Homicidal habits of trout in focus

Serra Moz

Sealiner
RAINBOW trout, introduced in 1897 for sport-fishing, have invaded streams and rivers in the Western Cape where they are making a meal of some of our threatened native fish, new research shows.

In some headwater streams – the uppermost reaches where rivers begin – of the Breede River catchment, rainbow trout appear to have gobbled up native fish to the point of elimination. Because of this, researchers suggest trout be eradicated from headwater streams where they directly threaten native species.

This research follows the Department of Environment’s inclusion of rainbow and brown trout as invasive aliens in draft regulations, which caused an outcry among the trout-fishing fraternity and trout industry. The Federation of Southern African Flyfishers demanded the government “prove... trout are invasive”.

Now the work by Jeremy Shelton and Michael Samways of the University of Stellenbosch and Jenny Day of the University of Cape Town appears to have done just that.

Published online by Springer International, the research says rainbow trout (Oncorhynchus mykiss) , which were widely introduced into the Cape Floralistic Region, “may pose a serious threat to endemic freshwater fishes in the region”.

Lead author Shelton said the severe negative impact of rainbow trout on native fish was well known globally and it had been listed by the World Conservation Union Global Invasive Species Programme as one of the world’s 100 worst alien invasive species. However, quantitative information about their impact in South Africa was scarce, hence the reason for the research.

The species introduced here is native to eastern Russia and western North America.

Researchers say native fish in the Western Cape are highly threatened for several reasons, but being preyed on by alien fish is the greatest threat.

The study looked at 24 headwater streams of the Breede River catchment, 12 of which had been invaded by trout and 12 which had not. Three species of native fish were recorded: redfin, Cape kurper and Cape Galaxias. The fish populations and many environmental variables were measured. The environmental conditions showed no consistent differences between the streams with or without trout.

However, researchers found an absence, or low abundance of, native fish in the streams with trout, 89 percent to 97 percent lower than the streams with no trout. Their results showed that trout might be capable of “completely eliminating native fish populations” from the headwaters of rivers in the Cape Floralistic Region.

“Our results indicate that in the headwater streams draining the upper Breede River catchment, native fishes appear to have been eliminated from sites with high-density trout populations,” Shelton wrote.

They also found low numbers of native fish in streams with low numbers of trout; and where native fish did occur with trout, there tended to be no small-sized individuals.

The researchers said the findings were important because the headwater streams were essential refuges for native fish populations. Their continuing to be refuges might depend on preventing any trout being introduced in these streams.

“The eradication of trout from headwater streams where their impacts are unacceptably high... should also be considered” Shelton said.
 
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