A conservation win: Removing non-native fish from the Thee River


Threatened fish species of the THEE River

The Thee River is a perennial tributary of the Olifants River, draining from the Koue Bokkeveld Mountains in the Western Cape, South Africa. It is a freshwater fish conservation priority, as it contains six of the endemic and threatened fish species of the Olifants River. 



In 2007 predatory non-native Spotted bass were first detected by Craig Garrow (Fynbos Fish Trust trustee) in this river. They had been illegally introduced – and this was immediately recognised as a potential death sentence for the fynbos fish here. Stomach content analysis of the Spotted bass showed that they were indeed feeding on indigenous fish, with 50% of their food made up of insects, and just under 40% made up of other fish. The remainder was made up of crab. (Bear in mind that prey on insects also indirectly impacts on indigenous fish species, who now have less food available for themselves.)

Mechanical eradication of invasive non-native fish had NEVER been completed with success in any South African river prior to this project. And literature was limited on what methods to use.

But in this case, chemical eradication with a piscicide was impossible, because the Critically Endangered Spotted Rock catfish occur here. These are only found in the Thee, and two other tributaries in the Olifants River.

Image above: Clanwilliam rock catfish from the Thee River



In 2010, a project was initiated by Craig and Riaan van der Walt (another Fynbos Fish Trust trustee) to remove all the Spotted bass from the Thee.

First they had to construct a temporary gabion barrier to halt Spotted bass invasion into upstream reaches.

The team then tested various mechanical methods to remove the Spotted bass. Those methods that worked were refined and adopted. 

These included:

– Multi‐meshed gillnets;
– 2m seine nets;
– Hand nets while snorkelling;
– Speargun;
– And an electro fisher.

These were undertaken by a team of two to four people, working around five hours a day (and sometimes at night).

Thee River

The Outcome

During the project, which ran from 2010 to 2014, 399 Spotted bass were removed from the river. Around 300 (75%) of these were captured by chasing them into gillnets or by catching them with hand nets while snorkelling. 

Extensive monitoring since 2015 has failed to detect any Spotted bass in the Thee River and the project has been declared a success – and has been published in a scientific journal. 

But it’s vital that controlling any further spread of Spotted bass here MUST be seen as a conservation priority going forward!

Success Story

What we LEARNT

The Thee River project was a success. But it may not necessarily work in all Cape Fold Ecoregion river systems. Here’s what contributed to the success of this project:

– The water was clear and shallow (this is not the case in all rivers and streams where fynbos fish occur);
– There was an accessible single river channel;
– There was very little instream vegetation, like Palmiet (Prionium serratum);
– Spotted bass were easy to catch;
– And we had a dedicated team helping;
– The installation of a temporary barrier helped stop the upstream movement of the invasive fish;
– A discrete lower boundary protected the intervention area;
– And landowner-support was vital in our removal efforts.

Image above: Removing spotted bass

Extensive monitoring since 2015 has failed to detect any Spotted bass in the Thee River and the project has been declared a success – and has been published in a scientific journal.


Help us protect these ancient species…

many of which are swimming on the edge of extinction.