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  National Geographic 2 Oct 06
Lice From Fish Farms Killing Wild Salmon
John Roach 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 aquaculture—the practice of raising fish in underwater cages or nets or in tanks—is 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.

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.

Changed Ecology

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," he continued.

In the wild, the salmon's migratory life cycle naturally separates adults from juveniles: Most adults are far out to sea when the juveniles swim from the rivers where they were born and into the ocean. As a result, wild juveniles are rarely exposed to the lice, Krkošek says.

But fish farms holding hundreds of thousands of adult salmon in open net pens have sprouted up in the narrow channels and inlets along the salmon migration routes in the coastal waters of British Columbia, Canada.

Clouds of sea lice form around the pens, forcing juvenile fish to swim through them on the way out to sea, Krkošek said. As the juveniles pass by the fish farms, the sea lice attack.

"The farms are changing the ecology of this parasite," he said.

Krkošek led the new study, which used a mathematical model to estimate the impact of fish farms on salmon populations. The model combined data on infection rates from fish farms with the effect the lice have on salmon. The team found that wild salmon mortality due to lice from fish farms ranged from 9 to 95 percent, depending on the time of the year.

Krkošek explains that early in the migration season, the sea lice are less abundant than they are toward the end of the migration season, which is also when the most juveniles migrate past the farms.

"We are erring towards 95 percent [mortality] towards the end of the season," he said.

The researchers say nonfarm sea lice infect some juveniles before they reach the fish farms. But infection rates due to natural encounters with sea lice are limited to about 5 percent of the population and only one louse per fish, they say.

"Once they pass the farms, we are getting [up to] over 90 percent prevalence. Some salmon populations are 100 percent infected, and they have 20, 30, 40 lice each," Krkošek said.

Research Implications

According to Krkošek, other farmed fish may be transmitting diseases to their wild cousins in a similar fashion. If so, ocean-based fish farms may not be the best bet for countering the effects of overfishing.

"This disease mechanism is likely to cause problems wherever aquaculture goes and interferes with natural systems," he said.

Hilborn, the University of Washington fisheries scientist, writes that the large-scale impacts of salmon farming on wild fish populations remain murky. But the data presented "would seem to support an important population level impact."
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