A consumer advocate says he's shocked about an alleged "loophole" in Canadian law that punishes grocery stores for changing a packaged-on date on perishable foods -- but allows grocery stores to change a best-before date.

Bruce Cran of the Consumer Association of Canada says the dual rules give too much leeway to grocery stores like the Real Canadian Superstore to change the marked dates, which may be misleading to some customers.

"This shouldn't be allowed to happen," Cran said. "Of course this is a loophole and should be closed as soon as possible."

Cran was reacting to a Coquitlam Superstore employee caught on video in a CTV News investigation changing the best-before date on food packaged to be "Fresh Atlantic salmon," moving it forward by four days.

It wasn't the first time -- on another visit to the same store, another employee was seen on cell phone video doing the same thing.

Cran maintains that no store should ever change a best-before date on its food. "I think they should apologize to the public very quickly and cease this procedure immediately," he said.

But grocery stores face different punishments based on the type of durable life date system the store uses.

There are two kinds of durable life labelling allowed by law in Canada for goods that are expected to expire in less than 90 days: a packaged-on date and a best-before date, said Ken Randa of the Canadian Food Inspection Agency.

Packaged-on dates refer to the date that a perishable food item was packaged, and to change that date would result in fines under the Food and Drugs Act.

"You can't change a packaged-on date," Randa said. "That's in legislation; that's cut and dried."

For example, a Superstore in Langford was caught changing the packaged-on date of several packages of New Zealand lamb. It was fined $20,000 per count, to a total fine of $100,000.

But a best-before date does not refer to a concrete date, said Randa. Instead, they are seen more as "best guesses" of when a product should be consumed. That can change based on the conditions the food is stored in, he said.

"There's no legislation that they can't," said Randa. "There's so many parameters involved in the handling. While we say you have to have a date we don't say how you determine that date."

Randa said the agency didn't consider that a loophole.

"Both systems give the Canadian consumer the same information so they can make a judgment call on how fresh their food is and whether they want to purchase it," Randa said.

Cran said he would lobby the agriculture ministry to change the rules.

Agriculture Minister Gerry Ritz referred calls from CTV News back to the CFIA. The Real Canadian Superstore emailed a statement to CTV saying the company's policy is to maintain original best-before dates.

Shoppers at the Coquitlam Superstore said they expected best-before dates would remain unchanged. Janet Walker told CTV News that she expected a retailer who was changing dates on packaging would be dealt with in court.

"The law is there to protect us, as a consumer," she said. "They should fix the law if there's a loophole in it -- very simple."

Watch CTV News at Six for a full report from Mi-Jung Lee