INTRODUCTION

Historically, short-finned eels (Anguilla australis) have been observed by the local community and Barwon Water operational staff in West Barwon Reservoir and the Barwon River immediately downstream. This includes observations of adult short-finned eels migrating out of the reservoir and downstream when it spills (Figure 1). Recent research in Lake Condah in western Victoria has also shown that adult short-finned eels migrate out of the lake and downstream when the lake spills (Koster et al. 2020). However, during recent reservoir spilling in winter/spring 2021, no short-finned eels were observed migrating out of the reservoir (Jayden Woolley, pers. comm.). Consequently, there is increased interest from the local community on the status of short-finned eel and other migratory fish populations in the reservoir.

Information on the status of fish populations in West Barwon Reservoir is limited, although previous studies in the upper Barwon catchment have recorded the nationally threatened Australian grayling (Prototroctes maraena), as well as short-finned eel, river blackfish (Gadopsis marmoratus), southern pygmy perch (Nannoperca australis), Australian smelt (Retropinna semoni) and galaxiids (Galaxias spp.) (Raadik 2000; Environous 2008; Alluvium 2021). The system retains some submerged aquatic vegetation, undercut banks, overhanging vegetation and riffle-pool sequences, which provide important habitat for fish and other aquatic animals (Hayley Vinden, pers. comm.). 

Objectives

The aim of this project is to conduct a survey to collect baseline data on short-finned eel and other fish populations in the West Barwon Reservoir. The information collected is a first step to inform the potential effect of the reservoir on migratory fish species, with a specific focus on short-finned eel populations throughout the system. More specifically:

• Is the dam likely to prevent eels from accessing the river upstream of the reservoir?
• Is the dam likely to be having any implications to eel populations below it in the Barwon River?
• Is the dam likely to prevent eels from migrating downstream out of the reservoir?

It is important to note that the surveys were conducted over only a few days and therefore provide only a snapshot of the species that may be present in the reservoir. More broadly, a lack of monitoring data to determine the status of, or trends in, eel populations throughout Victoria also precludes a comparison of eel populations to other systems in the State.

CONCLUSIONS AND RECOMMENDATIONS

• Improved fish passage past West Barwon Reservoir would increase opportunities for short-finned eel and migratory fish species to access habitats in the upper reaches of the system. Currently, upstream access to the 51 square kilometre catchment on the West Barwon River is impeded. Notwithstanding, the provision of fish passage over the reservoir is considered difficult (Alluvium 2021).

• Furthermore, other fish passage barriers in the catchment are considered higher priority for improving fish passage, particularly those in the lower reaches (e.g. Barwon Breakwater, Buckley’s Falls, Barwon barrages) because they restrict access to a greater amount of habitat for migratory fishes such as short-finned eel migrating upstream from the sea (Marsden et al. 2016).

• The CCMA has recently installed fish passage infrastructure at the lower Barwon at two key sites, the Lower Barwon Tidal Barrage and at the Reedy Lake Outlet regulator. This includes a cone fishway, Vertical Slot Fishway and elver ramps at the Barrage and a Vertical Slot Fishway and Elver Ramp at the Reedy Lake Outlet. Other barriers in the Lower Barwon that require further mitigation when funding is available includes numerous regulators and infrastructure associated with the lower Barwon Wetlands and Buckley’s Falls/Baum Weir (Denis Lovric, pers. comm.).

• More broadly, crucial gaps in our knowledge of the life history of eels that limit our ability to develop targeted conservation and management strategies need to be addressed (Righton et al. 2021).

• A critical life history phase of eels is the downstream migration of adults from freshwater into the sea to spawn. Environmental flows to stimulate adult eels to migrate downstream have been recently incorporated into streamflow management plans for coastal rivers such as the Barwon-Moorabool river systems. However, the linkages between river flow, including environmental watering events, and migration of adult eels have not been comprehensively tested or validated. Establishing an acoustic tracking program in the Barwon-Mooraboolriver system to investigate migration would help to provide evidence to validate and support refinement and implementation of environmental flows to assist the management and conservation of eel populations.

• The life history of eels is also characterised by extensive migrations into freshwater/estuaries from the sea by juveniles. The strength and success of these migrations determines the status and sustainability of adult eel stocks in freshwater into the future. Establishing a long-term monitoring program for the Barwon-Moorabool River system to determine annual recruitment trends of juvenile eels would allow for long-term analysis of trends and status of juvenile eel stocks and associated environmental drivers. This information is needed, for example, to improve our understanding of the effects of flow on juvenile migration, and to ensure environmental water entitlements are used to achieve the best environmental outcome with the water that is available.

• A long-term monitoring program of migratory fish populations in the Barwon-Moorabool river system would also help to fill knowledge gaps and develop targeted conservation and management strategies.