"Pike" is a poem which presents visual images that would make many people uncomfortable. That is his intention. People seem to forget that the earth is inhabited by other creatures and that, without them, mankind would not survive. Thus, why behave so irresponsibly towards these other creatures as if all others must submit to humankind?
Not the Pike! The Pike is "perfect" in his own environment and they even "dance on the surface." They are obviously majestic in their own world - "A hundred feet long"- and are threatening creatures with "The jaws' hooked clamp and fangs...." In stanza five as the fish eat one another.
Three we kept behind glass,
Jungled in weed: three inches, four,
And four and a half: fed fry to them-
Suddenly there were two. Finally one
There is a matter-of-fact attitude to this revelation. This is what they do. The short descriptions and the phrase, "Finally one," leads the reader to conclude that this is nature. The reader was warned at the beginning that these fish are "Killers from the egg," so those that survive are to be admired.