| Eastern freshwater cod | |
|---|---|
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Perciformes |
| Family: | Percichthyidae |
| Genus: | Maccullochella |
| Species: | M. ikei |
| Binomial name | |
| Maccullochella ikei Rowland, 1986 |
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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, that occur in the coastal Clarence River system of north-eastern New South Wales. Eastern freshwater cod are closely related to the Murray cod of the Murray-Darling river system, and are considered an icon of the Clarence River system.
A long-lived, slow-growing species, eastern freshwater cod are gravely threatened by poaching, habitat degradation and catastrophic natural events such as bushfires. Eastern freshwater cod are now protected by law.
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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. Eastern freshwater cod originally occurred in great abundance through all of the system, to significant altitudes, though reportedly were never found above the large falls many tributaries feature in their upper reaches.
The indigenous cod population of the Richmond River system (Richmond River Cod), now extinct, are believed to have been a population of eastern freshwater cod. The Richmond River has been restocked with fingerlings produced from Clarence River cod but there is no evidence that they are breeding (Anon, 2004).
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.
Eastern freshwater cod are a separate species of cod that originated from the Murray cod, Maccullochella peelii, that are present in tributaries of the Murray-Darling Basin on the western side of the Great Dividing Range. Murray cod entered an east coast river system, likely the Clarence, via a natural event somewhere between 0.62 and 1.62 million years ago (mean estimate 1.1 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 (Nock et al., 2010). (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:
Several genetic studies have found that eastern freshwater cod, in the southern-most of these four rivers, and Mary River cod, in the northern-most of these four rivers, are more closely related to each other than to Murray cod (Jerry et al., 2001; Bearlin & Tikel, 2002; Nock et al., 2010). This suggests that Murray cod only managed to cross into east coast river systems once. It is not clear which of the four river systems was the original entry point. Geomorphological evidence meant the Clarence River was long suspected as the entry point but latest genetic evidence (Nock et al., 2010) suggests the Mary River system could also have been the original entry point. It is worth noting that 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 freshwater cod gaining access to each one. The mean estimate for genetic separation of Mary River cod and Eastern Freshwater Cod is only 300,000 years (Nock et al., 2010).
DNA analysis reveals eastern freshwater cod of the Clarence River system went through a calamitous bottleneck a couple of thousand years ago, 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.)
Eastern freshwater cod are found in clear, flowing rivers and streams with rocky beds and deep holes within the Clarence River system, to significant altitudes. The size of the rivers and streams they inhabit range from very small to very large. Eastern freshwater cod are territorial and aggressive. The bulk of their diet is based on crustaceans (the large, clawed Macrobrachium shrimp and the small, clawless Paratya shrimp) and other fish, but they are powerful opportunistic predators and are also known to take insects (e.g. cicadas), frogs, snakes, lizards, birds and small mammals.
Growth is highly variable in eastern freshwater cod but overall is slower than in Murray cod. The oldest eastern freshwater cod yet aged is only 14 years of age, but this reflects limited sampling and many decades of overfishing and poaching, that have led to the loss of most large individuals (Butler & Rowland, 2008). There is little doubt the species has the potential to reach or even exceed the maximum age recorded in Murray cod (48 years).
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 (Butler & Rowland, 2008, 2009). This far smaller size at sexual maturity is likely an evolved adaptation to the rocky, low-nutrient and often quite small waterways eastern freshwater cod are found in. The trout cod, a Maccullochella cod also once found in small, rocky, low-nutrient streams in the upland reaches of the Murray-Darling system, displays a similar trait, also reaching sexual maturity at a far smaller size than Murray cod. 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 (Butler & Rowland, 2009). However, there are some indications that in contrast to Murray cod larvae, eastern freshwater cod larvae may not drift in river currents for several days after leaving their nest. This would be a notable difference in the larval ecology between the two species, however more research is required to confirm this.
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 (Butler & Rowland, 2009). There is now a total fishing closure on the Mann and Nymboida Rivers and their tributaries between 1 August to 31 October (inclusive) each year specifically to protect the eastern freshwater cod from accidental capture during their breeding season (NSW DPI, 2008).
It should be noted that anglers are not allowed to deliberately target eastern freshwater cod.
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 19th century/early 20th century (Anon, 2004).
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 the fingerlings being produced had very low levels of genetic diversity.
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.
Concern has been expressed over the future of all eastern freshwater cod stocks in the Clarence River system after the NSW Fisheries department allowed a stocking of Australian bass fingerlings from a completely different bio-region to proceed (Rowland, 2001). 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.