Sort of like a joke you have to explain, Legal Sea Foods reached out to Boston magazine last week about a blog post we ran (“Legal Sea Foods … Lousy Adsâ€�). Here’s what we got from Ida Faber, director of marketing:

“I think you missed the boat on our ads. You may not like them, but the truth is, they were intended to spark a debate on what fish are sustainable and which ones aren’t. Relying upon a single source of information is not going to provide the nuanced view we believe is necessary to understand seafood sustainability.”

While it’s easy to take issue with some of that statement — the intent wasn’t to sell something? — consider the debate sparked.

According to Faber, Legal Sea Foods sources their farmed salmon from True North Salmon Company, an arm of multinational aquaculture giant Cooke Aquaculture, which operates fish farms in Maine, Canada, Chile, and elsewhere.

Less than a year ago, Environment Canada raided eight of the company’s corporate offices looking for documentation of the use a cypermethrin, a banned pesticide used to wipe out swarms of sea lice that are often found in high concentrations near farming operations. Cypermethrin was determined to be a “probable” cause of a massive lobster die-off in parts of Canadian coastal waters in 2009.

For its part, the Cooke Aquaculture has categorically denied using the pesticide in Canada, and adding frustration, Environment Canada (the Canadian version of the EPA) hasn’t released any findings.

Sea lice remains one of the fish-farming industry’s major problems — infection and escape are the others — but there are few non-chemical options to control their numbers. (The harm is that sea lice can swarm to such numbers that they become lethal to fish; the lice aren’t harmful to humans, nor is cypermethrin harmful to salmon.) There is also mounting evidence that sea lice are developing immunity to government-approved and once-effective pesticides. Other chemicals with make-ups similar to cypermethrin aren’t specifically banned for use in Canadian aquaculture; some critics attribute this to a perceived conflict of interest — Health Canada is both responsible for promoting and regulating fish farming. Cooke Aquaculture defends its use of the legal substance.

Cooke Aquaculture continues to investigate greener ways to control the lice, but most efforts have proven less than effective (p. 30).

And this invites questions that the salmon is as utterly sustainable as Legal Sea Foods claims it to be.

UPDATE: This article was updated to clarify that sea lice and cypermethrin cause no harm to humans, and that Cooke Aquaculture denied the use of cypermethrin in Canada.