Rock fishways: Natural designs for an engineered world

Globally, fishways have been constructed as a technical solution to reduce the damaging effects of dams and weirs on fish movement. Some common design typologies are rock fishways and nature-like bypasses. While rock fishways are common in Australian rivers, there has been no general evaluation of their effectiveness, location, or costs to demonstrate their success and help guide future investment. Our objective was to develop an Australian inventory of rock fishways to visualise their location, major design elements, capital cost and frequency of evaluation. We also present two rock fishway case-studies that highlight key ecological and engineering design criteria, field evaluation of effectiveness and a summary of construction learnings. In Australia, there were 332 rock fishways, most typologies were lateral-ridge rock ramps (161), random rock fishways (123), nature-like bypasses (13) and hybrid designs (10). Approximately 20% of these have biological monitoring. Trends over time, included: (i) a slight flattening of fishway slope (from 1 V:20H to 1 V:30H) (ii) a progression from dumped loose rock to keyed-in formalised lateral-ridge fishways, increased use of natural sealing and (iii) an emerging tendency to hybrid concrete baffle and rock designs. Construction costs were available for 183 sites, with a total adjusted investment of USD$22 M and although highly variable the average cost per metre differential head was USD$110,000 (range USD$4500–286,000) per vertical-metre making rock fishways one of the most cost-effective designs available. Two field case-studies provided support for passage of a broad size range (15–410 mm long) of fish at a wide range of flows with hydraulic reliability over considerable design life-spans (i.e., >25 years). The publicly-available rock fishway database forms a platform for sharing technical information to promote more effective designs, contributing to continued global development of rock fishways and conservation of freshwater fish communities.