As the Tokyo Metropolitan Government works with industry groups to finalize the date for relocating the famed Tsukiji fish market next year, it is also mulling how to exterminate the hordes of rats infesting the current location.

“Rats won’t be spotted in the morning when the market is packed with people. … But they come out when it’s quiet, like on days the market is closed,” said Takao Katsumi, a metro government official in charge of inspecting the Tsukiji fish market.

The popular tourist attraction in Tokyo’s Chuo Ward has long been a home for rodents that feast off ample fish scraps and nest in the many nooks and crannies.

According to the metro government’s estimate in 2015, about 500 rats inhabit the market. The figure does not include the market’s seafood area where rodents are frequently spotted, so the actual total is probably much higher.

As the clock counts down to the razing of the world’s largest seafood market, the metro government and Chuo Ward are preparing to catch and exterminate as many rats as possible to prevent them from spreading and migrating to neighboring areas where high-rise apartment buildings and eateries are located.

“We’ve never experienced the closure of a facility this big. So it’s hard to even predict what could actually happen,” Katsumi said. “We need to step up our rat-control operations.”

When a building is demolished, rats usually move to neighboring areas, according to Tatsuo Yabe, head of the research group Nezumi Kujo Kyogikai (Rat Extermination Association). For example, when a former metro government office in Tokyo’s Marunouchi district was closed in 1991, the number of rats in neighboring buildings surged, he said.

Thus the rodents at Tsukiji — mostly brown rats — will probably migrate to neighboring areas if eradication efforts fail, Yabe warned. Those rodents can easily reach the upscale Ginza district, located only a few minutes’ walk from the market, he said.

“They may roam in nearby areas for a while, possibly for a year or so,” and will only settle when they realize there is enough food to feed all of the newcomers, said Yabe, who is also the author of “Nezumi ni Osowareru Toshi” (“Cities Attacked by Rats”). “It could coincide with the 2020 Tokyo Olympics, when many foreign tourists are expected to visit Ginza.”

One of the biggest concerns is that growth of the rat population may usher in infectious diseases, including leptospirosis spread by rodent urine either through direct contact or contaminated soil or water, Yabe said.

The bacteria sometimes enter human bodies through the skin or the mouth, causing headaches and fever, or more serious conditions such as jaundice or kidney failure, according to the National Institute of Infectious Diseases.

Leptospirosis was known as a deadly disease up until around the 1970s, when the nation’s overall public hygiene was worse than current standards.

Unlike black rats — the most prevalent species infesting houses and buildings — brown rats are bigger, filthier and more aggressive, feeding mostly on meat, Yabe said. “They sometimes even bite babies, enticed by the smell of breast milk.”

Brown rats — measuring around 22 to 26 cm in length— are commonly known as sewer rats because they scurry through sewage and dirty wet places, so they carry lots of germs on their feet, he said.

“And, like any other rats, sewer rats could multiply rapidly if the living environment is good. They give birth after only three weeks of pregnancy,” Yabe said.

To reduce the number of such unwanted pests, the Tsukiji Market Association has regularly conducted rat-sweeping operations during the Golden Week holiday in May and the Bon period in August, when the market is closed.

Chuo Ward, which oversees the Tsukiji outer market, which is packed with eateries and fish shops, also conducts rat-eradication operations usually during the fall or winter, catching around 300 per year, according to a ward official.

But they said they don’t have a clue on how effective those measures have been to reduce the rat population.

“Many people asked us how effective the measures were,” the Chiyoda Ward official said. “But to be honest, we don’t know, because we have no idea how many are out there. If the total number is 500, then I believe we can say it was pretty effective to catch 300 of them. But we don’t know that.”

As the only way to prevent the rats from spreading is to exterminate them, the metro government and the ward are planning to conduct intensive rat-eradication operations around the period when the market will be demolished.

The ward already has 83,000 sheets of glue traps purchased last year for distribution to local residents in preparation for the market relocation, initially scheduled on Nov. 7, the official said. It also has around 240 trap boxes.

Rat control operations will include placing snap traps inside sewage and setting trap boxes near garbage disposal sites or shrubs. Poisonous bait may also be thrown into rats’ nests.

The good news is sewer rats are more likely to take poisonous bait than are cautious black rats, according to Yabe.

“I recommend using both poisonous bait and traps, because rats are more likely to be trapped when they are poisoned,” Yabe said. It is also easier for exterminators to find and retrieve dead rats if they are in traps, he said.

Katsumi of the metro government said it will continue its efforts to catch rats before, during and after the expected demolition to minimize the impact to neighboring areas.

“It’s difficult to completely exterminate rats,” Katsumi said. “The important thing is to keep their number down as much as possible and to prevent rampant breeding. … We have to create an environment difficult for rodents to survive.”

The metro government hopes to relocate the market to the Toyosu site in Tokyo’s Koto Ward between September and October to avoid the sweltering summer season when food goes bad quickly. It plans to start demolition of the market in October, soon after the relocation.

The vacated site will be slated for use as a logistics base for the 2020 Tokyo Olympics.