An elongate, small to medium sized, slender fish with an almost cylindrical body. Maximum length ~350 mm; sexually dimorphic, females 150–300 mm, males <150 mm. The head is flattened on top and the eyes are small and set high on the head. The mouth is large, extending to below the front of the pupil, and the lower jaw protrudes. The first dorsal fin is short-based and rounded, and the second long-based and straight-edged. The caudal fin is truncate to rounded, and the anal fin is long-based and straight-edged, slightly longer than the second dorsal, and almost opposite in position.
Body colour varies with habitat but is mostly light brown on the back and yellowish-white ventrally, with a series of irregular dark brown blotches (or occasionally a solid dark mid-lateral band) on the sides.
Congolli is an unusual species in that there is sexual differentiation in migratory behaviour. Females are truly diadromous (move between freshwaters and marine for the purpose of reproduction), with adults residing in freshwater habitats of coastal rivers, but males are essentially estuarine/marine residents that only occasionally enter the lower freshwater reaches of rivers. In the Lower Murray the species occurs in terminal wetlands and a few lowland stream habitats where it is often found partially buried in leaf litter or sand, or associated with cover such as logs, rocks or overhanging banks.
Sexual maturity in females is reached at ~ 150 mm total length, at 3-4 years of age with spawning occurring in winter. Acoustic telemetry studies in coastal Vic and the lower River Murray SA, found that outside of the spawning season, adult females occupy restricted home ranges before undertaking rapid downstream spawning migrations from freshwater habitats, through estuaries and then out to sea between May and August. Female downstream migration is cued by time of year (June–July) and approximately full moon phases. In both coastal Vic and the Lower Murray no tagged female fish were recorded re-entering their original river after spawning, suggesting they may only spawn once and then die.
Nothing is known of male movement patterns, although spawning movements from estuarine to marine habitats are likely, given known female movement patterns. Spawning generally occurs from mid-July to late October, with young-of-year (20–70 mm length and ~120 days of age) then migrating from the Coorong upstream to freshwater habitats in late spring and summer.
The Congolli is predominantly an opportunistic benthic carnivore. It can also behave as an ambush predator, burying itself in the substrate and taking small fish. In the lower River Murray diet varies with age and between estuarine and freshwater habitats. Crustaceans form the bulk of the diet, with amphipods dominant in estuarine habitats and shrimps in freshwater habitats. Polychaetes are also consumed in estuarine habitats and aquatic insects in freshwater habitats, with fishes comprising an important dietary component for larger individuals (>80 mm) in both habitats. The diet in Tasmanian streams is similar comprising mostly small prey items such as aquatic insect larvae, (chironomids, caddisflies, mayflies), small crustaceans, snails and worms. Some plant material is also consumed. Congolli were recorded in the diet of 19% of medium Mulloway (400–700 mm length) in the Coorong.
The Congolli is found predominantly in coastal rivers in Tas, Vic, SA, and southern NSW. In the Murray-Darling Basin it only occurs in the Lower Murray drainage, where it has been recorded in the Murray River as far upstream as Echuca but is most common in streams of the Mt Lofty Ranges and the Lower Lakes (Alexandrina and Albert). Prior to construction of the Murray Barrages (1940s), it formed a notable seasonal component of the commercial fishery in the Lower Lakes, suggesting initial barrage construction impacted the species.
During the latter years of the Millennium Drought, the closure of the tidal barrages in the Lower Murray for a period of 1280 consecutive days (to limit saltwater incursion and retain freshwater for critical human needs) blocked the downstream migration of females to marine spawning habitats with an associated >90% reduction in the abundance of subsequent young-of-year upstream migrants. Following downstream fish passage mitigation, the breaking of the drought, and resumption of freshwater outflows through the Coorong, numbers have significantly improved in the lower river. No individuals were recorded from the Sustainable Rivers Audit (2004–2013) or the MDB Fish Survey (2014/15–2021/22).
Barriers to movement impede both downstream adult spawning migrations, and upstream young-of-year migrations. River regulation may also disrupt spawning migration cues.
Allen et al. 2002; Andrews 1996; Bice et al. 2012, 2016a, 2017, 2018, 2020a; Crook et al. 2010; Giatas & Ye 2015; Hortle & White 1980; Kuiter 1993; Lloyd & Walker 1986; Raadik 2008a; Scott et al. 1974; Wedderburn et al. 2014; Zampatti et al. 2010, 2011.
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