During ‘extreme dry’ conditions, when lake levels are low, key habitat for threatened freshwater fish in fringing wetlands and channels becomes disconnected. A loss of connectivity at these sites leads to water quality impacts, declining habitat, the threat of total desiccation and isolation of genetic units. As such, remnant refuge sites for key threatened species will be prioritised for the delivery of environmental water. A prioritisation matrix such as that outlined in Department of Environment and Heritage (2010a) will be used. This matrix will compare data collected through The Living Murray small bodied threatened fish condition monitoring and the DENR Drought Action Plan monitoring programs in the Lower Lakes to assess where threatened fish populations remain and which are most critical for protection through interventions such as the delivery of environmental water.
Criteria for prioritisation of refuge sites (DEH 2010a) includes:
• genetic uniqueness of threatened fish population within particular region
• costs required / volume required / community support for the project.
Small annual volumes are likely to be needed (i.e. <100 ML per site per year), but costs associated with water delivery are likely to be high, due to the long distances needed to be pumped to access water. Water needs to be delivered in small volumes on several occasions over the year, to maintain levels during the periods of highest evaporative losses.
Associated actions to do with water delivery to priority refuge during the ‘extreme dry’ scenario may include:
• site set-up at new refuge areas, including the installation of blocking banks, piping infrastructure and gauge boards
• more frequent monitoring (i.e. weekly) of water quality parameters at watered refuge sites
• targeted ecological intervention monitoring to assess if project objectives are being achieved
• organised rescue of threatened fish species at sites where intervention is not possible, as per Hammer et al. (2009)
• captive breeding programs for threatened fish protection for future re-stocking, as per Hammer et al. (2009).
During ‘extreme dry’ conditions, many fringing Lower Lakes wetlands will be disconnected and key wetland habitats will dry out. Along with protecting fish refuge sites, priorities for the delivery of environmental water will also include key fringing wetlands that support diverse aquatic vegetation and water bird communities. By delivering environmental water to key fringing wetlands and maintaining water levels for a period of 3–4 months over spring/summer, the seed/egg bank of the wetland will be stimulated to germinate/hatch, and provide aquatic plants and invertebrates for the use of aquatic species such as waterbirds and frogs. In order to prioritise which fringing Lower Lakes wetlands will receive environmental water, a number of criteria will be used to create a short-list. These include:
• the ability of the site to retain water
• whether the wetland has previously been a known hot-spot for submerged aquatic plants or waterbirds (i.e. from River Murray Wetlands Baseline Survey Project (SKM 2004; 2006) or past condition monitoring programs)
• community/landholder support for the project
• whether the wetland is within 1 km of lake water of depth ≥0.5 m (i.e. water is able to be pumped into the site).
Note that this prioritisation exercise has already been completed for the Lower Lakes fringing wetlands during the 2010–11 water bid.
Small volumes are likely to be needed for these activities, i.e. in the range of 200 – 500 ML/year per site. Water delivery will involve an initial fill-volume, and then a subsequent ‘top-up’ volume to compensate for evaporation and seepage; therefore at least two pumping events are required.
Associated actions with water delivery to key wetland habitats during the ‘extreme dry’ scenario may include:
• minor earthworks such as bank construction to ensure wetlands retain water
• targeted protection of submerged aquatic plants from waterbird grazing (if necessary)
• more frequent monitoring (i.e. weekly) of surface water level and quality
• targeted ecological intervention monitoring to assess if project objectives are being met.
In addition to small volumes for priority drought refuges and protecting key wetland habitats, small volumes should also be released through the barrage fishways for up to two months to enable a connection with the Lakes and Coorong. This can only be achieved if Lake Alexandrina is ≥0.5 m AHD, above Coorong water levels. Operating the fishways during the periods key to life stages of diadromous fish such as congolli and common galaxias will enable connectivity and the movement between the Lower Lakes and Coorong, and vice versa for recruitment, habitat selection and feeding. By opening the fishway in July/August, adult female congolli will migrate downstream to spawn. Opening the fishway again in December/January will allow recruits from the winter spawning cycle to move into the lakes to complete their lifecycle. Limited operation of fishways not only provides benefits to fish, but also creates localised estuarine conditions which are critical for sustaining a range of biota in the Coorong.
If the barrage fishways are unable to be used due to lake levels remaining <0.5 m AHD during the year, the Goolwa boat lock may be used as an alternative to allow fish passage between Lake Alexandrina and the estuary.
Environmental water may also be requested to maintain lake levels at >0.0 m AHD (if required), a critical threshold for maintaining connectivity between Lakes Alexandrina and Albert, and for preventing acidification.