Key words: irrigation, extraction, water diversion, fish screening, Native Fish Strategy.
Threats and Impacts: There is mounting evidence to suggest that significant numbers of fish are being lost from Australian rivers via water extraction. One of the most common tools used to combat this loss internationally are fish screens on irrigation intake pipes. Prior to this project there were no guidelines for intake screens in the Murray-Darling Basin (MDB).
Broad aim and specific objectives: This project aimed to undertake experiments to develop physical design criteria for fish screens at water diversions in the MDB as well as undertaking a scoping study of fish screening programs elsewhere in the world to make recommendations on how to best initiate a successful program in the Basin.
Methods: A combination of field and laboratory-based experiments at simulated intake screens was used to test how approach velocities (water velocities in front of and at right-angles to the screen face) and screening materials affected potential injury and mortality of native fish species. An experimental water pump was used, to which screens of varying mesh size could be fitted, enabling comparisons in fish entrainment to be identified. Dual-frequency identification sonar was also used to quantify the number and nature of fish interactions with the experimental screen. Lastly, a review of fish screening programs elsewhere in the world was undertaken to aid identification of key factors for successful implementation of an effective program in the MDB
Findings: It was found that the installation of fish screens has great potential to significantly reduce fish entrainment at intakes. Mortality at an experimental intake was reduced from over 90% (unscreened) to less than 2% (when screened) in the laboratory. Approach velocities of up to 0.4m/sec were effective in reducing entrainment of juvenile Golden Perch (Macquaria ambigua) and Silver Perch (Bidyanus bidyanus) in laboratory trials, with very little injury or mortality resulting from incidental screen contacts or impingement. In comparison, field observations of an assemblage of fish at a screen demonstrated that even modest increases in approach velocity (from 0.1 to 0.5 m/sec) produced a significant increase in the rate of screen contact for fish less than 150 mm, with the impact being more marked for smaller fish. There was little difference in the rate of screen contact or entrainment when using three different sizes of woven wire mesh. Together, these findings suggest that screening material may not be as important as approach velocity when designing screens for protecting fish.
A review of successful screen programs in the USA found that coordinating committees are a key factor to success. Government-irrigator cost-share programs have proven to be strong incentives to screen diversions elsewhere in the world and their use should be further explored for the MDB.
Lessons learned and future directions: This study highlighted that approach velocity is a key determinant of potential injury or mortality associated with contact with intake screens. Screen material was not found to influence potential injury or mortality of fish.
The findings of this project should be used to develop guidelines for water abstraction that will have significant benefits for native fish by reducing the number of fish lost to irrigation off takes and injury and mortality associated with fish contact with intake screens. This has the potential to significantly assist in native fish population rehabilitation by protecting smaller bodied fish (including early life history stages of large bodied species).
There is a need for a fish screening coordinating committee for the MDB to provide guidance regarding the setting and refinement of screen design criteria, identify funding opportunities and identify priority intakes for implementation.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy.
Contacts: Dr Craig Boys, Fisheries New South Wales. Tel: + 61 2 4916 3851, Email: firstname.lastname@example.org.