The natural distribution of the eastern freshwater cod (Maccullochella ikei) is the Clarence and Richmond Catchments of Northern NSW. It was once widespread and abundant in these catchments, and provided a good source of food to early settlers. They are now extinct in the Richmond catchment. Their abundance and distribution has been reduced considerably in the Clarence Catchment, and the species is endangered.

Fish populations declined drastically in the late 1920s and 1930s when there was a long dry period followed by bushfires and heavy rains. This resulted in massive fish kills, probably due to high turbidity, and low levels of dissolved oxygen in the river system. Dynamiting of rivers associated with railway construction,and release of contaminated water from mining tailings dams have also contributed to population declines.

Reasons for the long term decline in populations include the sedimentation of deep pools in the river system, loss of in-stream habitat (eg logs), over fishing, isolation of populations, and inbreeding.

Core habitat for eastern freshwater cod is deep pools in relatively fast flowing sections of the Clarence River system. The main remaining populations occur in the Nymboida, Boyd, and Mann Rivers.

The eastern freshwater cod is closely related to the Murray cod (Maccullochella peeli), the trout cod (Maccullochella macquariensis), and the Mary River cod (Maccullochella peeli mariensis).

Introduction of Murray cod to the Clarence would be a major threat to the eastern freshwater cod. A high percentage of abnormal and inviable hybrid larvae are produced when the two species are crossed. Introduction of the eastern freshwater cod to the Murray Darling System would be a similar threat to the Murray cod.