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European lake too clean for the fish


Lake Constance, viewed across Mainau, a German island at its western end.

Munich / DPA

Everybody knows that environmental pollution is bad for animals and plants. But in Lake Constance, bordered by three European nations, the opposite is proving true. At moderate levels, some types of environmental pollution are nourishing for fish.
Over the last 30 years, the lake wedged between Germany, Austria and Switzerland has become so clean that the fish are now considerably smaller and fewer in number.
The blame lies with the municipal sewer systems and efficient purification plants around Lake Constance, which prevent faeces and agricultural fertilizer from entering the lake. These contain phosphates — nutrients for plants both on land and in water.
Algae grow on the nutrients, and nourish plankton, which nourish Lake Constance’s most prized delicacy: the whitefish. So the low phosphate levels in the water have resulted in smaller and fewer whitefish.
With less prey swimming in the lake, the number of predatory fish also decreases.The other culprit is diatom algae, which lends mountain lakes their picturesque turquoise blue colouring.
“But the diatom algae break down phosphates,” says Gabi Schmidt, a fish farmer and member of the Bavarian state parliament. Nice for photographs, bad for fish. “I have been on the lake with professional fishermen three times. When you open up a fish, it has nothing in its stomach, nothing in its gut,” Schmidt says. “The fish are starving.”
In the late 1970s, the situation was the other way around: very high levels of phosphate threatened the lake’s ecosystem. Since then, the phosphate levels have been falling consistently, due to environmental regulations.
Professional fishermen are worse off. According to the records of the Swabian Fisheries Association, 2015 saw the worst year’s catch for six decades.The strictest environmental regulations in the whole of Europe are applied to Lake Constance, charges Roland Stohr, president of the Bavarian Professional Fishermen’s Association at Lake Constance.
The few remaining Bavarian professional fishermen caught around 47 tons of fish in the lake in 2015, down from 70 tons in 2011. “No professional fisherman can survive at the current levels,” says Stohr.
“Every year, the catch is reduced by 10 to 50 per cent. In the first three months of this year, it had dropped by another 40 per cent.”
The lack of fish has led to a peculiar phenomenon: the whitefish served in restaurants around Lake Constance are often now sourced from elsewhere. “A lot are imported from Poland or Sweden,” says a Green party representative for the Allgaeu region, UlliLeiner.
The threat to fishing from the lake’s water purity has spurred politicians to action. Some want the permitted mesh size of the fishing nets reduced, saying today’s whitefish reach adulthood at a smaller size than in the past.
“The whitefish now breed earlier, and at a smaller size,” says Leiner. “It would not endanger the fish population.”Another proposal is to release effluent from the purification plants closer to the lake surface so that it creates food for fish.
Currently the treated water is piped into the deep layers of water, and not near to the surface where the whitefish swim and feed. “This way, not one additional milligram of phosphate would be fed into the lake, but the fish would have more nutrients,” says Leiner.The environmental authorities are firmly against deliberately putting more phosphates into the lake.
“None of us wants the lake to become dirty again,” says a conservative lawmaker, Eberhard Rotter. But fishing and the whitefish are very important for tourism at Lake Constance: “We need a solution.”

A German fisherman raises his net at dawn on Lake Constance.

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