Climate affects fish-kill events in subtropical estuaries of eastern Australia

Fish kills following austral summer flood events (November-March) occur episodically in estuarine channels in Australia. We examined the climatic conditions associated with the historic record of such events in a subtropical region in eastern Australia to determine the effect of antecedent weather conditions on the probability of post-flood fish-kill events. Records, including regional county council logs, newspapers and New South Wales Fisheries annual reports, were analysed for reports of floods and fish kills following these events. Daily rainfall patterns preceding floods with fish-kill events tended to be drier than rainfall patterns preceding floods when a fish kill did not occur. Based on these observations, it is proposed that estuarine hypoxia resulting in a fish kill is increased by prolonged dry periods followed by rapid and intensive rainfall preceding the flood peak. This is most likely due to (i) accumulation of organic material on the floodplain due to vegetation stress and reduced decomposition processes in drier conditions; (ii) upstream migration of the salt wedge, allowing some estuarine fish species to maintain an optimum environment, followed by flooding with (iii) rapid consumption of dissolved oxygen during decomposition of accumulated organic material sourced from senescent vegetation; and (iv) fish becoming trapped upstream in discharging hypoxic floodwater during the flood recession phase.