HBRd Torrens xing

Torrens River crossing upstream of Henley Beach Road

Walkers along the Torrens banks between May Tce and Henley Beach Road in the past few days may have seen a number of dead or dying fish, particularly carp and possibly catfish. This is the EPA’s prompt response to our enquiry today:

We had a look at this and it appears that the recent rains have added a large organic load to the lowland Torrens River which, along with an obvious algal bloom, have resulted in the water holding minimal dissolved oxygen to sustain the many large carp and catfish you probably saw on your walk. While the current warm, sunny weather continues it is likely that more fish will continue to die. I would however expect the smaller fish to persist because they can usually get enough oxygen from gasping in the shallows, compared to larger fish that are more susceptible to these sorts of events. Council will be putting up signs and removing as many of the accessible fish as possible. Finally, these sorts of events happen occasionally in many lowland streams during the year, so it is not that unexpected for the bottom of the Torrens – and has occurred in the past through the same section of river.

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Dead carp, Torrens River lower reaches May 2014

To explain this further here is a paragraph from a 1988 EPA report: Dissolved oxygen (DO) is important for most aquatic organisms and varies with temperature, salinity, rainfall and runoff containing oxygen demanding organic material. Concentrations in unpolluted waters are generally between 7 and 10 mg/L. DO concentrations vary seasonally or even daily in response to temperature and biological activity. DO is used to indicate the degree of pollution caused by organic matter. DO concentrations below 5 mg/L are stressful to most aquatic animals.

Ambient Water Quality Monitoring of South Australia’s Rivers and Streams (Chemical and Physical Quality) EPA Water Quality Monitoring Report No 1, 1998  

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Dead catfish at stepping stones on Torrens May 2014

And another explanation of the impact of low dissolved oxygen, from a US website: Natural conditions such as high temperatures, large amounts of leaves and woody debris that fall into the river from streamside forests and an absence of waterfalls or riffles to aerate the water can lower DO concentrations.  Nitrogen and phosphorus (N & P) added to rivers from point sources such as waste water treatment facilities or nonpoint sources such as runoff from agricultural or urban areas may enhance the growth of algae in streams and rivers.  As algae complete their life cycle and die, they become a food source for bacteria which consume oxygen as they decompose the algae.  Large populations of bacteria feeding on algae are able to consume all the oxygen available in water – thus leading to the death of other aquatic organisms, including fish, which depend on the DO.  This is often described as a fish kill in the popular media.  http://www.nespal.org/dissolved%20oxygen.html

posted by Angela Gackle on 17/5/2014

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