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 (map of British Columbia).
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.
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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