Flow magnitude and variability influence growth of two freshwater fish species in a large regulated floodplain river

Fish are often targets for environmental watering outcomes under the premise that aspects of the flow regime are linked to key components of their life-history. This study examined the conceptual link between variability in river discharge and fish productivity by measuring annual growth patterns (generated using sclerochronology over a 22-year period) of two native freshwater cod Maccullochella spp. species over a range of flow conditions in a regulated Australian floodplain River. We found a positive relationship between fish growth, flow variability and river discharge. Flow variability during spring and summer-autumn, as well as their antecedent values, was particularly important in explaining annual growth of the nationally endangered Maccullochella macquariensis. Growth of Maccullochella peelii displayed similar patterns, though were more closely aligned with spring discharge. These results are consistent with the general view that increased river regulation, due to its suppression of flow magnitude and variability, has been a major contributing factor in the decline of native fish populations throughout the world. Our results provide support and guidance for the use of environmental water delivery, and have broad application to rivers worldwide for which any quantification of ecological impacts of regulation, and responses to water management remain scarce.