Let's unpack the assumptions:
a) that the fish going into salmon feed would not be consumed by people.
This is a handy assumption and perhaps the market is underdeveloped but I"m pretty sure starving people around the world would NOT turn their noses up at this source of protein, IF given the chance. The real issue is that the MARKET for feeding poor people is not a lucrative one. No government or private agency is stepping up to say "we'll pay for this fish and feed people with it." The market IS there for aquaculture.
b) that we are comparing apples to apples.
In fact, beef don't eat beef. Or shouldn't. Are we talking about industrial feedlot confined feeding operations or pasture-raised beef? Those conversion rates would look quite different, I believe, as would the impacts of those operations on the environment. CAFO farms are environmental nightmares. Downstream waste has been demonstrated as the likely source of at least two e.coli outbreaks. Sustainable pasture farms are not environmental nightmares. In fact, in proper balance, pathogens in the soil behind ruminants (not fed antibiotics to offset the health problems from feeding them corn and soy which they are not evolved to digest) is broken up by chickens. The pathogen cycle is broken.
Also, the data is preliminary but it appears that fish poop actually offsets the impacts of our industrial activity on land and "plays a key role in buffering the carbon dioxide that acidifies seawater." It helps mitigate climate change. But no fish means no fish poop. And less fish means less poop to offset the negative impacts of our land-based industry. http://www.newscientist.com/article/dn16432-fish-an-ally-against-climate-change.html
So the fish that are removed to feed farmed fish, are not able to help absorb the C02 from our emissions, including those of farming.
So are we comparing the type of salmon farming that operates as best-in-class against the worst CAFOs? What would this look like if we compared the much larger and more onerous industrial salmon farms as in Chile as against Joel Salatin style farm? We must be clear about WHAT is being compared to WHAT.
c) that we are feeding the world with aquaculture:
when I asked the salmon farm advocates in NB how they could back that statement, and specifically whether hungry nations are being fed this salmon - the answer, of course, was no. It's a very weak argument that doesn't connect dots but an uncritical ear hears "world hunger" and "salmon farm" as if the latter were the answer to the former. Ask them how. Doesn't compute.
Will aquaculture play a role in feeding the world? Undoubtedly.
Is it now a significant source of developed nations' seafood supply? Absolutely.
Can it be done in more environmentally sound ways? YES.
Is SALMON the best species to farm? Doubt it.
Are open oceans the best way to do it? Probably not.
We must look to set international standards for land-based closed containment aquaculture of the SPECIES that have been successfully grown w/o a NET LOSS to the oceans.
That is bigger and much more complex question than simply "Farmed salmon is better (more efficient) than beef."