Lice From Fish Farms Killing Wild Salmon
John Roach
for National Geographic News
October 2, 2006

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.

(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.

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 Krkosek, 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,
Krkosek 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, Krkosek said. As the
juveniles pass by the fish farms, the sea lice attack.

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

Krkosek 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.

Krkosek 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," Krkosek said.

Research Implications

According to Krkosek, 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."

http://news.nationalgeographic.com/news/2006/10/061002-sea-lice.html