- Place your fish over the circle (=Arrange the fish to be the top object)
- Make a copy of your circle at the same place, but bring it to the top
- Draw a shape X that can be used as a clipping mask for the top circle. Use it to hide that part of the top circle which is not needed not be visible
The X can be simple, no need to follow the outline of the fish. See an example. There are 2 circles (actually rings); one is behind the fish and one is in front of the fish. The shape X is the rectangle at the left.
X is placed, but not still used as a clipping mask:
X is the clipping mask for the top circle:
As well you can have only one circle. Take a piece of the fish to hide the unwanted part of that circle.
Not asked, but maybe interesting The fish is taken from MS Office clipart. The ring is a little circle which was revolved to a ring using Effects > 3D > Revolve. See the settings (the used circle is visible as thin blue outline in the middle):