Authorities on a remote island in Japan where there are more cats than people appear close to solving the mystery behind a dramatic drop in its feline population.

Animal welfare groups voiced concern that foul play was responsible after the number of cats on Umashima, an island of 30 people about 10km (six miles) off the south-western city of Kitakyushu, fell from 90 in 2014 to just 30 this year.

Fears that the island’s cats had been poisoned rose after residents found slices of fish laced with a mysterious blue substance in multiple locations, including agricultural land, and said they had seen several stray cats in physical distress.

In recent years, once-healthy cats were seen foaming at the mouth and collapsing, while in 2017, five animals were found dead or close to death near the island’s harbour, the Mainichi Shimbun reported.

Umashima has always been home to a large feline population, although it is less well known than Aoshima, an island in Ehime prefecture whose 100-plus cats outnumber human residents by a ratio of six to one.

In 2014, a local animal welfare group neutered 79 of the 90 cats on Umashima amid complaints about the smell and the nuisance created by the growing feline population.

That, however, did not explain the sharp fall in the cat population, according to Kunihisa Sagami, director of the foundation that neutered the cats. “It’s not a normal decrease, and there’s no doubt that an outside, human element, such as animal cruelty, is causing it,” he told the Mainichi.

Days after the deaths drew media interest, a Japanese TV network this week appeared to have solved the mystery, interviewing a local man who said he had placed fish laced with an agricultural chemical in his fields to stop crows eating his potato crops. “It was never my intention to harm any cats,” he told the local RKB News channel.

Sachie Yamazaki, a member of Scat, a local nonprofit group that campaigns against cruelty to animals, said police were analysing samples of the fish to see if it contained poison. “There aren’t that many crows on Umashima, and the food found in fields was clearly intended for cats,” Yamazaki, who visited the island last week, told the Guardian.

Animal welfare activists reportedly considered evacuating the remaining cats until their safety can be guaranteed, while local people voiced anger over the possibility that the cats had been harmed deliberately.

“I feel angry and sad. If someone is responsible for this cruelty, I really want them to stop,” a resident told the Mainichi.