A small to medium-sized fish with a yellowish-brown to olive green back and sides, often spectacularly mottled (‘giraffe’ spots), and a creamy or light-grey belly. Maximum length 325 mm Total Length and 195 g; usually < 240 mm and 60 g. The pelvic fins have been 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 1–3, usually 2, spines.
The body is covered in very small scales with a thick mucous coating. Readily distinguished from the Northern river blackfish by its possession of 1–3 spines in the dorsal fin.
The Two-spined blackfish is restricted to cool, clear upland or montane streams with abundant instream cover, usually in the form of boulders and cobble. It is generally not found in very small, headwater creeks with shallow depth and very fast flows and is more commonly present in the medium to large streams. It also occurs in upland reservoirs with suitable rocky margins. It is usually found in forested catchments, where there is little sediment input to the stream from erosion or agriculture.
Its diet is dominated by aquatic insect larvae, particularly mayflies, caddisflies and midges, and occasionally fish and crayfish. Young-of-year and juvenile blackfish eat proportionally more mayfly and midge larvae than adult fish, which consume larger items such as caddisfly larvae and terrestrial invertebrates.
The Two-spined blackfish is benthic and nocturnal. Its movement is extremely limited—the home range of adults is estimated at ~15-23 m in streams but moves further in reservoirs. Fecundity increases with length and spawning occurs in November– December. 80–420 eggs are laid in a single mass. Larger individuals spawn earlier in the season than small individuals and females commence breeding in their second or third year (at > ~120 mm length). The spawning site is usually in the gaps between cobbles or boulders where the eggs are attached to the upper surface of a rock.
The eggs are large (~3.5 mm diameter), yolky and adhesive and are fanned and guarded by the male fish until the larvae have almost fully utilised the yolk reserves and are free-swimming. Hatching occurs after approximately 16 days at a water temperature of 15°C. After approximately three weeks both the male parent and the larvae leave the spawning site.
In a reservoir population on the Cotter River, ACT, a radiotracking study found that adult fish undertook nocturnal migrations from daytime shelter habitat to fringing reedbeds where they were assumed to be feeding. At the end of the night fish then returned to their daytime shelters. In the upper Cotter River, the species is host to a small (~3-5 mm) leech with medium and larger fish often having numerous small bites (red ‘dots’), particularly on the lower sides and belly.
This species is only known from the Murray-Darling Basin, where it has been recorded from north-east Vic, the ACT and southeast NSW. In Vic it is present in the upper sections of the Goulburn, Broken, Ovens, Mitta Mitta and upper Murray catchments. In the ACT it is only currently present in the Cotter catchment, although it was previously present in the Murrumbidgee and Paddys rivers, and possibly the Naas/ Gudgenby system. In NSW it is known from the Goodradigbee, upper Murray above Lake Hume, Tumut and Goobarragandra catchments. A small remnant population was still present in the Murrumbidgee River above Adaminaby in 1999, but it is unknown whether it still persists. The highest altitude that the species is recorded from is 1070 m. (Cotter Catchment, ACT).
When it is present, the species is often abundant, but it has declined in several catchments in the ACT and NSW. A recent study reported two closely related species of Two-spined blackfish, with one species widespread (G. bispinosa) but the other undescribed species currently only known from the Goulburn catchment in Vic. 2520 individuals were recorded from the Sustainable Rivers Audit (2004–2013) across 7 river valleys, with 52%, 29% and 18% from upland, slopes, and montane elevational zones respectively, and only 1% from lowlands. 1803 individuals have been captured from 7 river valleys in the MDB Fish Survey (2014/15–2021/22).
Significant threats include cold-water pollution, smothering of eggs and spawning sites by sediment, and interactions with trout, particularly predation and competition for food. Rapidly fluctuating water levels in reservoirs can expose egg masses.
Climate change will increase frequency and severity of bushfires with rainfall following the 2019-20 bushfires resulting in catastrophic sedimentation that severely impacted some populations. The upper Cotter River population was severely reduced with abundance of both blackfish and Rainbow trout declining by 80% immediately following post-bushfire rainfall events which infilled streams with sand silt and gravel. For blackfish, mainly large adults survived, and there was a strong recruitment pulse in 2021, however instream habitat (substrate diversity and pool depth) was still greatly impacted 2 years post-fire.
ACT Government 2018; Broadhurst et al. 2011, 2012; Dennis et al. 2016; Ebner et al. 2009a; Hammer et al. 2014; Jackson et al. 1996; Kalish et al. 1998; Koehn 1990; Lintermans 1998, 2002; Lintermans et al. 2019a; O'Connor and Zampatti 2006; Sanger 1990; Todd et al. 2017; M. Lintermans unpubl. data.
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