Today comes news that a new study has more convincingly than ever before demonstrated that salmon farms are a threat to wild salmon.

The study published today in the peer-reviewed Proceedings of the National Academy of Sciences found that sea lice, a salmon parasite, escape from adult fish kept in net pens along the coast of British Columbia and infect young salmon.

The sea lice are naturally occurring. What's not natural, the researchers found, is the mixing of adult fish with juvenile fish on their way out to sea. In the natural order of things, the adults would rarely or never be in contact with small fish going out to sea.

For a big fish, a few parasites isn't that big of a deal. But for the tiny fish headed for sea, which may be only a few inches long, the introduction of several parasites that can grow to perhaps half an inch each can be lethal. The parasites open sores in the young fish's skin, for starters. But just imagine carrying around something that's a fair-sized fraction of your body size inside your guts. Reminds me of Alien.

And it's long been known that the sea lice can jump from farmed to wild salmon, but the aquaculture industry has always maintained it would have only a minor effect. This study led by scientists at the University of Alberta's Centre for Mathematical Biology found that they can kill up to 95 percent of the young pink and chum salmon headed out to sea.

From Martin Mittelstaedt's account in the Globe and Mail:

The research found that the number of juvenile salmon killed by the sea lice increased over the migration season, rising from a low of 9 per cent early in spring when parasite populations were low to the 95 per cent figure in late spring when the area was teaming with lice.

The results were based in part on counting the sea lice on more than 14,000 juvenile salmon, fish that were only a few months old and only a few centimetres long.

Other good stories came from Jeff Barnard at the Associated Press, Cornelia Dean at the New York Times, Canwest News Service's Keith Gerein and CTV.ca.

My colleague Joel Connelly, a columnist, weighed in on the fish farm debate here a while back.