Spatial and temporal patterns in fish assemblages following an artificially extended floodplain inundation event, northern Murray-Darling Basin, Australia.

Water extraction from dryland rivers is often associated with declines in the health of river and floodplain ecosystems due to reduced flooding frequency and extent of floodplain inundation. Following moderate flooding in early 2008 in the Narran River, Murray-Darling Basin, Australia, 10,423 ML of water was purchased from agricultural water users and delivered to the river to prolong inundation of its terminal lake system to improve the recruitment success of colonial waterbirds that had started breeding in response to the initial flooding. This study examined the spatial and temporal patterns of fish assemblages in river and floodplain habitats over eight months following flooding to assess the possible ecological benefits of flood extension. Although the abundances of most fish species were greater in river channel habitats, the fish assemblage used floodplain habitats when inundated. Young-of-the-year (4-12 months age) golden perch (Macquaria ambigua) and bony bream (Nematalosa erebi) were consistently sampled in floodplain sites when inundated, suggesting that the floodplain provides rearing habitat for these species. Significant differences in the abundances of fish populations between reaches upstream and downstream of a weir in the main river channel indicates that the effectiveness of the environmental water release was limited by restricted connectivity within the broader catchment. Although the seasonal timing of flood extension may have coincided with sub-optimal primary production, the use of the environmental water purchase is likely to have promoted recruitment of fish populations by providing greater access to floodplain nursery habitats, thereby improving the ability to persist during years of little or no flow.