DNA analysis reveals eastern freshwater cod of the Clarence River system went through one or more population bottlenecks before European settlement. This was likely due to a sequence of catastrophic drought, whole-of-catchment-scale bushfire and severe, widespread ash-induced fish kills, 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.