If you are considering releasing your captive turtle or tortoise, please consider the following:
Clearly no turtle or tortoise should be released if the same species is not native to the area being considered for release.
Our understanding is that both the captive and the wild population might be at risk if you release a turtle or tortoise that has been in captivity for very long.
Depending upon how the captive has been housed, the captive might not have a fully developed immune system. Turtles kept indoors usually do not. Even if kept in a natural, outdoor habitat, it may not have been exposed to the diseases that are common in the native population and may not have developed immunity to them.
The wild population may be put at risk by a released captive. Captive turtles may harbor diseases unknown to their keepers. Clearly the wild population can be hurt by releasing such an animal. This appears to have happened with populations of the Desert Tortoise.
Studies have indicated that released box turtles seldom do well.
From Dr. Dave McShaffrey, Associate Professor of Biology, Marietta College:
Growing scientific and anecdotal evidence suggests that releasing [box] turtles into different populations can be deadly to those populations. Even apparently healthy turtles can harbor diseases and parasites that other populations of turtles might not have a defense against. Remember what happened to Native Americans after Columbus arrived? If you find a turtle in the road, the best thing you can do by far is move it to the side it was heading.