Mary River cod
Mary River Cod (Maccullochella peelii mariensis) are a Maccullochella cod found in the coastal Mary River system of southern Queensland, Australia. Mary River Cod are significant because they are the most northerly of the four Maccullochella cods found or once found in coastal river systems of eastern Australia.
A large fish, reliably recorded to at least 27 kilograms in the early years of European settlement. Very similar in appearance to Murray Cod and Eastern Freshwater Cod; a striking looking, deep-bodied fish with dark green or black mottling. Curiously, Mary River Cod have a slightly shorter, thicker caudal peduncle (tail wrist) than the other cod species.
Mary River Cod are subject to the usual story of woe. Incredibly abundant at the time of first European settlement, they were grossly overfished with nets, lines and explosives by the early European settlers and, as with other Maccullochella cods, were even used as pig feed. This overfishing, combined with the massive siltation of their habitats by land-clearing, destruction of riparian vegetation and cattle trampling river banks, and dams and weirs blocking migration, rapidly caught up with this large, slow-growing, long-lived Maccullochella cod species, as it has with all its close relatives.
Taxonomically, Mary River Cod are designated as "Maccullochella peelii mariensis", which denotes that they are a sub-species of the Murray Cod, Maccullochella peelii peelii. This was done on the basis of examination of muscle proteins and enzymes. (This designation of Mary River Cod as a sub-species of Murray Cod was actually the cause of the Murray Cod's recent name change from Maccullochella peeli to Maccullochella peelii peelii.)
Two recent genetic research projects using mitochondrial DNA, which offers far more resolution, indicate that Mary River Cod were in fact more closely related to Eastern Freshwater Cod, Maccullochella ikei, of the Clarence River (the most southerly of the four coastal cod populations) 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 (Clarence, Richmond, Brisbane and Mary Rivers) were all sub-species of Eastern Freshwater Cod of the Clarence River. 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 gaining access to each one.
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.
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