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Abstract

Chowilla icon site fish assemblage condition monitoring 2020.

Abstract

The Chowilla Anabranch and Floodplain system is the largest remaining area of undeveloped floodplain habitat in the lower River Murray. Chowilla consists of a range of aquatic habitats that are now rare in the region, including permanently flowing creeks, and these habitats support a diverse native fish community. The Chowilla Floodplain, however, has become increasingly degraded as a consequence of changes to the natural flow regime, grazing and an extended period (2001-2010) of low flows in the Murray-Darling Basin. In order to 'enhance and restore' the environmental values of the Chowilla Floodplain, an Asset Environmental Management Plan was developed as part of the Chowilla Integrated Natural Resource Management Project. Subsequently, a Chowilla Floodplain Environmental Water Management Plan (MDBA 2012) has been developed with refined ecological objectives as follows: * Ecological Objective 10: Maintain or increase the diversity and extent of distribution of native fish species. * Ecological Objective 11: Maintain successful recruitment of small- and largebodied native fish. 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 region (i.e. fastflowing and slow-flowing creeks, backwaters and the River Murray main channel). 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 2020, a total of 25,925 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 (87.2% of total catch) (Nematalosa erebi), Australian smelt (4.5%) (Retropinna semoni), unspecked hardyhead (3.5%) (Craterocephalus fulvus) and Murray rainbowfish (Melanotaenia fluviatilis) (1.1%) the most abundant native species. Of the non-native species, common carp (1.6%) (Cyprinus carpio) and eastern gambusia (0.4%) (Gambusia holbrooki) were the most abundant, whilst goldfish (Carassius auratus) and redfin perch (Perca fluviatilis) collectively comprised 0.48% of the total catch. The fish assemblage in 2020 was similar to that sampled during previous low flow years, with generally high numbers of small- to medium-bodied native species (e.g. bony herring) and low abundances of non-native fishes (e.g. common carp and goldfish). Data from 2005-2020 indicate that Objective 10 and 11 of the Icon Site management plan are being met. Over the 16-year sampling period, species diversity in each mesohabitat and for each year was similar, whilst the extent of most species throughout the available aquatic mesohabitats either increased or was maintained. Most species were widespread throughout the available aquatic mesohabitats, although some species were specific to one or more mesohabitats. The native species Murray cod, golden perch (Macquaria ambigua), silver perch (Bidyanus bidyanus), Australian smelt and freshwater catfish (Tandanus tandanus) characterised fastflowing mesohabitats and the non-native species, goldfish, characterised backwaters. Recruitment indices indicate that small- to medium-bodied native species - Murray rainbowfish, Australian smelt, and bony herring - successfully recruited in 2020, whilst unspecked hardyhead recruitment was limited. From 2015-2020, with the exception of 2017, recruitment of Murray cod to reproductive maturity (400-600 mm TL) has been less than the reference value, suggesting a decline in Murray cod recruitment to maturity. In 2020, however, recruitment to young-of-year (YOY) fish (<200 mm TL) was similar to 2018, the highest since monitoring began in 2005. In 2020, golden perch recruitment to YOY was not evident, likely due to the absence of hydrological conditions required to stimulate spawning. Recruitment of the nonnative common carp was also low, again reflective of low flows and an absence of floodplain inundation in 2019/20.