The overwhelming way in which nature, through the creatue of the pike that this excellent poem focuses on, is presented as a dangerous, scary and terrifying place. The pike itself is depicted as a relentless predator, that will continue to follow its murderous instincts even in death, as the following image shows:
Two, six pounds each, over two feet long
High and dry and dead in the willow-herb-
One jammed past its gills down the other's gullet:
The outside eye stared: as a vice locks-
The same iron in this eye
Though its film shrank in death.
This memory that the poet has of coming across two dead pikes, who had fought each other to the last, coupled with his experiment of putting three pikes in a tank and seeing how, one by one, they disappeared until only one remained, "With a sag belly and the grin it was born with," serve to create a menacing, frightening impression of nature. The last few stanzas show how the pike is not only a violent predator in its own world as the poet himself feels almost like an intruder as he fishes for pike:
Owls hushing the floating woods
Frail on my ear against the dream
Darkness beneath night's darkness had freed,
That rose slowly toward me, watching.
He is scared to fish for the pike yet he is unable to stop himself, but all the time he imagines this ancient and ruthless predator watching him and his pathetic attempts to fish with his violent eye. The lingering image of the pike coldly and cooly watching the poet in a calculated way serves to consolidate the presentation of nature as being a violent force of ruthless and calculated predatory instinct.