The Fynbos Fish Trust

More than 90% of the Western Cape’s total river habitat is invaded by non-native fish. This has been utterly devastating for our native fish species.

For thousands of years, these tiny Fynbos Fish species lived in our Western Cape rivers with few predators. But in the late eighteenth century, the first non-native fish (carp and goldfish) were introduced to our rivers. Around 100 years later, Brown trout, and then Rainbow trout were introduced for angling purposes.

Since then, Largemouth, Spotted and Smallmouth bass arrived from North America. Bluegill sunfish, Mozambique tilapia and Banded tilapia were then introduced as fodder for the bass. Throughout the mid 1900s, the South African government spread trout and bass throughout the Western Cape. In the 1960s, Sharptooth catfish were introduced.

All in all, these introductions (both from overseas and translocated species introduced from nearby river systems) have decimated our Fynbos Fish populations. As an example, in the De Hoop Vlei in the early 2000s, Mozambique tipalia numbers exceeded the indigenous Cape kurper by a ratio of 100:1.

There are a number of reasons why non-native fish are so detrimental to native fish species:

• They are fish-eating fish – and find the Fynbos Fish to be easy pickings, given that native fish are poorly adapted to coexist with these predatory alien species.
• Non-native fish, when not eating native fish species, also compete with them for other food sources. In many instances, Fynbos Fish are completely outcompeted by the sheer number of invasive fish.

The Western Cape is a rain-scarce region. In order to provide water to our cities and agricultural sector, dams are built to store water, and in many instances, entire tributaries are pumped dry. As a result, rivers that should flow throughout the year, such as the Olifants River in the Cederberg, are now not flowing in summer.

Fynbos Fish rely on environmental cues which guide them to act. Temperature and flow changes let them know that it’s time to start spawning, or that they need to migrate. But as the flow reduces, temperatures rise above optimum levels. And as a result, Fynbos Fish experience shorter spawning seasons, and a smaller population. What’s more, drying rivers trap fish in isolated pools – many of which dry up completely over the summer months.

Barriers like dams also prevent species migration. And when irrigation water is released from dams during summer, flows increase, and temperatures decrease. That’s the opposite of the natural situation for these fish, and the cooler water temperatures can easily kill young fish.

Many of the Western Cape’s riparian zones are completely overrun by invasive non-native plants and trees. These trees outcompete natural vegetation – and this is detrimental for Fynbos Fish. Why? Because Fynbos Fish enjoy a level of protection from indigenous plants, which invasive trees don’t provide. Invasive trees also have weak root systems, and during times of floods, these are easily uprooted. This destabilises the riverbanks and causes extensive erosion.

Fynbos Fish also live in rivers and streams with a low nutrient value. So they rely on the leaves from natural plants, and are not adapted to consume the leaves of alien trees. What’s more, natural vegetation provides protection from pesticides released during agricultural activities.

Without the natural plants, these pesticides impact on the quality of water, and on the ability of these Fynbos Fish species to survive.

 Many years ago, bulldozing was fairly common in the Western Cape rivers. Many landowners simply didn’t realise the impact of bulldozing on their river systems – nor on their native fish populations. Today bulldozing, while illegal, still occurs. It displaces the Fynbos Fish from their habitat, and often completely removes their natural habitat.

Agricultural pollution is the biggest pollution threat to Fynbos Fish. 

In many instances, rivers and streams in the Western Cape are exceptionally pure, containing low levels of nutrients. Therefore, even a small increase in nutrients will throw the delicate ecosystem off-balance. Algae grows faster than it would naturally, and consumes oxygen in the water. This reduces the oxygen available for the Fynbos Fish species – killing them.

Pesticides are widely used in agriculture – but with devastating effects on our natural fish populations. These pesticides are in many cases very toxic to fish and other aquatic invertebrates. They reduce the life span of fish, can cause male fish to become hermaphrodites (having both male and female sex organs), or result in deformed offspring. Growth hormones used for livestock also often end up in our Western Cape river systems. These result in Fynbos Fish growing up to 50% their natural size.

Recent studies show that native species are more sensitive to rising temperatures and a decreased rainfall than invasive fish species.

And that’s exactly what climate change experts are predicting for the Western Cape in the decades to come (in fact, these changes are already increasingly evident). The research, undertaken by Dr Jeremy Shelton and Dr Helen Dallas, published in the Journal Aquatic Conservation: Marine and Freshwater Ecosystems, found that our Fynbos Fish will become increasingly vulnerable as the climate changes. The predicted loss of surface run-off will place even more pressure on already stressed water resources. And humans and our Fynbos Fish will compete for the same stressed water sources.

The climate change hotspots that they identified include:

– The Olifants Doring River
– The Upper Berg River
– And the Upper Breede River