>> All those sites are talking about the desastrous impact of farmed
>> salmon on wild ecosystems. >Thanks, but I wasn't talking about
>> "ordinary" farmed salmon, but about the supposedly "naturally
>> fed" farmed salmon that is sold in some organic food stores here.

i never heard of organic farmed salmon here, and the organic movment
is
very much present in BC.
Organic will mean no antibiotics but will not exclude processed fish
mealand grains .
I will be curious to know how they can manage to avoid antibiotics
with
the number of fish living in concentration pens (they don't have free
range salmons yet) from the Bc salmon farmers association (BC now
produces over 39,000 tonnes of farmed salmon valued at $300 million
per
year-more than three times the total value of the province's
commercial
salmon fishery in 1997. ...If placed side by side, all the salmon farm
sea cages would occupy only about 70 hectares,)
As mathematician you might have a clear idea how crowded those pens
are.
The cost in netting plus protection from predators will make a less
crowded organic production very costly.
Even if they could feed them with their naturel  unprocessed food
(crustaceans and small fishs ) their living condition will affect
their
health ( lack of exercise make them obese ) don't let your self fooled
by the mass production organic food marketting ,
They will soon end up to the same place than normal commercial
practices
with slight flavor of " naturel practices".
What makes the food so unaturel  and unhealthy is the mass production
of
food by a very small number of producers organic or commercial.
The demand for better quality of foods is growing so much (at least in
North america ) that it will be hard to keep strict standards.
Governments
( i should say economic interests)  are very much interested in
swallowing
the organic movment and at setting up more elastic standards.

jean-claude