The largest Australian freshwater fish, reaching 113.6 kg and 1,800 mm Total Length (TL) in modern times. Easily identified by its large mouth, cream to white belly and green mottled pattern on the body and head. Adults have a broad head with a concave profile. The spiny front portion of the long, single dorsal fin is lower than the softer rear portion. The tail is rounded.
Murray cod is an icon of the MDB and forms the basis of a popular recreational fishery in south-eastern Australia where it is often stocked into rivers, dams and lakes. The species is important in Aboriginal lore: a huge Murray cod is responsible for forming the Murray River and all its fishes. Generally associated with deep holes in rivers, Murray cod prefer habitats with instream cover (rocks, stumps, fallen trees or undercut banks). Juvenile cod have been shown in aquaria trials to prefer rocky substrates. A ‘sit and wait’ predator, its diet contains fish, crayfish, freshwater prawns, shrimps, mussels, Murray crayfish, aquatic invertebrates, frogs, the odd water dragon, snakes, birds, turtles, water rats, and even Platypus. In the 2021 mouse plague in inland eastern Australia, mice were commonly found inside cod. In the Murray River individuals have even been recorded with golf balls in their stomach: they will eat just about anything. Larval Murray cod commence feeding while drifting and in the Broken River diet at this early life stage consisted mainly of benthic items such as cladocera, copepods and chironomid larvae. Larvae in the southern MDB generally drift, dispersing actively downstream for several days (but cod will also breed in still waters such as dams where drift does not occur); juveniles and sub-adults in the mid-Murray were relatively sedentary with a range of ~350 m and average daily movements of <20 m in autumn. In a study on the Murray and Ovens rivers ~50% of adult Murray cod migrated upstream to spawn. This movement can be up to 130 km and generally occurs in late winter/early spring when river levels are high. After spawning these fish rapidly move downstream again, with >80% returning to the same area they previously occupied, usually to the same snag. About 25% of fish did not follow this ‘upstream then downstream’ pattern, and moved little, and 25% made single direction movements (in both directions). Murray cod can also move downstream to spawn. In Mullaroo Ck individual cod undertook spawning migrations to the same spawning sites in consecutive years. In the Edward-Wakool river system, acoustic tracking of Murray cod found activity to peak during low light levels (dawn, dusk, night) at water temperatures of ~20°C.
The species matures at ~4–5 years of age and 500–600 mm TL (but smaller fish are mature in the northern MDB), and spawns in spring and early summer when water temperatures exceed ~15°C. Spawning time doesn’t vary greatly between lowland and upland locations in the southern MDB (late Oct–mid Dec), but spawning occurs earlier in the lower Murray (late Sept–Nov) and northern MDB (late Aug–Oct). Eggs are large (3–3.5 mm diam.), adhesive and usually deposited onto a hard surface such as logs, rocks, or clay banks. The male guards the eggs during incubation and they hatch after 5–13 days. In hatcheries polygamy has been recorded, as has multiple spawning by both sexes within a season and repeated mating between pairs of fish across multiple seasons. Evidence from the upper Murrumbidgee suggests that multi-year pair bonds can occur. The larvae are about 6–9 mm long at hatching and have a large yolk sac. Larvae drift downstream for 5–7 days, particularly by night in spring and summer (Oct–mid-Jan), peaking from late Oct–Nov in the southern MDB, with drift apparently not obligate in the northern MDB.
Murray cod is a long-lived species: average weight for modern fish from rivers aged 5, 10, 15, 20 and 30 years is approximately 5, 10, 15, 20 and 36 kg respectively. However, 8-30,000 years ago fish were larger and generally lived longer with cod reaching lengths of at least 2,200 mm. The oldest modern cod accurately aged was 49 years old, 1,270 mm long and weighed 32.5 kg, but younger fish may be larger: one was 29 years and 34 kg. In the Murray River, increased annual growth of Murray cod was best explained by higher flow variability during spring and to a lesser extent in summer-autumn, highlighting the importance of variable environmental flow regimes. As with Trout cod, Murray cod growth was low during the periods of low, stable flow during the Millennium Drought.
