The verse paragraph uses metaphors. The metaphors help use to visualize the shape of the tuna and how it coarses through the sea.
The third verse paragraph compares the tuna to a "dark bullet", and a "sea-javelin" showing us how streamline and fast it is.
The word "barrled" is a metaphor that shows us the tuna is accelerating like bullet barreled out of a gun. The phrase "locked into the current" says that the fish is moving with the current of the sea and cannot escape it.
In conclusion, the third verse paragraph talks about how swift and aerodynamic the tuna is. It also talks about how free-flowing the tuna is.
Only you:
dark bullet
barreled
from the depths,
carrying
only
your
one wound,
but resurgent,
always renewed,
locked into the current,
fins fletched
like wings
in the torrent,
in the coursing
of
the
underwater
dark,
like a grieving arrow,
sea-javelin, a nerveless
oiled harpoon.