More than the sum of their parts - Environmental flows increase fish movement and fishway functionality

The ecology of riverine fish is intrinsically linked to attributes of the natural flow regime such as duration, timing, and magnitude. The flow regimes and hydrological connectivity of many rivers globally has, however, been severely altered by river regulation, detrimentally impacting fish populations. Environmental flows and fishways are increasingly being used to reinstate important aspects of the flow regime, improve instream connectivity, and recover fish populations. The present work examines the potential benefits that the combination of fishways and environmental flows may have in terms of greater overall environmental outcomes than fishways alone. We used multiple lines of evidence to correlate fish movement through multiple fishways in three lowland Australian rivers with increases in discharge mainly driven by environmental flows. Specifically, fish movement rates (from PIT tagged fish and fishway trapping) were modelled against stream discharge and water temperature, while the internal hydraulics (turbulence, water velocity and discharge) of a low level (<3 m head) fishway was modelled over three different flow scenarios — low, medium, and high discharge — to document functional changes that improved fishway efficiency with increased discharge. Fishway trapping and PIT tag data indicated that native fish movement increased with increases in discharge associate largely with environmental flows, and medium-large magnitude flow events resulted in a greater movement response. Environmental flows raised river levels and improved the internal hydraulics of one low-level vertical-slot fishway, specifically benefiting passage of the smallest fish species and size-classes. Management interventions, such as environmental flows and fishways, come at considerable cost and require empirical data regarding their benefits, yet the potential benefits of combining interventions is rarely quantified against key objectives. This study presents evidence demonstrating that environmental flows cue fish migration and improve fishway efficiency and thus promote functional fish populations.