Chowilla icon site fish assemblage condition monitoring 2018.
The Chowilla Anabranch and Floodplain system (hereafter Chowilla) comprises the largest remaining area of undeveloped floodplain habitat in the lower River Murray. It encompasses a series of anabranching creeks, backwaters, wetlands and terminal lakes that bypass Lock and Weir No.6 (hereafter Lock 6) on the River Murray. Chowilla is part of the Riverland Ramsar site and listed in the directory of important wetlands in Australia as Wetland of International Importance for nationally threatened species, habitats and communities (Environment Australia 2001). Chowilla is also recognised as an Icon Site under the Murray-Darling Basin Authority's (MDBA) The Living Murray Program (TLM) (MDBA 2016). To assist with monitoring of Ecological Objectives 10 and 11, quantitative fish surveys have been undertaken annually in the Chowilla system since 2005. Sites have been selected to represent all aquatic mesohabitats present within the Chowilla region (i.e., fast-flowing and slow-flowing creeks, backwaters and the River Murray main channel). This paper reports the results of the project. Due to variation in sampling efficiency during high water levels in 2011, data from this year were excluded from quantitative analyses. Since 2013, additional targeted surveys have been conducted for Murray cod (Maccullochella peelii). In 2018, a total of 16 724 fish from 14 species were sampled from 21 sites within Chowilla and the adjacent River Murray main channel. The fish assemblage consisted of 10 native and 4 non-native species, with bony herring (67%) (Nematalosa erebi), Australian smelt (14%) (Retropinna semoni), unspecked hardyhead (9%) (Craterocephalus stercusmuscarum fulvus) and Murray (Melanotaenia fluviatilis) (3%) the most abundant native species. Whilst non-native species common carp (Cyprinus carpio), goldfish (Carassius auratus), eastern gambusia (Gambusia holbrooki), and redfin perch (Perca fluviatilis) compromised 5% of the total catch. The fish assemblage in 2018 was similar to that sampled in the region during previous low flow years, with generally higher numbers of small- to medium-bodied native species (e.g. bony herring and Australia smelt) and low abundances of non-native fishes (e.g. common carp and goldfish).