Two short stanzas, and a small drama of hunting is already folded inside them. An eagle has taken hold of something, and the poem's "possessed" can be read two ways: the eagle has seized its prey, or the fish itself seems to be in the grip of a force it can't see. The line "far beyond the sand" suggests we are looking at a scene that has moved well past the shore, out into open water or air.
The second stanza shifts to the fish, which "goes on." That phrase is odd and a little haunting. The fish seems to keep moving, unaware or unable to stop, "lost in too much nothing." The speaker doesn't describe fear or violence. Instead the tone is hushed and detached, almost like watching from a distance. The mood is eerie and quiet, a feeling of vast emptiness in which something small is carried along by something much larger. The poem leaves us to decide whether that is tragic, peaceful, or simply how nature works.