Aquatic fauna and flora surveys at the Lennox Weir, Busselton

S. Beatty; R. Paice; K. Hastings; V. Summer; L. Ma; D. Morgan

Water Corporation proposes to undertake infrastructure works on the Lennox Weir, near Busselton, Western Australia. Due to the potential impact on aquatic fauna and flora associated with the proposed works, the Water Corporation engaged the Harry Butler Institute (Murdoch University) to carry out a desktop review and field survey for flora and fauna upstream and downstream of the structure. The study aimed to provide an assessment of the likely impacts of the proposed works on prevailing native species, and develop recommendations to mitigate any potential identified impacts. A desktop assessment of the fauna and flora communities around Lennox Weir was initially conducted. Subsequently, standardised surveys were conducted for flora and fauna at Lennox Weir in August 2019, again in October 2019 for flora, and finally in January (into early February) 2020 for flora and fauna. The proposed replacement or removal of the Lennox Weir was found to have a moderate to high risk of localised impacts on freshwater fishes upstream; based on the four storm surge scenarios modelled by GHD (2018a). Contrary to recent impact assessment by GHD (2018b), high abundances of native fishes were found to be present upstream of the weir in summer, which could be impacted by sudden increases in salinity predicted by the storm surge modelling. The upstream spatial extent of this impact would depend on the storm surge scenario. However, under all scenarios freshwater fishes are likely to be impacted (including potential high levels of mortality) in zone 2 (from the weir to 500m upstream) and a proportion of zone 3 (between 500‐ 1100m upstream). However, while the known acute salinity tolerances of resident native fishes would be exceeded in those sections, the behavioural responses of the fishes may help to partially or totally) mitigate the impacts. This could include moving upstream away from the incoming saline water, or more likely, utilising a freshwater lens that may be present. However, the hydrological modelling by GHD (2018a) did not consider or model the potential for a freshwater surface layer to form. Additional modelling would be required to better predict whether these factors could occur to mitigate the impacts on freshwater fishes. The assessment also revealed that the Threatened (EPBC Act 1999) Carter’s Freshwater Mussel Westralunio carteri was present in the Lennox River from just upstream of the weir to at least the Vasse‐Yallingup Siding Rd. However, its relative abundance increased significantly with distance upstream of the weir and the lowest abundances were found in zones 2 and 3; the sites predicted to be most affected by increased salinity under the modelled storm surge scenarios. As concluded by the previous impact assessment (GHD 2018b), the risk to the species of the increase in salinity would be high to moderate in zones 2 and 3 with residual salinity after 24 hours under all four storm surge scenarios projected to exceed its known tolerances in zone 2 and a proportion of zone 3. However, there remains uncertainty of this impact owing to the fact that modelling of residual salinity was only undertaken for a 24 hour period. Previous acute salinity trials of the species revealed the initial deaths occurred after 10 days at 8 mg/l, highest salinity level tested. The impact on the species in terms of mortality would therefore likely depend on the longer‐term residual salinity levels following storm surges. The impact on the South‐west Snake Neck Turtle Chelodina colliei upstream of the Lennox Weir was assessed as low in the current study. While there may be a short‐term decline in abundances of salt‐intolerant freshwater fishes and invertebrates associated with the modelled storm surges, there were high abundances of alternative prey items present particularly the Blue‐spot Goby Pseodogobius olorum and South‐west Glass Shrimp Palaemon australis. While the Smooth Marron Cherax cainii was not detected during the current sampling in the impacted reach (only visually observed upstream at the Vasse‐Yallingup Siding Rd), additional anecdotal information from the landholder suggested they may be present in low abundances within zone 3. If present, those individuals would have a high risk of impact under all scenarios and would likely walk out of the river to avoid the salinity. The Lennox Weir survey area occurs within a Conservation Category Wetland area. Upstream and downstream vegetation differed in terms of community structure and condition. The upstream vegetation is an example of very old remnant riparian vegetation and included poorly represented community types. The vegetation provides habitat for the Critically Endangered Western Ringtail Possum, with extensive scat noted throughout the survey area. The P4 species Eucalyptus rudis subsp. cratyantha occurs in riparian vegetation upstream and downstream and on adjacent land north‐east of the weir. Changes to the salinity regime associated with altering the function of the weir are likely to impact this vegetation.

Aquatic fauna and flora surveys at the Lennox Weir, Busselton

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