Regulated floodplains - A trap for unwary fish

Loss of river-floodplain (lateral) connectivity has impacted on fish communities and fisheries around the world. However, evidence of the impacts of reduced lateral connectivity on Australian native fish remains scant. To document these impacts, isolated pools located immediately downstream of two major regulators (or weirs) that control flows from the Murray River to an extensive off-stream floodplain, were pumped out five times between 2001 and 2006. A total of 20980 fish (16228 native, 4752 non-native) comprising 15 species (five non-native) were collected. Five fish species collected are classified as threatened, while turtles and macroinvertebrates were also present. Regulator operation appeared to affect the number of fish becoming stranded, with the highest number coinciding with longer regulator openings and extended floodplain inundation. Hundreds of floodplain regulators exist on Australian rivers, and all have the potential to impact upon native fish. Trial of several potential management and engineering solutions is recommended to improve river-floodplain connectivity in lowland rivers.