A large, deep-bodied fish with a large mouth reaching to below the back of the eye. Maximum size 16 kg and 850 mm; usually <5kg. The head profile is straight, and the upper jaw overhangs the lower. The tail is rounded and the pelvic fins are located below the pectorals. The overhanging upper jaw and a speckled body pattern which is blue-grey rather than yellow-green, distinguishes this species from the otherwise similarly-shaped Murray cod. Most individuals have a dark stripe through the eye, although this feature is also present in young Murray cod.
Only formally recognised as a separate species from Murray cod in 1972, some aspects of the biology of Trout cod are poorly known. The species is usually associated with deeper water (pools) and instream cover such as logs and boulders. In the Murray River, where it co-occurs with Murray cod, it occupies slightly faster-flowing locations toward the middle of the river in faster flowing waters.
Sexual maturity is reached at 3–5 years of age when fish are 0.75–1.5 kg and spawning occurs in late spring (mid-October–mid-November). Fecundity is ~1200–11,000 eggs per female. Eggs are large (2.5–3.6 mm diam.), adhesive, and probably deposited on hard substrates such as logs and rocks. After 5–10 days, larvae hatch at about 6–9 mm in length, and then drift downstream.
The diet includes fish, yabbies, mudeyes, aquatic insect larvae, shrimps, freshwater prawns and terrestrial organisms. A study based in the Murrumbidgee River found that Trout cod diet comprised smaller items than did Murray cod and was more like the diet of Golden perch than Murray cod.
Adult Trout cod move smaller distances, less often and have higher site fidelity than Murray cod, Golden perch or Carp. Research in the lowlands of the Murrumbidgee River has demonstrated that adults occupy small areas of less than 500 m centred on a ‘home snag’, and occasionally undertake exploratory movements of 20–60 km involving a return to their home. Fish are most active during low light (dusk to dawn).
In the Murray River, increased Trout cod annual growth was best explained by higher flow variability during spring and summer-autumn, with lower growth in years of stable low flow (e.g. the Millennium Drought), highlighting the importance of variable environmental flow regimes.
Because of early confusion regarding the identification of Trout cod, information on the historic distribution of the species is unclear. Trout cod was originally described from the Macquarie River, where it has not been recorded since the 1820s, other than a single unconfirmed record from the Turon River in the 1970s. It has not been recorded from the Darling-Baaka River. Formerly widespread in the southern Murray-Darling Basin (Murray, Murrumbidgee and Macquarie rivers, NSW/ACT; Ovens, Goulburn, Campaspe, King,
Buffalo, Mitta Mitta rivers, Vic; Murray River, SA), the species declined significantly in the 1970s. Before the mid-2000s there were only three self-sustaining populations of Trout cod remaining. The largest is in the Murray River between Yarrawonga and Barmah (approx. 200 km of river), the others were small, translocated populations present in Cataract Dam, and about 15 km of the upper Sevens Creek near Euroa in Vic.
Over the last 3 decades, however, there has been a concerted recovery program that has yielded considerable success. Long-term stocking since the late 1980s has reintroduced Trout cod to several locations in the Basin including sites on the mid and upper Murrumbidgee, Macquarie, Ovens, Goulburn, Tumut and upper Murray drainages. The species has been stocked in Bendora Reservoir, ACT, and Talbingo Reservoir, NSW. Self-sustaining populations have now established in the Ovens River in Vic and the mid-Murrumbidgee in NSW, with occasional breeding recorded in the upper Murrumbidgee and Goulburn rivers and Bendora Reservoir. Stocking of ~300,000 individuals in the Macquarie River from 1991-2018 and the upper Murrumbidgee from 1988-2008 failed to establish populations. The mid-Murray population downstream of Yarrawonga is steadily increasing and there are recent captures from near Gunbower Island on the Murray, indicating that the population is still expanding downstream. However, there is little evidence that the Yarrawonga fishlift is facilitating upstream expansion of this species. Trout cod abundance has been shown to increase following restoration actions such as resnagging, weir removal and riparian rehabilitation and protection.
A total of 73 Trout cod were recorded from the Sustainable Rivers Audit (2004–2013) across 4 river valleys, with 42 and 29 coming from the Ovens and Central Murray valleys respectively. 95% of captures were from the lowland altitudinal zone. 130 individuals have been captured from 5 river valleys in the MDB Fish Survey (2014/15–2021/22), with again the catch predominantly (94%) coming from the Ovens and Central Murray valleys.
Threats include interactions with alien species such as trout and Redfin perch, and habitat modification (desnagging, sedimentation, clearing of riparian vegetation, river regulation and cold-water pollution from dams). Overfishing of remnant populations also contributed to declines; nothing is known of post-angling mortality for Trout cod, but it could be expected to be similar to Murray cod where one study estimated it to approach the harvest take in some instances, and another study estimated ~15% in summer months. Trout cod are a targeted catch and release fishery in both NSW and Vic, so optimal catch and release practices are required to facilitate establishment and then expansion of stocked populations. Similarly, misidentification of Trout cod as Murray cod (and subsequent retention) can hamper recovery programs.
There is recent evidence of increased hybridisation with Murray cod, and bird predation of predator-naïve, ongrown adults in a reintroduced population in the upper Murrumbidgee. Proposals to establish stocked recreational impoundment fisheries risk competing with conservation stockings for scarce hatchery production. Inappropriate stocking of Murray cod (outside its known historic range) increases the risk of hybridisation. Hybridisation, hatchery production and recreational angling need to be carefully managed if reintroductions are to be successful and releasing hatchery ongrown fish is unlikely to be a successful conservation stocking alternative.
Extreme events associated with Climate Change (e.g. increasing severity and frequency of bushfires) can have catastrophic impacts on small, isolated populations.
ACT Government 2018; Baumgartner 2007; Couch et al. 2016; Douglas et al. 1994, 2010, 2012; Ebner et al. 2007b, 2009b; Ebner and Thiem 2009; Hall et al. 2012; King et al. 2016; Koehn & Harrington 2006; Koehn & Nicol 2014, 2016; Koehn et al. 2008, 2013, 2019, 2020a; Lintermans et al. 1988, 2005b, 2015; Lyon et al. 2012, 2018a,b; Miles 2022; Nicol et al. 2007; Raymond et al. 2019; Silva et al. 2020; Thiem et al. 2008; Tonkin et al. 2017c.
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