Murray cod was formerly widespread and abundant in the lower and mid-altitude reaches of the Murray–Darling Basin. In 3 catchments with terminal wetlands (the Lach- lan, Macquarie and Gwydir), there has been limited mixing with fish in the broader MDB with these 3 populations genetically distinct. There was significant commercial harvest of the species beginning in the 1860s and continuing for the next 140 years. For example, a single individual fishing with set lines in the 1920s/30s captured 600 pounds (272 kg) of cod from Kow Swamp near Gunbower on the Murray River over 5 days. Then 3 months later he captured another 1,412 pounds (641 kg) from the same location over 14 days. Of 49 cod captured, the average size was 17 kg. Commercial fisheries data indicate populations declined in the 1920s and then again dramatically in the 1950s. The species now has a patchy distribution and reduced abundance across its historic range and is currently listed as nationally threatened. Murray cod abundance has been shown to increase following restoration actions such as resnagging, weir removal and riparian rehabilitation and protection. A total of 1,345 Murray cod were recorded from the Sustainable Rivers Audit (2004–2013) across 20 river valleys (missing Paroo, Castlereagh, Wimmera), with 50%, 30%, 11% and 8% from lowland, slopes, montane, and upland elevational zones respectively. 1,484 individuals have been captured from the same 20 river valleys in the MDB Fish Survey (2014/15–2021/22).
For several years it has been anecdotally reported that that some Murray cod populations are again increasing, and a recent comprehensive analysis of NSW population data between 1994 and 2022 by NSW Fisheries confirms this. Although trends in relative abundance varied strongly among valleys and years, overall, there was a statistically significant increase in abundance across the time series in NSW (NSW DPI unpubl. data). Murray cod was recently included in a list of 29 nationally listed threatened animals that now do not meet the criteria for listing.
The total NSW recreational catch of Murray cod during 2017/18 was 416,677 with 9% being kept and 91% released. In the Millennium Drought Murray cod recruitment was minimal in the weir-pool habitats of the Lower Murray in SA but was consistent during this period in the flowing creeks of Chowilla system. Murray cod were historically common in the Paroo catchment, but since the 1980s seems to have disappeared and are likely functionally extinct in this drainage. Reasons for the decline of Murray cod (and Silver perch and Freshwater catfish) in this essentially unregulated river are unknown, but speculated to be related to overfishing, blackwater, or Carp.
Threats include: overfishing, habitat destruction through wood removal (instream and floodplain) and sedimentation; damage to larvae going over weirs and losses into irrigation networks and pumps; river regulation (altered flows and cold-water pollution, construction of weirs and regulators); black- water following floods.
Allen-Ankins et al. 2012; Baumgartner 2007; Baumgartner et al. 2006, 2007; Couch 2018; Couch et al. 2020; Disspain et al. 2012; Douglas et al. 2010; Dyer et al. 2014; Ebner 2006; Gilligan et al. 2019a, Hall et al. 2012; Humphries 2005; Hutchison et al. 2020; Jones & Stuart 2007; Kaminskas & Humphries 2009; King et al. 2016; Koehn 2009; Koehn & Harrington 2005, 2006; Koehn & Nicol 2014, 2016; Koehn et al. 2009, 2020a; Leigh & Zampatti 2013; Lintermans & Phillips 2004, 2005; Marett 1943; Nock et al. 2010; Raymond et al. 2019; Rourke et al. 2009, Rowland 1989, 1992, 1998a,b, 2005, 2020; Saddlier & O’Mahony 2007; Thiem et al. 2018, 2022; Todd et al. 2005; Tonkin et al. 2017c, 2020, 2021; Trueman 2011; Woinarski et al. 2023; Zampatti et al. 2014; A. Couch unpubl. data.
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