A remarkable 24 of the 27 fish species found in the Cape floristic region (CFR), which is mainly situated in the Western Cape, occur nowhere else in the world. They are commonly known as fynbos fish, as they thrive in streams that flow through areas dominated by fynbos. Nowadays, however, many of these endemic fish can be found only in the upper reaches of isolated mountain streams as their natural distribution ranges have shrunk, in many cases dramatically, and their populations have crashed in the last 50 years.
Invasive alien fish
By far the most significant threat to these fish are invasive alien fish species introduced into the rivers of the Western Cape as far back as 200 years ago for angling and food. The indigenous species of the CFR were deemed too small for these purposes. Many of these aliens – specifically rainbow trout, smallmouth bass, sharptooth catfish (barbel), carp, bluegill sunfish and Mozambique tilapia – have had a severe impact on indigenous fish and associated food webs.
Researchers estimate that more that 80% of the river habitat in the CFR has been invaded by these aliens. Fish conservation experts agree that one of the best ways to increase fynbos fish populations is to remove alien fish from priority river areas, so that the indigenous fish can reclaim key parts of their habitat. This is known as rolling back the invasion.
Lower Rondegat River project
To this end, CapeNature, in co-ordination with the Cape Action for People and the Environment (CAPE) programme, has launched a project in the Cedarberg’s Rondegat River to remove alien smallmouth bass and bluegill sunfish from the lower Rondegat River and reintroduce the highly threatened indigenous Cedarberg fish to an important part of their natural distribution range.
The project entails treating a stretch of the river with a piscicide, CFT Legumine (see box). According to Dr Olaf Weyl, senior aquatic biologist at the South African Institute of Aquatic Biodiversity, a very small population of smallmouth bass has made its way up the Lower Rondegat River over the past 80 years, voraciously consuming most of the fynbos fish and invertebrates. Although these aliens have been no more than a few dozen in number, they have destroyed the biodiveristy of this section of river. “Not all invasions involve large numbers,” says Dr Weyl.
“The Rondegat River has a relatively small fish carrying capacity, and I’d imagine that this invasion of smallmouth bass would constitute only about 30kg of fish.” Dean Impson, CapeNature’s freshwater fish scientist and leader of the Rondegat River project, explains that the aim is to remove all invasive smallmouth bass from a 4km stretch of the Lower Rondegat River between a waterfall and a weir. “The intervention is intended to transform this aquatic desert into an extended river habitat for the critically endangered fynbos fish,” he adds.
A future for fynbos fish
After the first application of piscicide, which took place in February, Dr Weyl supervised a neutralisation zone downstream of the treated area, adding diluted potassium permanganate (Condy’s crystals) to the river to neutralise the rotenone (see box). At the same time, Dr Weyl is studying the seasonal insect abundance of the river and will return to conduct post-treatment surveys.
The pilot project in the Rondegat River is the first of four similar projects that aim to reintroduce endemic fynbos fish to these mountain streams. According to CapeNature, local landowners support the project because the alien smallmouth bass in the Rondegat River have no angling value, whereas a rehabilitated river with indigenous fish is more appealing to ecotourists. “We expect full recovery of the treatment area in the lower Rondegat River to take up to three years, resulting in substantially higher numbers of the threatened indigenous fish,” says Impson.
For more information, contact Dr Olaf Weyl on 046 603 5834 or Dean Impson on 021 866 8019 or email: [email protected]