My 2 labs have started chasing “game” while on walks. What could we do to stop this?

I have two beautiful three year old labs. They are not trained except for the basic commands. I walk them in an open mesa area near my home every morning. The problem is they have begun not only chasing young rabbits, but when they do catch one they pick it up and clamp down, thus causing internal organ damage, which of course kills the rabbit. I am a nature lover and very upset by this behavior! I have thought of getting a shock collar and using this devise to get them to quit chasing the rabbits when they get too close. Do you have any thought on this?

Thanks,
Judy