“Young otters are often taken from the wild while their mother is killed trying to defend her litter,” said Paul Yoxon, head of operations at the International Otter Survival Fund, a nonprofit group based in Scotland. “The fact that there are so many newborns available also suggests that traders have no concern as to whether the animals survive or not.”

The otter trade has spread from Thailand, most notably to Japan. According to Traffic Japan, a group monitoring the illegal wildlife trade, a popular television series helped kick off the trend by featuring a pet otter. Social media stars followed up with videos of visits to otter cafes, some of which have gotten millions of views.

“The problem with otters is that just normal people, even my friends, are now interested in keeping them as pets,” said Yui Naruse, a researcher at Traffic Japan. “We have this cuteness culture that is really deeply rooted in Japan, and that plays a strong role in this trend.”

In 2018, Ms. Naruse and her colleagues conducted an online survey and found 85 otters for sale around Japan. Nearly half the retailers claimed that their animals were captive-bred in Japan.

But Ms. Naruse and her colleagues found no evidence of captive breeding in the country, strengthening their suspicion that otters are being smuggled in from abroad. (Japan’s native otter subspecies was declared extinct in 2012.)

According to Traffic’s research, 70 percent of otters seized in Southeast Asia in 2017 were destined for Japan; authorities seized at least 39 otters coming into Japan or bound for the country from 2016 to 2017. In a widely publicized case last October, a Tokyo district court prosecuted two men for smuggling five baby otters into Tokyo from Thailand.