Responses of fish assemblages to flow variability in the Gwydir Wetlands ecosystem, 2008-2009: Final Report to the New South Wales Department of Environment, Climate Change and Water

Abstract

Monitoring of fish assemblages was undertaken across three lower Gwydir channels in May-June and August 2009, in order to establish assemblage structure patterns during a low-flow period. This work was funded by the Australian Government's Water for the Future program, through the NSW Rivers and Environmental Restoration Program SPII whose overarching objective is to guide the 'Better use of environmental water for maintenance of the ecological character for stressed wetlands'. The sites and the monitoring (fyke-netting) protocols mirrored those utilised in earlier (2006 to 2009) research. Assemblage structure in mid 2009 reflected longitudinal patterns between sites within each channel, including more species and abundances of less-common upstream. However, assemblage structure was generally more homogeneous than during spring-summer periods. For example, differences between the three channels were less clear than those established from longer-term monitoring, and separation of May-June from August samples was only evident for one of the channels (Lower Gwydir River). Dominant species were either present mid year as just mature fish (e.g. Australian smelt, 'Retropinna semoni') or with only limited numbers of juveniles, suggesting that annual recruitment largely occurs during spring-summer. Additional data from December 2008 and February and September 2009 were added to the mid-year data in order to explore preliminary relationships between fish abundance and prior discharge. Relationships were variable, both between sites and when differing discharge intervals (30, 60, 90, 120 days) were selected. Relationships were more commonly inverse, suggesting either the importance of low-flow periods for sustaining lower Gwydir fish populations or that periods of higher flow may be flushing more fish downstream of study sites. These findings, and those from other recent descriptions of lower Gwydir fish assemblages, allow recommendations for ongoing monitoring of these channels, including the importance of determining longitudinal patterns in assemblage structure, monitoring sites mid-year as well as before/after discrete flow events, and ageing early lifehistory stages to develop links between spawning activity and individual flow pulses. Recommendations for environmental flow management in lower Gwydir channels include limiting releases to spring-summer, establishing a greater stage height and variability of environmental flow releases, and taking a cautious approach when making any future adaptive management changes to environmental flow releases given the narrow set of ecological objectives associated with past releases.