Fish are covered in jackets of slime. They have, for obvious evolutionary reasons, relatively few grab-points. In the belly they are convex on all sides so any

Fish are covered in jackets of slime. They have, for obvious evolutionary reasons, relatively few grab-points. In the belly they are convex on all sides so any squirm makes them narrower than your grasp, and they get free. The gills are a concavity that can be grasped.