ok, you have to have animation first... You set keys for your dolphin, causing it to move... right?

You will want to render however many frames your dolphin is moving. However, keep in mind that how quickly the animation passes is dependant on how many fps you have it set for. Default is usually 24fps or 30fps.

So, for example, if you have your dolphin keyed to move in an undulating circular motion for 15 frames, it'll be moving lightning fast, as that's just half a second.

Good luck!