Fisheye and Crucible are separate products. They can be run separately, and they can also be run together.
We recommend that you run Crucible together with Fisheye. If you choose to run Crucible alone without Fisheye, you will have access to your repositories via the "light" Fisheye implementation bundled with Crucible. However, a number of Fisheye's advanced features will not be available to you, including pre-caching repository content (for improved performance), the ability to search and browse through repositories and Fisheye's activity graphs.
For more information, please read the following FAQ: What happens if I decide to stop using Fisheye with Crucible