Development of fish screening criteria for water diversions in the Murray-Darling Basin.
Native fish populations in the Murray-Darling Basin (MDB) are believed to have undergone a 90 % decline since European settlement. Whilst multiple key threatening processes have contributed to this decline, the impact of fish losses at water diversions has been largely underestimated and remains unaddressed. There is mounting evidence that significant numbers of fish (ranging from hundreds to millions) are being lost from rivers through water diversion. Whilst the total extent of impacts remain uncertain, given the extent of water extraction across the MDB, it is likely that fish entrainment is significant and will need to be addressed if a recovery in native fish populations is to be achieved.
Fish screens are an important fisheries management tool used in many parts of the world to protect fish populations whilst allowing flows to be delivered to irrigators. Although many different screening approaches are currently applied throughout the world, most of which would be suitable for application in the MDB, it is essential that technologies are designed with the needs of local fish species in mind. In particular, screens will need to meet certain design criteria (e.g. maximum velocities at the screen face, or be made out of suitable material) that ensure that fish are excluded from diversion and do not suffer any injury or mortality.
Currently no criteria or guidelines exist for the design of screens suitable for Australian native fish, and this study has been the first to collect data relevant to the MDB. A combination of field and lab-based experiments at simulated intake screens was used to test a variety of approach velocities (velocities in front of and perpendicular to the screen face) and screening materials. It was found that the installation of fish screens has great potential to significantly reduce fish entrainment at intakes, and in some cases mortality at an experimental intake was reduced from over 90 % (unscreened) to less than 2 % (when screened).
The probability of screen contact was found to increase with decreasing fish size. Lab and field trials suggested that approach velocities (8 cm) in front of the screen face should not exceed 0.1m/sec for screens in the MDB. This is aligned with accepted screening criteria in other parts of the world and there are currently many screening solutions available that are capable of meeting this in a cost effective manner. An approach velocity of 0.1 m/sec will protect a diverse range of species and size classes of fish, including the some of the most vulnerable in the population. We observed that even modest increases in approach velocity above 0.1 m/sec produced a significant increase in the rate of screen contact for fish smaller than 150 mm.
Several screening materials were tested and this appeared less important than approach velocity in determining the level of fish protection afforded by screens. Therefore, guidelines adopted from other countries can help inform screen material, maintenance and performance standards. Approach velocity, however, will need to be a feature of primary consideration when developing new guidelines for the MDB, offering protection to the most vulnerable species and age classes of fish in the population. Ongoing field and lab-based research program will be needed to adaptively inform further criteria development. Fish screening coordinating committees and 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. Diversion management plans for all catchments in the Murray-Darling Basin, backed by adaptively implemented guidelines will provide a robust framework with which to address further native fish declines.