In answering this question you will want to focus on the way that the pike is presented as a ruthless, terrifying predator, that even is able to scare the speaker himself. Consider how the pike is introduced in the first stanza:

Pike, three inches long, perfect 
Pike in all parts, green tigering the gold. 
Killers from the egg: the malevolent aged grin. 
They dance on the surface among the flies.

There is somthing about the alliteration of the "p" in "Pike" and "Perfect / Pike" and the way that it is separated by other words and then the line break that gives a vaguely menacing tone to the fish. In addition, note how the pike has "green tigering the gold" of its colour, comparing it to another violent creature, the tiger. The third line assures us that all pike are "Killers from the egg." They are born to be violent killers, and the way that the pike is described as having a "malevolent aged grin" likewise serves to present it as a figure that inspires horror in its victims and those who look upon it.

Perhaps the most terrifying imagery that is used to present the pike is in the final stanza, where the poet himself goes fishing, describing how his hair is "frozen on my head" at the thought of what he might come across. The final line of the poem imagines the pike watching the human as an invader in his territory and presents the pike as a menacing, violent and deadly entity that is not deterred even by the strength of humans:

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.

The image of the pike rising towards the speaker, coldly watching him calculatingly ends the poem on a note of fear and terror.