Editing Eastern freshwater cod (section)

==Origins==
Eastern freshwater cod are a species of cod that is descended from the [[Murray cod]], ''Maccullochella peelii'', 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 mya), as estimated by [[DNA]] divergence rates.  Subsequent isolation from Murray cod populations, the [[founder effect]], [[genetic drift]], and [[natural selection]] lead to [[allopatric speciation]].<ref name=nock_2010/>

In addition to eastern freshwater cod of the Clarence River system, cod are/were to be found 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 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 and its Logan-Albert and Coomera River sub-systems, 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 southernmost of these four rivers, and Mary River cod, in the northernmost of these four rivers, are more closely related to each other than to Murray cod.<ref name=nock_2010/><ref name=jerry/><ref name=bearlin/> 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, but geomorphological evidence suggests the headwaters of the Clarence River.<ref name=nock_2010/> The mechanism by which Murray cod crossed the Great Dividing Range are considered to be either river capture, where the headwaters of an easterly-flowing coastal river cuts back (in the erosional sense) and intercepts and "captures" the headwaters (and fish fauna) of a previously westerly flowing stream, or a "wet divide", where extreme flooding or a very wet climatic phase created swamps in a topographic low point that effectively connected the headwaters of western flowing and eastern flowing streams.<ref name=unmack/><ref name=faulks/>

After crossing the Great Dividing Range, drops in sea level during glacial periods ([[ice ages]]) and/or "lateral" river capture events linked these four coastal rivers at times, allowing freshwater cod to gain access to these rivers and colonise them. The mean estimate for genetic separation of Mary River cod and eastern freshwater cod is only 300,000 years.<ref name=nock_2010/>

DNA analysis reveals eastern freshwater cod of the Clarence River system went through one or more [[population bottleneck]]s before European settlement. This was likely due to a sequence of catastrophic drought, whole-of-catchment-scale bushfire and severe, widespread ash-induced [[fish kill]]s, similar to those which occurred in 1936 (see below). Eastern freshwater cod numbers recovered from these pre-European bottleneck events, and the fish were abundant by the time of European settlement, but the species appears to have lost some [[genetic diversity]]. Further, very significant losses in genetic diversity occurred when European settlers caused catastrophic declines, and to a lesser extent by recent stockings of hatchery fish with low genetic diversity.