After Santiago kills the great marlin in The Old Man and the Sea, by Ernest Hemingway, he lashes it to the side of his boat. The fish is eighteen feet long, longer than the boat, and Santiago knows he will not make good time going home because of the added weight and unwieldiness of the fish. He does not get very far before the first sharks strike the fish.
In keeping with the simile in the quote you mention, the marlin is laid out like a long, narrow trough along the side of the boat. The first sharks to arrive took great chunks of the marlin's flesh; the shovelnose which comes next is approaching a rather tattered and ragged carcass but is undeterred.
The next shark that came was a single shovelnose. He came like a pig to the trough if a pig had a mouth so wide that you could put your head in it. The old man let him hit the fish and then drove the knife on the oar down into his brain. But the shark jerked backwards as he rolled and the knife blade snapped.
The comparison of this shark to a pig approaching a trough is apt because this shark, like pigs, is not discriminating about what it eats. (Remember what is generally placed in pig troughs--at least those which are non-commercial--is all the scraps and leftovers a family has and is called "slops.") Pigs eat anything and they do so greedily, just as this shovelnose intended to do before Santiago killed it. The image of a bunch of pigs vying for a spot at the trough to eat whatever slops are there is appropriate for this and all the other sharks that attack Santiago's fish.
*On a personal note, I find it a bit disconcerting but quite fortuitous to write about this subject while watching Shark Week on the Discovery Channel.