Substrate roughening improves swimming performance in two small-bodied riverine fishes: implications for culvert remediation and design.

Worldwide declines in riverine fish abundance and diversity have been linked to the fragmentation of aquatic habitats through the installation of instream structures (e.g. culverts, dams, weirs and barrages). Restoring riverine connectivity can be achieved by remediating structures impeding fish movements by, for example, replacing smooth substrates of pipe culverts with naturalistic substrates (i.e. river stones; culvert roughening). However, empirical evaluations of the efficacy of such remediation efforts are often lacking despite the high economic cost. We assessed the effectiveness of substrate roughening in improving fish swimming performance and linked this to estimates of upstream passage success. Critical swimming speeds () of two small-bodied fish, purple-spotted gudgeon (; 7.7-11.6 cm total length, BL) and crimson-spotted rainbowfish (; 4.2-8.7 cm BL) were examined. Swimming trials were conducted in a hydraulic flume fitted with either a smooth acrylic substrate (control) or a rough substrate with fixed river stones. Swimming performance was improved on the rough compared to the smooth substrate, with ( = 0.28 ± 0.0 m s, 2.89 ± 0.1 BL s, = 0.36 ± 0.02 m s, 3.66 ± 0.22 BL s, mean ± s.e) and ( = 0.46 ± 0.01 m s, 7.79 ± 0.33 BL s; = = 0.55 ± 0.03 m s, 9.83 ± 0.67 BL s, mean ± s.e.) both experiencing a 26% increase in relative . Traversable water velocity models predicted maximum water speeds allowing successful upstream passage of both species to substantially increase following roughening remediation. Together these findings suggest culvert roughening may be a solution which allows hydraulic efficiency goals to be met, without compromising fish passage.