The New Seaway Fish Market will reopen its doors Friday morning, but it won't entirely be business as usual for the store, a fixture in Toronto's historic Kensington Market for more than 50 years: All its slippery inventory will be extra fresh.

That's because New Seaway has had to be completely restocked, following a rat infestation that shut things down for several days this week and stirred up a tide of unwanted publicity.

"I'm really shocked," said proprietor Kim Chou, who acquired his fish shop five years ago and concedes he has not looked after it as diligently as he might. "This building is at least 100 years old, and gradually it got damaged. I have to say I didn't really pay attention, that's why the rats found entry to my store."

Story continues below advertisement

The intruders were Norway rats, brown and sleek, and there were probably dozens of them on or around the premises, said Jim Chan, who manages the food safety program for Toronto Public Health.

By no means is the incident a first.

In recent weeks, Mr. Chan's staff has closed three other rodent-plagued food outlets downtown – one in Kensington, another on Spadina Avenue and a third on Gerrard Street – and he explains that what's going on is no great mystery.

It's the rapidly changing weather, coupled with rats' talent for crawling through extremely small holes.

Story continues below advertisement

"When it's cold, rodents such as rats like to burrow into structures, to wherever it's warm," he said. Then, when things warm up, they're on the move again. "It all makes them mobile."

Indeed, just a day later, Wanda's Pie in the Sky, a cafe just around the corner from the New Seaway, was also shut down, besieged not by rats but by mice.

Mr. Chou's problem surfaced on the weekend, when a passerby on Baldwin Street snapped a picture of three rats in the front window, posted it on the social media website Reddit and followed up with a complaint to Public Health.

Inspectors moved in the same day, and not for the first time. Back in November, the New Seaway was warned it had a rat issue that needed dealing with, but was granted a conditional pass and allowed to stay open. The rats, however, multiplied.

Story continues below advertisement

"From the inspector's report, the evidence indicates a heavy infestation," Mr. Chan said.

The main ports of entry were cracks and holes in the west wall, behind the ice machine, Mr. Chou suspects, and since then, along with the pest-control people, he's been fixing things.

So too has Wanda's Pie in the Sky, also expected to reopen shortly.

So how bad is Toronto's problem? Mr. Chan cites a report estimating that for each city resident there may be several rats, which would put the population in the millions.

Story continues below advertisement

"It would be next to impossible to answer that question," said Matt Avery, a technician with Reliable Pest Control, which has been in the rodent-destruction trade for more than 70 years, adding that the most humane way to get rid of them is with poison, which sends them to sleep.

The two most common species, he says, are the Norway rat and the roof rat, which look alike but have different habits.

But a city overrun by rodents? Maybe not.

Last year, Toronto Public Health shut down a total of 34 food establishments city-wide, 45 per cent of the time because of rodents. It sounds like a lot. "But it's not that bad because we inspect over 17,000 food establishments in a year," Mr. Chan said.

Mr. Chou, meanwhile, is looking ahead, anxious to put the rat issue firmly behind him.

Story continues below advertisement

"We found it, we fixed it and we will monitor it," he said.