Feral cats, who raid garbage cans and poach food from outdoor restaurant tables, have for years been a nuisance across the Big Island.

But two nonprofit groups are fighting back, and they say their efforts are working.

The Hawaii Island Humane Society and AdvoCats are spaying and neutering the animals to control their population and protect domesticated cats from diseases such as feline AIDS and leukemia.

"We have a terrible feral cat problem," said Kona Councilwoman Brenda Ford, who last year spent more than $10,000 of county money to help control the feral cat population. "You have all these scrawny, sickly cats around commercial areas and hotels. It's just a bad situation."

Feral cats are the offspring of lost or abandoned pet cats who are not spayed or neutered and are estimated to number between 500,000 and a million on the Big Island.

The Humane Society last month ran a spay and neuter campaign ahead of the year's first kitten season.

Clinics in Kona and Keaau performed surgery on 259 feral cats and 120 domestic cats, said Executive Director Josi Morgan. Feral cats that tested positive for infectious diseases were euthanized.

Morgan said the number of kittens turned in to the shelter has dropped significantly since the society began its spay and neuter program three years ago.

Islandwide, the Humane Society euthanized about 10,000 feral cats during the 2006-07 fiscal year at a cost of more than $30,000, she said.

In the nine years AdvoCats has been fixing felines, Karen Klein, the group's president, said it has spayed or neutered more than 5,000 animals at clinics across the Big Island.

"I've seen colonies just disappear," she said.

Klein said the group of about 30 members relies on donations to run spay and neuter clinics.

Mainland veterinarians occasionally volunteer their time while visiting the island on vacation, she said.

"It's just an ongoing job," Klein said. "Our local vets are overwhelmed."

AdvoCats has also helped several resorts start feeding programs in an attempt to keep the hungry animals from roaming the resorts' grounds for food.

At least a half dozen resorts along the Kona and Kohala coasts have adopted the colonies, spaying and neutering them and setting up dedicated "kitty cafes" with regular feedings.

"They are really happy with these programs," Klein said. "Some of the hotels are even telling their guests when the feeding times are so they can come out and watch. People don't realize how useful the cats can be, controlling rats, mongoose and other rodents."