Origins
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:
- Clarence River system, northern New South Wales (eastern freshwater cod, Maccullochella ikei)
- Richmond River system, northern New South Wales (Richmond River cod, Maccullochella ikei)
- Brisbane River system, southern Queensland (Brisbane River cod, Maccullochella sp.)
- Mary River system, central Queensland (Mary River cod, Maccullochella mariensis)
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.)
Read more about this topic: Eastern Freshwater Cod
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