Lines 45-55 Summary

Get out the microscope, because we’re going through this poem line-by-line.

I admired the sullen face,
the mechanism of his jaw,

Line 45 tugs at a pretty human feeling. Think sulky, grumpy ("sullen") old man.
But in 46 the fish is back out of the realm of the living, and described as something mechanical.
Notice that the speaker seems to admire both of these features.

and then I saw
that from his lower lip
—if you could call it a lip—

…"and then"! Folks, here comes a turn in the poem.
But line 49's little aside is a moment that drags out the suspense of the "and then" moment.
Lines 48 and 49 is the umpteenth example of Bishop going back and forth between humanizing the fish and describing it as a mere object. First the fish has a lip and then…maybe not.

grim, wet, and weaponlike,
hung five old pieces of fish-line,

The fish has five broken-off (presumably from other close encounters with fishers) pieces of fishing line stuck into his lower lip.
Bishop is really pushing the tough, old soldier image here. This fish has been through a lot.
Can you believe there are more things hanging off this fish? He must be pretty heavy!

or four and a wire leader
with swivel still attached,
with all their five big hooks
grown firmly in his mouth.

Here's Bishop's commitment to accuracy again. She modifies the description to make sure it's clear to the reader.
This fish has not only fish-line trailing from his lower lip, but actual hooks and pieces of fishing equipment grown into his jaw. Tough guy! It's like he has a bunch of piercings.
Can you believe how carefully the speaker is examining this fish?! At this point we have to consider that the speaker is thinking of something other than how good the fish will taste fried with tartar sauce.