Andrew Marttila An orange cat on Ainoshima, one of several "cat islands" in Japan.

Animal photographer Andrew Marttila says when he first heard about Japan’s “cat islands,” he knew he had to go.

He’s best known for his portraits of felines and his partner, Hannah Shaw, founded the cat rescue group Kitten Lady, so it made sense for them to plan a trip.

“Cat islands” — places where populations of free-roaming kitties boom in the absence of predators — have drawn growing numbers of international feline-loving tourists for several years. Photos of huge groups of cats ― like these images from an island called Aoshima ― periodically go viral and fueling interest in visiting the region.

Japan has 11 cat islands in all, typically the result of fishermen bringing cats to shore to control rodent populations.

Andrew Marttila Two cats on Ainoshima.

In November, Marttila and Shaw traveled to Ainoshima, an island off the city of Fukuoka that measures a little more than a half a square mile, inhabited by around 500 people and hundreds of outdoor cats. The place is known as “Cat Heaven.”

But what the couple found was a little more complicated.

“For us cat lovers, there’s something pretty special about an area littered with dozens of cats,” Marttila told The Huffington Post in an email. “What you’re not seeing, however, are all the cats and kittens suffering from very treatable illnesses.”

Lack of spaying and neutering, plus an abundance of food provided by tourists, contributes to a growing cat population. But with no veterinarians or real framework for their care, this leads to what Shaw, writing for Paw Culture, called “a constant cycle of birth, early death, and more birth.”

Andrew Marttila Cat rescuer and advocate Hannah Shaw holds a young cat on Ainoshima.

She wrote that while the cats that survived to adulthood seemed healthy, there was a high mortality risk for the younger felines.

“Roughly one-third of the cats were young kittens struggling with untreated upper respiratory infections,” Shaw wrote.

“Eyes and noses crusted, the kittens huddled together on the warm pavement.”

Andrew Marttila A kitten apparently suffering from an upper respiratory infection on Ainoshima.

A 2014 Japan Times article gave a similar account, quoting Japanese cat scientist Akihiro Yamane as observing many of the cats die in kittenhood, and adult males suffer brutal injuries over fights for mates and territory on the crowded island.

Marttila said that people on the island were resistant to the idea of veterinary care and were “more keen to allow nature run its course.”

And while he acknowledges that yes, it’s natural for animals outside to get sick and die, the island is teeming with cats in the first place only because of human activity. As Shaw wrote in her essay, “human intervention is already impacting the growth of the population, just not in a way that benefits anyone.”

Andrew Marttila A cat apparently taking a bath on Ainoshima.

Marttila also stressed that he did not mean to criticize Japan in particular, since the United States is rife with its own animal welfare problems. And not all Japanese cat islands are the same. On Tokonoshima Island, for instance, which is home to around 3,000 cats, the government is implementing a “trap-neuter-return” program. This involves humanely trapping the cats, neutering or spaying them and giving them necessary vet care and then returning them to their outdoor homes.

Spaying and neutering cats cuts down on overpopulation and curbs stressful behavior like fighting and mating. On Tokonoshima, officials started the program in part to protect an endangered species of rabbit threatened by the cats.

When it comes to cat islands, Marttila believes potential visitors should be aware of what they may be getting into.

“Just be prepared to see the full gamut of beautiful to utterly depressing,” he said. “Having a more realistic expectation of what occurs on Ainoshima would have better braced me for the experience.”

You can see some of Marttila’s photos from the island below, and view more of his work on his Instagram and website.

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

Andrew Marttila

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Correction: A former verison of this story called Fukuoka a “town,” but it’s a city.