Endemic fish threatened by predatory impacts of rainbow trout

Post category:2014 / News
Reading time:3 mins read

23 June 2014 | By Jeremy Shelton

Rainbow trout (Oncorhynchus mykiss) is among the most widely introduced fish in rivers of the Cape Floristic Region, and may pose a serious threat to endemic fish such as redfin minnows, members of the genus Pseudobarbus, which are a unique and vulnerable group of freshwater fishes. In the Cape Floristic Region alone, there are twelve species of redfin minnow, some of which live in only a single river or stretch of river. Nine Cape minnow species face a serious threat of extinction by introduced predatory fish, such as trout.

Jeremy Shelton sampling fish in the headwater streams of the upper Breede River catchment (Photo credit: Dr MS Bird)

In a recently published article, C·I·B researcher Jeremy Shelton with colleagues at the University of Stellenbosch and University of Cape Town, used a combination of comparative and experimental approaches to put a figure to the predatory impacts of trout on native fish populations in the headwaters of the upper Breede River catchment. Jeremy found that densities of Breede River redfin (Pseudobarbus burchelli), Cape kurper (Sandelia capensis) and Cape galaxias (Galaxias zebratus) were 89–97% lower in streams invaded by trout, than in streams without trout. The native fish were not found at all in seven of the twelve invaded sites that Jeremy searched. However, native fish were found in all of the sites that had not been invaded by trout. It seems that the smaller, younger fish are hardest hit, too, and native fish of ≤ 40mm in length were seldom found in trout-invaded streams, but were abundant in streams without trout.

According to Jeremy, “This study found evidence that trout have depleted the number of endemic fishes through size-selective predation. Managers should prevent new trout introductions, and should consider eradicating trout populations where they pose a threat to already vulnerable native species.” This project is one of the first quantitative assessments of native versus invasive fish presence and numbers, and will provide biodiversity managers with reliable information for decision making.