This report represents a continuation of “Impacts and Solutions: A scoping study on the relative impacts of irrigation infrastructure on fish in the Fitzroy Basin” (Hutchison et al. 2022). This original study was initiated to understand how different types of irrigation infrastructure and hydrological conditions contributed to entrainment of fishes into irrigation systems. This knowledge assisted with development of a prioritisation matrix, to direct mitigation measures to where there was the greatest need.

In the current study, supplementary data was collected to boost the statistical power of the original data set. The aim was to determine if the patterns observed in the original study were maintained and supported by statistical significance to provide more confidence in the recommendations from the original report. The increased data set and revised statistical analyses would also assist with refinement of the original prioritisation matrix.

Analysis of the enhanced data set largely confirmed the patterns observed in the original study. A comparison of a gravity fed and pumped offtakes from Fairbairn Dam in the original data set indicated that fish were entrained at greater rates through the gravity fed diversion than the pumped diversion (Hutchison et al. 2022). With the inclusion of the supplementary data, this was statistically significant for several species and size classes, and across all species combined. Inlet flow rates had little impact on this result. Gravity fed diversions should be a high priority for mitigation with modern self-cleaning screening systems.

For irrigation systems that pump water from rivers, there was a general tendency for increased numbers of fish to be entrained per unit time as pumping rate (ML per day) increased. The number of different species entrained per unit time also increased with pumping rate. This trend was consistent across many species, although not statistically significant for most due to high variability in catch rates between different flow events. The steepness of the relationship between pump rate and fish entrained per unit time varied between species. When considering entrainment rates on a volumetric basis, there was still a positive relationship evident, but the upward trend was more gradual than might have been expected. These results suggest that pumping rate does have an impact on entrainment rates, but the effect of pumping rates is probably less important than some other factors such as flow type and intake location and depth.

The current study confirmed that flow type has a significant influence on entrainment rates. In the original study, entrainment rates were found to be very low on overbank (flood) flows. This trend continued with the addition of supplementary data. Low entrainment rates on flood flows are probably partly due to the dilution effect of large volumes of water and partly due to fish inhabiting habitats inundated over the banks and away from the irrigation intakes. When water levels drop back to the main river channel, fish are more vulnerable to entrainment. In the original study it was suggested that natural within bank flows had a tendency for marginally higher entrainment rates than allocated (supplemental) within-bank flows. With the addition of the supplementary data, it is apparent that natural within-bank flows can lead to entrainment rates approximately double that of entrainment rates on allocated (supplemental) flows. This is probably because fish migrations are more likely to be triggered on natural flow events. In general, larvae are entrained at higher rates on natural flow events, although carp gudgeon larvae are entrained more frequently on allocated flows. This probably reflects the fact that species with pelagic larvae tend to spawn on natural flow events.

The current study also confirmed that for most species and size classes of fish (including overall entrainment rates for all species combined), bankside shallow intakes generally entrained far fewer fish than other inlet types (e.g., bankside deep intakes, mid-channel deep intakes and intakes from side channels perpendicular to the river). Inlet location and depth appears to be more important than pump rate.

Based on these findings a prioritisation matrix has been developed to assist with decision making on where investment in mitigation measures (e.g. screening with modern self-cleaning fish screens) should be directed to have the most impact. The matrix is virtually identical to that produced by Hutchison et al. (2022) with some minor changes to one of the factors.

The factors used in the matrix include pumping rate, pump intake position and depth (intake configuration), flow types pumped and annual licensed allocation (total volume licensed to pump of any flow type).The metrics used for three of the four factors remain unchanged from Hutchison et al. (2022), but within the factor flow type, the weighting for natural within-bank flows has been updated to reflect that these flows on average have double the entrainment rate of allocated (supplemental) flows. By cross multiplying the metrics for these different categories, an overall score can be derived for different pumps in a river. The highest scoring pumps will be those predicted to be in the greatest need of mitigation action. However, for mitigation actions, feasibility and cost, based on the specific site characteristics also need to be considered. In some cases, better outcomes for fish per unit cost may be achieved by screening several slightly lower ranked pumps, rather than expending a large amount of resources on a single highly ranked pump that is logistically difficult or expensive to screen. A more cost-effective time to consider screening can be when pumps have reached the end of their useful life and require replacement.

The following recommendations have been derived from this research project.

1.      Data from this project suggests that gravity fed diversions have a high impact, but only one such diversion was monitored. Further investigations into impacts of riverine gravity fed diversions are recommended.

2.      Pumped diversions can be prioritised using a four-part scoring system that considers flow type being pumped, intake configuration (location and depth), pump rate and total volume pumped per annum. Consideration also needs to be given to the costs and feasibility of screening a site as part of the prioritisation process.

3.      Future pumped irrigation developments should consider factoring in screening at the design and construction phase when it will be most cost-effective to install screens, compared to retrofitting them later. When existing pumps reach the end of their useful life, screening of the replacement pump should also be considered because it will be more cost effective.