JOHN YELD
Environment and Science Writer
A 4KM stretch of the Rondegat River in the Cederberg has been treated with the highly toxic fish poison rotenone to kill alien invasive small-mouth bass.
This week’s piscicide treatment was the first operation in a pilot project that is paving the way for the “Save the fynbos fishes” programme aimed at eventually removing alien-invasive fish and restoring the rich but severely threatened indigenous fish life in all critical conservation stretches of the Cape’s river systems.
Of the 27 indigenous species found in the fynbos region, 24 occur nowhere else on Earth.
Less than an hour after the piscicide started being fed from seven “drip stations” into the targeted section – about midway between the popular Algeria campsite and the Clanwilliam Dam – on Wednesday morning, dead and dying bass were spotted.
“This stuff is lethal. I’ve counted at least 15 dead fish,” Cederberg wilderness reserve manager Patrick Lane remarked soon after the start.
He was among 35 CapeNature staff from across the province who were involved in the operation, which was monitored by two international experts in piscicide application and independent scientists from the SA Institute for Aquatic Biodiversity (SAIAB) in Grahamstown, as well as fishermen and local farmers.
Dean Impson, CapeNature’s freshwater fish ecologist, who has been central to the co-operative project that has been eight years in the making, said yesterday the international experts – Dr Brian Finlayson, of the US, and Jarle Steinkjer, of Norway – had given the operation the thumbs-up.
Between them the pair have more than 60 years’ experience in freshwater fish management and have literally written the rotenone “standard operating procedure” manual.
“They were very happy, although we had to continue the deactivation process longer than expected,” Impson said.
About 14 litres of rotenone was applied over six hours at the drip stations.
A “deactivation” station was set up below the last drip station, where potassium permanganate (Condy’s crystals) was added to the river to neutralise the rotenone at the end of the treatment zone.
It had initially been expected to continue running for another six hours to neutralise the rotenone.
This station was managed by Dr Olaf Weyl, senior aquatic biologist at SAIAB, who leads the monitoring programme which has involved years-long baseline studies in advance of the poisoning and that will continue for two or three years.
The project was initially severely criticised by freshwater anglers who believed their sport was under threat.
The alien fish were introduced from the late 1800s and early-to-mid 1900s for angling.
Talks between CapeNature and the freshwater fishing forum have reduced some of the tension, and the authority has given the assurance that prime sections of fishing rivers popular with anglers will not be touched.
Impson said yesterday he had not yet had figures for the number of fish killed. There had been some deaths of adult indigenous yellowfish, but this had been anticipated since these adults were able to coexist with the bass, unlike the juveniles and smaller indigenous species.