If the only reason you're thinking of capturing and returning the turtle to
the turtle pond is the food issue, I wouldn't give it a second thought. I,
too, am fanatical about "taking care of" every living thing that ends up in
my pond and make sure (to the best of my ability) that they have everything
they could need or want. But I don't think the turtle will starve to death,
nor even go hungry. There is SO MUCH in a pond that a turtle can (and
should) go after himself, as only he knows what his body needs. Unless he
was a "pet" from a hatchling, I'm sure his instincts are still alive and
well and his survival skills are strong. Heck, even if he WAS a pet from
very young, I think they instinctively know what to do. and if you're still
worried, just make sure you toss some food his way whenever you see him. My
turtle LOVES chicken, so maybe throw in some chunks that are too big for the
koi to steal? It's amazing how big a chunk a turtle can bite onto and then
tear away at with his front claws. And if he's NOT finding enough to eat
there? He'll move on, I'm sure.
Good luck, whatever you decide to do.
Sue