulo Branco, António Pinheiro

Keywords: Potamodromous cyprinid species; Low-head ramped weirs; Upstream migration; Retrofitting; Discharge

Abstract: Portuguese rivers have more than 8000 small barriers (< 5 m high), of which low-head ramped weirs are one of the most common designs. The negotiation of these weirs, where water passes over the ramp but does not generate a waterfall, is usually site- and species-specific, being often insurmountable for fish due to excessive velocity and/or insufficient depth along the ramp. The addition of substrates (retrofitting), couple with variations in flow discharge to make them more friendly for fish passage, is an issue of increasing interest, though few studies have been performed so far. The aim of this study was to evaluate the performance, assessed through the number of fish approaches, attempts and successful negotiations, of an experimental low-head ramped weir (3.00 m long x 0.60 m wide) to enhance passage of a cyprinid potamodromous species, the Iberian barbel (Luciobarbus bocagei). Special attention was given to test the effects of substrate placement (i.e. cobble, 100-300 mm; RD) and discharge (Q) on fish passage performance, by testing two configurations: i) a Control (no substrate), and ii) a retrofitted scenario (Nature), with cobbles randomly embedded on the bottom of the ramp, both ran under two different Q (55 and 110 L.s-1). Each combination of RD and Q was replicated 20 times, using individual adult fish (n = 80 fish). Results showed that the total number of approaches, attempts and successful negotiations increased significantly under the Nature scenario (PERMANOVA, RD: F = 3.9, P < 0.05), and with the decrease of Q (PERMANOVA, Q: F = 26.9, P = 0.001). Transit time (T) and fish effort (measured by lactate content from blood sample; LA) were Q-related (PERMANOVA, T: F = 16.5, P = 0.001; PERMANOVA, LA: F = 4.4, P < 0.05) registering, with increasing Q, an increase of T, and a decrease of LA due to the reduction of fish movements by more than 50%. These outcomes may help to design more efficient passage structures for potamodromous fish and restore connectivity at these small barriers.

DOI: https://doi.org/10.3850/IAHR-39WC25217119202264

Year: 2022