for National Geographic News
Clouds of sea lice billowing from fish farms infect and kill up to 95 percent of the wild juvenile salmon that swim past the farms on the way out to sea, according to a new study.
The finding is further evidence that aquaculturethe practice of raising fish in underwater cages or nets or in tanksis dangerous to wild fish populations, according to the researchers.
The fish-farming industry has kept a steady supply of cheap salmon on supermarket shelves as wild salmon populations have crashed in recent decades from overfishing.
(Related: "Salmon Farm Escapees Threaten Wild Salmon Stocks" [June 16, 2003].)
But the farms are controversial. One thorny debate is over whether the practice enhances the spread of deadly diseases to wild salmon populations.
The answer is yes, suggests a new study of Canadian farms, to be published tomorrow in the journal Proceedings of the National Academy of Sciences.
"The results will undoubtedly intensify the debate," Ray Hilborn, a fisheries scientist at the University of Washington in Seattle, wrote in an accompanying commentary.
Sea lice are common on adult salmon. But at 15 to 40 pounds (7 to 18 kilograms) and covered in scaly armor, the mature fish face little threat from the tiny lice.
Juvenile salmon, however, are only about an inch (2.5 centimeters) long and lack scales.
"The lice inflict really severe damage on the surface of the fish," said Martin Krkošek, a mathematical biologist at the University of Alberta in Edmonton, Canada.
"Their feeding activity results in big lesions, puncture wounds, open sores. Eventually the fish die," said Krkošek, who received partial funding for the project from the National Geographic Society. (National Geographic News is part of the National Geographic Society.)
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