The sustainability of fisheries resources is dependent on the diversity, complexity, availability and connectivity of habitats to support the survival, growth and reproduction of fish populations. The Curdies Estuary, in south-west Victoria, is a popular fishing destination containing key recreational fisheries, despite its environmental condition being very poor and a history of habitat loss and degradation. In the early to mid-1900s, instream woody habitat (IWH) was removed from the Curdies Inlet upstream towards Timboon to facilitate boat navigation and support water carriage of farming resources and passengers. Removal of IWH and damage to native riparian vegetation are now listed as potentially threatening processes in the Victorian Flora and Fauna Guarantee Act 1988. Waterway management activities, including installation of IWH and replanting riverbanks, are now actively carried out to improve waterway health and habitats that support production of fish resources.
In this study, we mapped and assessed current IWH distribution and density in the Curdies Estuary to inform reinstatement of IWH. The spatial location of IWH masses and their size and complexity, water depth, boat ramps and jetties were surveyed along 19.3 km from the Curdies Estuary mouth to upstream of Curdievale. This data was used to produce an IWH density map and an interpolated water depth map for the study reach.
IWH were generally small and isolated in volume and distribution in the Curdies Estuary. The Curdies Inlet, the reach between the Curdies Inlet and Curdievale, and the reach upstream of Curdievale had no IWH to low densities of IWH. The most upstream 3 km of the study reach had relatively higher densities of IWH. Riparian vegetation was intact in this section and likely contributed to the IWH inputs observed. Seagrass and rocky outcrops were observed in the Curdies Estuary and these also provide important instream habitats for fish. Water depth varied across the study reach, but was relatively deeper in the reach above Curdies Inlet and just upstream of Curdievale. Curdies Inlet was consistently shallow. The Curdies Estuary had lower average IWH density compared to average densities recorded in other Victorian estuaries that have also been desnagged.
Identifying the locations for IWH installation can be informed by combining the IWH density and water depth data. Deeper areas with low IWH densities could be targeted to provide refuge areas for fish during low tide or low water flows. Installing IWH in sections upstream and downstream of Curdievale would increase IWH available for fish populations in the Curdies Estuary, as well as improve ecological connectivity between the upper estuary and Curdies Inlet. Rather than installing IWH in the Curdies Inlet, restoring and expanding seagrass beds could be considered.
Rehabilitating IWH in the Curdies Estuary will improve the availability and connectivity of habitat for fish species that occupy or utilise the estuary, including key recreational fish, such as Estuary Perch and Black Bream. We predict that increasing IWH densities will benefit fish communities and the fishery, and provide overarching ecosystem advantages; however our knowledge of how fish populations utilise and respond to IWH installation is limited. Therefore, monitoring is recommended to inform future actions in an adaptive management framework.
Other factors, such as permits and approvals, water flows, riparian vegetation condition, land tenure, infrastructure (such as boat ramps and jetties), and waterway use and user groups, should also be taken into account when making decisions on IWH installation.