Abundance trends in floodplain fish larvae: the role of annual flow characteristics in the absence of overbank flooding

Temporal variability in flow is central to the functioning of river ecosystems, with current conceptual models emphasising the importance of flow events, namely overbank flood pulses and in-channel flow pulses, in enhancing riverine fish production. However, whilst the benefits of flood pulses have been widely documented, there is an overall dearth of information with regard to the role of flow pulses in enhancing fish spawning and recruitment. To test the validity of applicable conceptual models of floodplain fish production in response to alternating low-flow and flow-pulse years in the absence of a flood pulse, fish larvae were sampled every three to five weeks from 2002 to 2007 in an anabranch system of the highly-regulated and semi-arid lower Murray River (southeastern Australia). Small-bodied native fish (mainly, un-specked hardyhead, carp gudgeon, flathead gudgeon and Australian smelt) spawned successfully in every year irrespective of hydrological conditions, with Australian smelt being particularly abundant in two of the three low-flow years following a flow-pulse event. However, spawning in the majority of these small-bodied species was enhanced by the two flow pulses. On the contrary, large-bodied silver perch and golden perch only spawned during one of the two spring flow pulses, emphasising the importance of both timing and duration of a flow pulse along with its coupling with temperature. The findings of the present study support recent views that a combination of conceptual models of floodplain fish production is likely to apply in temperate to semi-arid floodplain rivers. Management measures aimed at the benefit of native fish communities should account for both flow and flood pulses through the release of environmental water, and this should be supported by long-term studies able to span the components of a river's flow history.