A pale olive-green or brown to almost black fish, often with a diffuse marbled pattern. Maximum size of the northern form is about 350 mm, but it is commonly 200–250 mm and about 100 g. The pelvic fins are reduced to a pair of fine, white, divided filaments located under the throat. The dorsal fin is low and long, reaching almost to the tail. The mouth is large. The dorsal fin has 6–13 spines. The body is covered in very small scales with a thick mucous coating. The northern form is readily distinguished from the Two-spined blackfish by its possession of more than three spines in the dorsal fin.
There is a long-recognised undescribed species complex within Gadopsis marmorata consisting of two species (northern and southern) which differ mainly in maximum size attained. Only the northern form occurs in the MDB. The northern or MDB form of Gadopsis marmorata has recently been further subdivided with 2 taxa (species) recognised, one of which occurs only in the Wimmera River within the MDB (and the coastal draining Glenelg and Bool Lagoon drainages), with all northern blackfish outside the Wimmera drainage in the MDB belonging to the other taxon. The ecology of the Wimmera River taxon is assumed to be similar to the more broadly distributed form, and the two taxa are considered together in this species account.
The Northern river blackfish is found in a diverse range of stream types, from upland and lowland small creeks to large rivers. It prefers habitats with perennial flow and good instream cover such as woody debris, aquatic vegetation or boulders. Most aspects of its ecology are similar to that of the Two-spined blackfish, which often replaces this species in montane habitats.
Spawning occurs from October–January when water temperatures exceed ~16°C. There is no evidence of a spawning migration in this species. The spawning site is usually inside hollow logs, although rocks and undercut banks may also be used. Fecundity is low (generally ~200–500 eggs) and increases with fish length. The eggs are large (~4 mm diameter), demersal, adhesive and hatch after 14 days at 15°C with the larvae about 6–8 mm long. The male guards and fans the eggs and rarely leaves the spawning site. The larvae remain at the spawning site for about 3 weeks after hatching.
An opportunistic carnivore, the Northern river blackfish consumes aquatic insect larvae, crustaceans, terrestrial insects that fall on the water surface, and occasionally other fish.
Its movements are restricted—home range is estimated at 10–26 m. It is benthic and nocturnal. However, in a highly modified, small rural stream, it has been shown to use different habitats between day and night: undercut banks during the day, and open water at night.
The Northern river blackfish is known from the Murray and the mid to upper reaches of the Murrumbidgee, Macquarie, Lachlan, Gwydir and Namoi drainages in NSW. Predominantly a species of the lowlands and slopes, it is found in the montane zone of the Gwydir and Namoi valleys. It is locally common in the upland reaches of the Condamine and Border Rivers drainages of Qld, the northern-most extent of any of the blackfish species. In SA it has disappeared from the Murray and is now confined to small, localised populations in the Eastern Mt Lofty streams. In Vic it is known from all the major tributaries of the Murray. Once considered to be highly threatened across the MDB, its numbers and distribution have declined, but it is no longer listed as threatened across most of its range.
However, the incomplete taxonomy (cryptic taxa); lack of targeted monitoring; and considerable inter-annual fluctuations in abundance in some locations are likely masking local declines. SA populations remain precarious with noted declines in abundance over recent decades, and Qld population are restricted to cooler headwaters, with nowhere to go as temperatures rise. In the Sustainable Rivers Audit (2004–2013) Northern river blackfish were the most widespread blackfish species with 1448 individuals captured across 15 river valleys (including the Wimmera with its undescribed taxon). 58% were captured in slopes, 26% in lowland, and 16% in montane elevational zones. 583 individuals have been captured in 11 river valleys in the MDB Fish Survey (2014/15–2021/22).
Major threats are smothering of eggs and spawning sites by sediment, and interactions with alien species such as trout and Redfin perch, particularly predation and competition for food. Habitat modifications such as coldwater pollution, desnagging and altered flows through river regulation are also likely to impact on this species. In Qld, the species is likely restricted by high summer temperatures exceeding the species thermal tolerances, which has been exacerbated by loss of stream shading through riparian vegetation removal.
Rising temperatures from climate change will worsen the outlook for northern MDB populations. During the Millennium Drought, blackfish populations in Eastern Mt Lofty streams in SA were in danger of extinction as streams dried up, with the provision of supplementary water at one location allowing fish to survive the drought. Similarly, the 2017-19 drought resulted in blackfish rescues in the upper Condamine drainage in QLD.
Balcombe et al. 2011a; Ebner et al. 2020; Hammer 2004; Hammer et al. 2009, 2013, 2014; Jackson 1978a,b; Jackson et al. 1996; Khan 2003; Khan et al. 2004a,b; Lintermans et al. 2019c; Lloyd and Walker 1986; Miller et al. 2004; Shelley et al. 2021; Turschwell et al. 2017, 2018, 2020.
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