Eastern freshwater cod

Taxobox
name = Eastern freshwater cod
status = EN
status_system = iucn2.3
regnum = Animalia
phylum = Chordata
classis = Actinopterygii
ordo = Perciformes
familia = Percichthyidae
genus = "Maccullochella"
species = "M. ikei"
binomial = "Maccullochella ikei"
binomial_authority = Rowland, 1986

Eastern freshwater cod, "Maccullochella ikei", also known as eastern cod, are a large and striking predatory freshwater fish of the "Maccullochella" genus and the Percichthyidae family. They are closely related to the Murray cod of the Murray-Darling river system.

Range

Eastern freshwater cod are native to the Clarence River system in northern New South Wales, Australia. The Clarence River system is an extensive East Coast drainage with many tributaries of differing size. By Australian standards the Clarence River and its major tributaries the Mann and Nymboida Rivers are extremely large rivers with extremely large flow volumes.

Appearance and size

Eastern freshwater cod have been recorded to 41 kg, but are more common at sizes less than 5 kg. Eastern freshwater cod are a small to medium sized groper-like fish with a deep, elongated body that is round in cross section. They have a broad, scooped head, and a large mouth lined with pads of very small needle-like teeth. The jaws are equal or the lower jaw protrudes slightly. Their eyes are slightly larger and more prominent than in Murray cod.

In eastern freshwater cod the spiny dorsal fin is moderate in height and is partially separated by a notch from the high, rounded soft dorsal fin. Soft dorsal, anal and caudal (tail) fins are all large and rounded, and are dusky grey or black with distinct white edges. The large, rounded pectoral fins are usually similar in colour to flanks. The pelvic fins are large and angular, set forward of the pectoral fins, and are usually a translucent greyish-white colour, tending toward opacity in large fish. The leading greyish-white coloured rays on the pelvic fins split into two trailing filaments. These filaments are significantly longer than in Murray cod.

Eastern freshwater cod are vary from cream or greyish-white to yellow on their ventral (“belly”) surface. Their back and flanks are usually an intense yellow or gold in colour, overlain with a dense pattern of black to very dark green mottling. The effect is a marbled appearance sometimes reminiscent of a leopard's markings. Colouration does vary considerably however and eastern freshwater cod from dark, heavily shaded habitats can be very dark or almost black in colour.

Habitat, diet and spawning

Eastern freshwater cod are found in clear, flowing rivers and streams with rocky beds and deep holes within the Clarence River system. The size of the rivers and streams they inhabit range from very small to very large. Eastern freshwater cod are territorial and aggressive. They are known to prey upon other fish, frogs, crustaceans and snakes. A long-lived, slow-growing species, they are gravely threatened by overfishing, habitat degradation and catastrophic natural events such as bushfires. Cod populations in the Richmond River system, now suspected to have been a strain or sub-species of eastern freshwater cod, are extinct, but have been re-established with fingerlings produced from broodfish from the Clarence River system.

Eastern freshwater cod are now protected by law.

Eastern freshwater cod are sexually mature at 4 or 5 years old, and at sizes as small as 700 g, the latter being markedly different to Murray cod. This far smaller size at sexual maturity is an evolved adaptation to the rocky, low-nutrient and often quite small waterways eastern freshwater cod are found in. Eastern freshwater cod spawn in early spring when water temperatures reach 16°C, using rock structures as sites for the adhesive eggs. Most other aspects of their spawning, including the guarding of eggs and newly hatched larvae by the male fish, are similar to Murray cod. It is important for anglers to avoid any accidental captures of eastern freshwater cod in winter when they are developing their roe, or in early spring when spawning is occurring, as research indicates this results in resorbed roe or abandoned nests respectively and a failed spawning effort. Anglers are not allowed to deliberately target eastern freshwater cod.

Origins

Eastern freshwater cod are a separate species of cod that originated from the Murray cod, "Maccullochella peelii peelii", that are present in tributaries of the Murray-Darling Basin on the western side of the Great Dividing Range. Murray cod crossed the Great Dividing Range and entered the headwaters of the Clarence River system through a natural river capture event somewhere between 0.8 and 1.7 million years ago, as estimated by DNA divergence rates. Subsequent isolation from Murray cod populations, the founder effect, genetic drift and natural selection all led eastern freshwater cod to diverge from and become a separate species to Murray cod. (See allopatric speciation.)

In addition to eastern freshwater cod of the Clarence River system, there are/were cod in several other coastal river systems. In total, at the time of European settlement of the Australia in the 18th century, naturally occurring cod were present and extremely abundant in four East Coast river systems:

* Clarence River system, northern New South Wales (eastern freshwater cod)
* Richmond River system, northern New South Wales (Richmond River cod)
* Brisbane River system, southern Queensland (Brisbane River cod)
* Mary River system, central Queensland (Mary River cod)

It is not clear whether Murray cod only crossed the Great Dividing Range once or several times to found these four coastal cod populations. Eastern freshwater cod of the Clarence river system may be the parent species of all coastal cod populations including Mary River cod, "Maccullochella peelii mariensis", although this is not currently reflected in the taxonomy.

