The deformities in whitefish from Lake Thun in the Bernese Oberland are most likely caused by the plankton they feed on, a five-year study has found.This content was published on May 7, 2008 - 18:12
Around 40 per cent of the fish show abnormalities in their reproductive organs, on a scale and in a variation that has not been seen anywhere else in the world, the authorities said on Wednesday.
Nearly two-dozen studies were conducted to get to the bottom of what has been causing the deformities.
The experts have been confronted with fish with subdivided organs, missing testicles or ovaries, reproductive parts protruding through the stomach, and in some cases androgynous samples.
Of greater concern to the authorities was the extent the lake might have been contaminated, with possibly serious consequences for the population of the region.
More than 400,000 people around the capital, Bern, drink water from the lake or the bodies of water downriver from it.
"We didn't know five years ago if the deformations were caused by organic matter, hormones or something else harmful to humans," Peter Friedli of the cantonal fishery department told swissinfo.
It was first feared that chemicals leaking from thousands of tons of army munitions dumped in the lake between 1940 and 1963 could be causing the changes to the stocks of whitefish.
The water was also tested for abnormally high concentrations of other substances, water temperature changes and disease. And the spawning grounds have been analysed for genetic alterations.
A controlled breeding programme returned the only positive result. Researchers fed plankton from the lake to some fish and fodder to others.
The fish receiving the plankton developed abnormalities to their reproductive organs on a scale similar to wild fish in the lake.
However, since plankton consists of thousands of small plant and animal organisms, experts are still swimming in murky waters.
"Each species of the whitefish in the lake eats different animals in the plankton," Friedli said, explaining that for this reason it will be extremely difficult to pinpoint the source of the problem.
"We now know that something happens to the plankton eaten by the whitefish and this is the reason for the deformations, but we don't know how this happens.
Needle in haystack
"We haven't found the needle in the haystack yet, but we're a lot closer," Friedli summed up.
No significant fluctuations to the number of deformed fish have been reported in the eight years since the problem first appeared.
The authorities ruled out that abnormalities were present before 2000 since samples from fishermen's catches have been analysed on a monthly basis since the 1980s.
On a positive note, there has been no influence on the fertility of the fish and therefore there has been no change to the size of the population of whitefish, the most common variety in the lake.
And it can still be eaten without fear of any harmful side effects – good news for lovers of fish dishes and the handful of professional Lake Thun fishermen who live from their catch.
swissinfo, Dale Bechtel in Spiez
The whitefish is the most common variety in Lake Thun and accounts for around 90% of the catch of the professional fishermen working on the lake.
400,000 inhabitants of canton Bern get their drinking water from the lake and the rivers and bodies of water downstream.
Even though the five-year whitefish study has come to an end, some research projects are ongoing.
For example, scientists will carry out further controlled breeding programmes, continue to investigate lake sediments and the presence of organic chemicals.
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