In the first four stanzas, the speaker describes the pike as a perfectly ruthless species, "a life subdued to its instrument," functioning as a ruthless animal because of, and according to, its design. The speaker notes having three pike in an aquarium and finally having only one because they had eaten each other. Noting, "indeed they spare nobody," the speaker describes an image of a dead pike on the shore ("high and dry") which had been eating another pike.
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.
In the last four stanzas, the speaker talks about fishing for pike in a pond. Recalling these images of the ruthless pike, the speaker is almost too scared to fish. The poem ends when a pike slowly rises to the surface and gazes back at the speaker, thus reversing the gaze from speaker-pike to pike-speaker. This is significant because it equalizes the two: pike and speaker. While humans tend to believe that they are above and better than all other species in the animal kingdom, this suggests that all, humans included, are equal.
Throughout the poem, the speaker marvels and is horrified at the pike's ruthless nature; yet, the speaker has been equally ruthless in taking pike out of its natural world and imprisoning them in a glass aquarium and, in the end, even more ruthlessly, fishing for the pike. Thus the speaker (the human) is also capable of living a "life subdued to its instrument." That instrument could be the fishing rod or, more generally, the instrument is the human mind/nature and the human tendency to feel superior to other animals and therefore feeling justified in imprisoning and killing them. This is perhaps what the pike feels in its own ruthlessness. And in that final reversal of gazing between the pike and the speaker, maybe the speaker recognizes that the two are not so different.