drifting from the noses of the cowed, compliant fish. (8)
The fish are trapped in the aquarium, just as the slaves weren't free. The use of "cowed" and "compliant" shows us Lowell is making a connection between the behavior of the fish and the behavior slaves are forced into. We'll never see aquariums the same way again (thanks a lot, Lowell).
Behind their cage,yellow dinosaur steamshovels were grunting (13–14)
This is sorta like the fish. Lowell shows the steam shovels working from behind a cage. This hits on the trapped thing again, and introduces the idea of labor (something slaves were no strangers to). The idea of them grunting adds to the connection of the slaves (a human connection), but it's also pretty silly to picture.
He rejoices in man's lovely,peculiar power to choose life and die— (37–38)
This is a nod to the somewhat positive fate of the soldiers—that they were free men, and were free to choose to fight for their country, and to die for it. The lovely part is that they were free. The peculiar part is that once they were free they gave up their lives. Seems like a pretty steep price to pay for freedom.