Aquatic fauna survey of Thegoa Lagoon October 2004

"October 2004".

Project Number: Aquatic fauna survey of Thegoa Lagoon - M/BUS/69-1.

MDFRC item.

A baseline survey of water quality and aquatic fauna was conducted at four sites within Thegoa Lagoon on 6-8 October 2004. The pH in Thegoa Lagoon varied between sites within the lagoon, with the range of pH values recorded similar to those recorded during a 1998 MDFRC aquatic survey of Thegoa Lagoon (Plummer et al. 2000). Electrical conductivity (EC) increase with distance from the inlet channel (located at the eastern extremity of the lagoon), as surface salts were mobilised and accumulated in the inflowing water. Inflow of fresh water from the Murray River through the inlet channel appeared to dilute EC in the eastern end of the lagoon. EC measurements taken in the eastern end of the lagoon were lower, and those in the western end of the lagoon were comparable, to those recorded during 1998/99 (Plummer et al. 2000). Dissolved oxygen was significantly lower at Site 3 than the other three sites due to a dense covering of the floating macrophytes Azolla filiculoides, which restricted light penetration and photosynthetic production of oxygen within the water column, and atmospheric diffusion. Dissolved oxygen concentrations recorded during the 2004 survey of Thegoa Lagoon were considerably lower than recordings made in 1998. Turbidity was significantly higher in the eastern arm of the lagoon (near the inflow channel) then other sites in the lagoon, possibly due to inflow water scouring and resuspending sediments in the inflow channel. All of the native and introduced fish species recorded in this survey are considered to be common and widespread within the lower Murray Darling River system (McDowell, 1996). Australian Smelt, Carp Gudgeon complex and Fly Specked Hardyhead were the most abundant native species sampled, while Gambusia were the most abundant introduced species in the wetland. Two native species captured in the 2004 survey (Flathead Gudgeon and Dwarf Flathead Gudgeon) were not captured in previous MDFRC aquatic surveys of Thegoa Lagoon. Only one Bony Bream was recorded, while Golden Perch and Crimson-spotted Rainbowfish were absent from the 2004 survey (all three species were recorded in the 1998/99 survey). None of the fish species captured are listed as endangered, rare or vulnerable. However, the absence of large bodied native fish such as Bony Bream and Golden Perch was notable. This could be a consequence the method of re-filling via the eastern regulator given that historically, during high flows in the Murray and/or Darling Rivers, the lagoon filled from both the eastern and western ends. Bony Bream have been shown to avoid dark tunnels, such as the regulator pipe on the inlet to Thegoa Lagoon. The absence of Bony Bream and Golden Perch could also be related or the timing of the re-filling (Golden Perch and Bony Bream may not have been moving when the Lagoon re-filled). Alternatively, considering the general shallowness of the lagoon, a lack of suitable habitat may limit re-colonisation of the wetland by these larger species. It is also possible that although a range of sampling equipment was utilised in this survey, small numbers of each species could be extant in Thegoa Lagoon, yet by chance avoided capture. The exotic species Redfin was absent in the 2004 survey, despite the capture of three individuals in the 1998/99 survey. The authors wish to note however that a single small Redfin specimen was captured by MDFRC staff in a bait trap deployed at Thegoa Lagoon in November 2004, four weeks after the actual survey. As there had been no inflows to the lagoon subsequent to the October 2004 survey, the capture of this specimen indicted that Redfin did re-enter the Lagoon during the 2003/04 refilling phases, albeit in small numbers. The capture of large Carp in the 2004 survey demonstrated that although the dry phase had been effective in eliminating the pest species from Thegoa Lagoon, the re-filling via the eastern regulator did not prevent the re-entry of Carp. Although fewer Carp were captured in the 2004 survey (11) than previous surveys in 199/99 (25 in November 1998, and 27 in June 1999), the observation that adult fish were 'running ripe' indicates Carp numbers within Thegoa Lagoon may increase over the coming months.