Trout impact on distributional patterns of native fish species

Post category:2021 / News
Reading time:3 mins read

20 September 2021 | By Lerato Maimela

A recent fish survey by C∙I∙B PhD student, Lerato Maimela (University of Pretoria) and C∙I∙B Core Team members, Chris Chimimba (University of Pretoria) and Tsungai Zengeya (SANBI), in the headwaters of the Blyde River, Mpumalanga Province, revealed that Rainbow trout (Oncorhynchus mykiss) invasions have reduced the abundance and divided the community structure of native fish species.

For over a century, alien predatory fish such as Rainbow trout have been introduced into lakes and rivers in South Africa for sport fishing and aquaculture. However, most of the habitats that are suitable for trout, such as mountain headwater streams, are also inhabited by endemic and range restricted river minnows. The upper catchments of the Blyde River headwaters are home to the endemic and range restricted river minnow, the Treur River barb (Enteromius treurensis). Populations of these native species historically used to occur throughout the catchment but have since been significantly reduced and fragmented by the introduction of alien predatory fish.

The fish survey revealed that there was low abundance of native species, specifically, the Treur River barb that had 96% of its population reduced in areas were Rainbow trout occurred. In contrast, native species populations were abundant in several sections of the upper catchment that are free of trout invasions. These river sections have managed to remain invasion free largely because of natural biogeographic barriers such as waterfalls that have prevented upstream migration by introduced Rainbow trout, and there are also located in protected areas were human facilitated fish introductions are prohibited.

This finding is consistent with other studies that have shown that introduced alien predatory fish can have a significant effect on fish communities and highlighted the need to prevent human-facilitated introductions in biodiversity sensitive areas such as mountain headwater streams that are inhabited by endemic and range restricted minnows.

“We believe that the further spread of trout into biodiversity sensitive areas such as mountain headwater streams that are currently invasion free and inhabited by endemic and range restricted species should be discouraged,” says Lerato Maimela, “as some of the species are critically endangered and will likely go extinct if no conservation measures are implemented.