Two recent genetic research projects using mitochondrial DNA, which offers far more resolution than older techniques, indicate that Mary River cod are in fact more closely related to eastern freshwater cod of the Clarence River system than they are to Murray cod. If this finding is correct then it suggests that Mary River cod should be renamed "Maccullochella ikei mariensis" to denote that they are a sub-species of eastern freshwater cod. If this finding is correct it also, logically, suggests that Murray cod only crossed the Great Dividing Range via a natural river capture event once, into the Clarence River system, and that the four coastal cod populations present at the time of European settlement (Clarence, Richmond, Brisbane and Mary Rivers) all are/were sub-species of eastern freshwater cod of the Clarence River system. Dramatic drops in sea-level during glacial periods (aka "Ice Ages") and/or "lateral" river capture events could easily have seen these four coastal river systems linked at times and eastern freshwater cod of the Clarence River system gaining access to each one.

Confusing the situation further is a calamitous bottleneck eastern freshwater cod of the Clarence River system went through a couple of thousand years ago, as revealed by DNA analysis, in which the majority of the population perished. This was likely due to a sequence of catastrophic drought, whole-of-catchment scale bushfires and widespread ash-induced fish kills. Eastern freshwater cod recovered from this event and were in abundance by the time of European settlement, but appear to have lost much of their genetic diversity in this event. (The genetic diversity of eastern freshwater cod has been further reduced by catastrophic declines caused by European settlers and the stocking of hatchery fish with poor genetic diversity.)

With the new resolution that DNA technologies are providing us with, many questions are being asked about eastern freshwater cod and the other coastal cod populations (though the original Richmond and Brisbane River cod strains are now extinct). Researchers and native fish enthusiasts look forward to the insights that future, high resolution DNA studies will provide us on the origin of the coastal cod populations.

Conservation

Early records reveal eastern freshwater cod were extremely abundant in the Clarence River system as the time of European settlement. These records reveal eastern freshwater cod were so abundant they were caught "on demand" for diners at a riverside hotel, and were sometimes used as pig feed, the latter being quite shocking treatment and waste of a strikingly beautiful and valuable fish.

A number of factors have led to the endangerment of eastern freshwater cod. One factor is gross overfishing, including with lines, nets, explosives and spears. As a very slow growing, top predator with low fecundity ("fertility") eastern freshwater cod are even more vulnerable to overfishing than their relative Murray cod. Another factor was severe whole-of-catchment scale bushfires in the 1930s which caused very large, widespread ash-induced fish kills. Gross habitat degradation and siltation by poor farming practices (such as clearing riverbank vegetation and allowing stock to trample river banks) is another factor that has destroyed many eastern freshwater cod habitats, and unfortunately some of these poor farming practices continue unchecked today. Finally, serious cyanide pollution from mining caused serious fish kills in the late 1800s/early 1900s.

All of these factors have reduced the once abundant eastern freshwater cod to an endangered species with a limited distribution in the Clarence River System.

The same factors have led to the extinction of Richmond and Brisbane River cod and the endangerment of Mary River cod as well.

Eastern freshwater cod were recognised as a potentially separate and endangered species of cod in 1984, and were declared a protected species in that year. Subsequent research confirmed they are indeed a separate and endangered species of cod; they remain a protected species. Disturbing levels of illegal poaching are taking place however and are not being tackled.

A restocking programme was undertaken by the government of New South Wales from 1984 to 1989. The government-run stocking programme was ill-advisedly closed after 1989 and contracted to a private operation, which produced and stocked fingerlings until the late 1990s. The eastern freshwater cod stocking programme was then suspended after genetic research indicated inbred fingerlings with extremely poor genetic diversity, that could threaten the genetics of remnant wild populations, were being produced and stocked.

No eastern freshwater cod are currently being bred or stocked.

The NSW fisheries department has allowed the private operation that produced eastern freshwater cod fingerlings to retain broodfish and produce captive-reared eastern freshwater cod for the table fish market — a decision that most people involved in conservation of Australian native freshwater fish consider indefensible and vehemently object to.

There are grave concerns over the future of all eastern freshwater cod stocks in the Clarence River system after the NSW Fisheries department allowed a foolhardy stocking of Australian bass fingerlings from a completely different bio-region to proceed. The fingerlings were contaminated with and introduced the banded grunter, "Amniataba percoides", to the lower reaches of the river. The banded grunter is an extremely aggressive small native fish species. It is feared banded grunter may yet invade the main freshwater reaches of the Clarence River system with devastating impacts on eastern freshwater cod.

References

* Listed as Endangered (EN A1acd v2.3)
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* Bearlin, A.R. and Tikel, D. (2002) Conservation genetics of Murray-Darling Basin fish: Silver perch ("Bidyanus bidyanus"), Murray cod ("Maccullochella peelii"), and Trout cod ("M.macquariensis"). In: "Managing Fish Translocation and Stocking in the Murray-Darling Basin (workshop proceedings), Canberra, 25-26 September, 2002". World Wildlife Fund, Sydney.
* Jerry, D.R., Elphinstone, M.S and Baverstock, P.R. (2001) Phylogenetic Relationships of Australian Members of the Family Percichthyidae Inferred from Mitochondrial 12S rRNA Sequence Data. "Molecular Phylogenetics and Evolution" 18: 335–347.
* Rowland, S.J. (1993) "Maccullochella ikei", an endangered species of freshwater cod (Pisces: Percichthyidae) from the Clarence River System, NSW, and "M.peelii mariensis", a new subspecies from the Mary River System, QLD. "Records of the Australian Museum" 45: 121-145.
* Rowland, S.J. (1996) Threatened fishes of the world: "Maccullochella ikei" Rowland, 1985 (Percichthyidae). "Environmental Biology of Fishes" 46: 350