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SEDGEFIELD NEWS - Recent reports of fish dying off in Groenvlei, Sedgefield caused a stir – even making national news headlines – and had scientists baffled at what exactly was causing the fish to go belly-up on the outskirts of the Slow Town. On Wednesday 17 October, an article appeared on the Knysna-Plett Herald’s website indicating that CapeNature scientists had visited the site, taken samples of the water and fish and sent them for testing.

Although CapeNature scientists have said that a mass of fish dying like this could be brought on by man's intervention, or by toxins introduced by humans, all indications are that the fish dying occurred as a result of natural causes. CapeNature warned residents not to eat the dead fish.

On Tuesday 23 October CapeNature spokesperson Marietjie Engelbrecht said that CapeNature was now confident that natural processes are responsible for the incident. "We can confirm that no further fish deaths have been recorded since the weekend, which is a strong indicator that the situation is normalising. CapeNature will continue to monitor and collect water samples at Groenvlei," she said.

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The reasons for the "fish die-off" which was first observed and reported on 6 October, followed by more dying between 8 and 10 October, were unknown at that early stage according to CapeNature. ''There is not an obvious reason,” it read, but Engelbrecht said at the time that there were indications that the growing carp population might have been a contributing factor.

Carp to blame?
''Carp were illegally stocked into Groenvlei in the 1990s, also presumably for angling purposes,"' she explained, adding that while carp are a popular angling species, they are renowned worldwide for causing water-quality problems through their destructive feeding habits. 

Fish in Groenvlei, Sedgefield died most probably due to a natural occurrence according to CapeNature. Photo: Supplied

''They have the ability to make clear water bodies such as Groenvlei more turbid with reduced aquatic plant beds, thereby changing the biodiversity and functioning of the aquatic food web,” Engelbrecht said.According to Engelbrecht, the occurrence was characterised by substantial numbers of largemouth bass, bluegill sunfish and indigenous fish dying. She said that CapeNature fish scientist Dean Impson and Dr Kevin Christison, a specialist fish-disease scientist of the department of agriculture, forestry and fisheries (DAFF), visited Groenvlei to assess the extent of the fish dying and try to understand why it was happening.

During their visit, said Engelbrecht, dead and decaying bass and bluegill were noticed floating near reed beds, especially on the western shoreline of Groenvlei. 

“There were no fresh dead bass or bluegill, indicating that the worst had passed. Carp and tilapia seem to be unaffected,” she said, explaining that these are very hardy fish able to survive poor water-quality conditions.

'Poor water quality'
Engelbrecht said that water quality measurements done by the Garden Route District Municipality and Sanparks at the time indicated poor water quality (e.g. low dissolved oxygen levels, 3.9mg/l) and elevated water alkalinity (pH 9.2) in some of the shallow bays on the western shoreline. 

The combination of these factors, she said, is a known fish stressor. “There was also a strong smell of what may have been hydrogen sulphide, a known fish toxicant, which is produced by decaying animal and plant material,” she added.

A worldwide phenomenon
According to Impson, fish dying in such numbers occurs worldwide and could be the result of natural and/or human-made factors. He said mortality from natural causes is generally far more common and can be caused by many factors, including oxygen depletion, toxic gases and substances, toxic algal blooms and the turnover of the water column. 

“Bacterial kills are seldom sudden and there is usually a gradual build-up of fish losses,” he added.

Human-made factors include the discharging of pesticides, fertilisers and other chemicals into water bodies that may act directly or indirectly on the fish.

According to Engelbrecht fishing at Groenvlei is currently allowed, but CapeNature encourages people not to consume the fish for the time being.

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