Stilled legendary depth:
It was as deep as England. It held
Pike too immense to stir, so immense and old
That past nightfall I dared not cast
But silently cast and fished
With the hair frozen on my head
For what might move, for what eye might move
The still splashes on the dark pond,
Owls hushing the floating woods
Frail on my ear against the dream
Darkness beneath night's darkness had freed,
That rose slowly towards me, watching. [1960; pp.85-6]
That 'deep as England' is the key: this fish is the grotesque pre-ichthus that lurks beneath Christian England. That sense of this country as a place where the sleep of trees produces monsters (owls hushing the floating woods). Don't stir up the ancestral waters; you won't like what you rouse.
There's context, too: Hughes's 1960 'three-inch long' pike has a 'silhouette/Of submarine selicacy and horror./A hundred feet long in their world'. HMS Dreadnought, Britain's first nuclear-powered submarine was 265 feet long, and was launched by the Queen on Trafalgar day in 1960. What secret business was it about? Hush those floating woods ... don't tell them, Pike.