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The Occurrence of Fish Kills, and their Causes missing in the Darwin-Katherine-Jabiru Region of the NT



The Occurrence of Fish Kills, and their Causes missing in the Darwin-Katherine-Jabiru Region of the NT


Townsend, S.


E-Publications; E-Books; PublicationNT; Report ; 36/1992




Made available via the Publications (Legal Deposit) Act 2004 (NT).





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Power and Water Authority

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Report ; 36/1992

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Technical Report WRD92036 Viewed at 15:07:57 on 29/07/2010 Page 13 of 54. 13 run-off into receiving waters have caused fish kills, for example the Donkey camp Pool fish kill (Townsend et al. 1992). These events occur predominately in hilly areas where run-off can attain high velocities. The waters carry a substantial organic load and have high instantaneous/post instantaneous oxygen demand. They can significantly depelete the oxygen content of receiving waters, though not always causing fish kills. Denisty differences between inflm-l and receiving waters limit mixing, and consequently the assimilation capacity of the receiving waters. Catchment practices which increase the erodability of soils will also impact on run-off water quality and the frequency of fish kills. Floodplain fish kills hava also been ascribed to low dissolved oxygen concentrations. Vegetation and soils, when initially inundated exert a high oxygen demand, dissolved oxygen concentrations can decrease to <2 mg/l (unpublished data DPIF) , and may remain low due to decomposing macrophytes. The slow passage of water over the floodplains also contributes to the low oxygen content of the floodplain waters. Depleted oxygen concentrations in recently inundated floodplains have been observed for many systems (Welcomme 1979). Fish kills not only occur on the floodplain but also as a result of floodplain run-off into watercourses. The Shady Camp Billabong fish kills in the latter part of the wet season are an example (unpublished data, DPIF). Not all fish kills are associated \vith run-off events. A small

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