Red-eared sliders for generations have been the most popular turtles sold in pet stores, and they also have been frequently dumped into the wild — so much so they have become common in some Hawai'i watersheds.

That's both illegal and a potential threat to native stream animals, but people who have brought the turtles home as pets have few options as the animals outgrow their aquariums.

"They don't get sweeter as they get older. They can get snappy," said Christy Martin, public information officer for the Coordinating Group on Alien Pest Species.

Martin recommended that unwanted turtles be taken to a humane society, where they have at least a chance of being adopted.

"There's really nothing else you can do with them. That's the most responsible thing to do," she said.

In years past, the animals were sold as juveniles not much bigger than a silver dollar, but after the turtles were linked to the bacterial disease salmonella, the U.S. Food and Drug Administration in 1975 banned the sale of red-ear sliders less than 4 inches across. Officials thought the bigger size would make them less appealing to buyers and prevent children from putting the baby amphibians in their mouth, reducing the occurrence of salmonella.

Pet stores in Hawai'i and elsewhere still legally sell the sliders, which can reach nearly a foot in shell length. Since that's more turtle than most folks can handle, and since there is little demand for the larger animals, many have been released into the Hawaiian environment.

"That's not a good idea because of the impacts they could have on native species," said Philip Thomas of Hawaiian Ecosystems At Risk, a government-funded organization that supports alien pest control efforts.

The state has listed them an "injurious species" because young turtles could feed on 'o'opu, native freshwater gobies that live in Hawaiian streams.

The turtles, whose scientific name is Trachemys scripta elegans, have a green to brown shell, and their green heads have a characteristic red stripe behind the eye from which they get their name.

They are known to be in Kawai Nui marsh and surrounding streams on O'ahu, have been found in at least two parts of Maui, and are in the Hanalei and Wailua river basins on Kaua'i. But they're probably in many other waterways as well.

"I've had reports from all over the island," said Don Heacock, aquatic biologist with the state Division of Aquatic Resources on Kaua'i.

Horticulturist Keith Robinson recently found a 7-inch turtle walking on a road in Wainiha Valley this week, suggesting they also may be in that valley's stream on Kaua'i's north shore. He said people who saw that turtle recalled having seen others in the Hule'ia River near Lihu'e and in a drainage ditch near Kekaha.

Red-eared sliders are primarily meat-eaters when young and eat a mixture of meat and vegetation as adults, according to Sean McKeown's book, "Reptiles and Amphibians in the Hawaiian Islands." Young red-eared sliders can become carriers of salmonella if they eat tainted meat. Humans can contract the disease from handling the turtles.

When the FDA banned the sale of the smallest turtles, it found that 14 percent of all cases of salmonella poisoning in the United States was associated with the handling of pet turtles. Most of the victims were young children.

Red-eared sliders can live for up to 25 years. They require fresh water, a place to get out of the water to bask, and proper food. As they grow older, they need considerably more room than the average home aquarium provides.

The organization Tortoise Trust urges people not to buy them in the first place, since little thought generally is given to providing care into their adulthood, if they survive.

"There is a massive surplus of unwanted adult turtles looking for good homes, yet thousands of tiny hatchlings continue to be bred each year, making an already desperate humanitarian situation even worse," the organization said on its Web site, tor toisetrust.org.