Why does the shark present a greater challenge to Santiago than the marlin?

For one thing, the mako shark is bigger, stronger, and faster than the marlin. This fact alone would present a huge challenge to even the most experienced of fishermen. But Santiago's attitude toward sharks also creates difficulties. Santiago has a much better understanding of marlins than he does of sharks; he has a deep, abiding respect for marlins. They are noble creatures, worthy opponents of a skilled fisherman. Sharks, however, are just scavengers. Santiago finds it harder to deal with them because he does not feel any natural bond with them as he does with the marlin. He can kill them, but he does not see this as the vanquishing of a noble foe in an epic battle of wills. Instead, Santiago's scrap with the mako sharks is just a brutal, debilitating slugfest, the maritime equivalent of an unseemly street brawl.