The total number and condition of fish extracted via high and low flow irrigation pumps was assessed over a 2-year period in the Namoi River, Australia. A combination of boat electric fishing and fyke netting was used for 10 weeks during the peak irrigation period to determine species and size classes susceptible to entrainment during water abstraction. Over 2300 fish passed through the pump outlets over the study period, with many individuals (7.5% of total) both killed and injured. The maximum number of fish entrained in a single day was 232. Mortality was significantly higher from the high volume pump site, but only large (>200 mm long) or small (<50 mm long) fish were killed. Medium-sized fish (50-200 mm long) largely survived the abstraction process; although 70 were injured (3% of total), only one was killed (0.07%). The Electric fishing surveys showed that only four species were present in storage dams, suggesting that survival through the pump systems may be size and species specific. Fish that survived the water abstraction process had no opportunities to return to the main river system and were effectively classified as lost from the main river population. The development of suitable mitigation measures, including operational changes and screening, are suggested as mechanisms to prevent extraction and minimise any adverse impacts arising from irrigation development.

Determining factors responsible for increases in the mortality of freshwater fish larvae are important for the conservation of recruitment processes and for the long-term sustainability of freshwater fish populations. To assess the impact of one such process, Murray cod (Maccullochella peelii peelii Mitchell) and golden perch (Macquaria ambigua Richardson) larvae were arranged into treatment and control groups and passed through different configurations (overshot and undershot) of a low-level weir. Passage through an undershot weir resulted in the death of 95 ± 1% golden perch and 52 ± 13% Murray cod. By comparison, mortality was significantly lower in the overshot treatment and both controls. The relatively large number of undershot weirs within the known distribution of these species could impact upon recruitment over a large scale. It is therefore recommended that water management authorities consider the potential threats of operating undershot gated weirs on the survival of larval fish until further research determines appropriate mitigatory measures for these and other species.