after he has returned home, when he has another dream of the lions. I think that the dream combined with his battle with the fish shows readers that Santiago might be old, but he still has the fighting spirit of a lion. His repeated attempts to go fishing and then to not give up on his big catch show Santiago's pride, and that should also clue readers into the lion symbol, as a group of lions is called a "pride".

The second symbol that frequently comes to my mind is the mast. I believe that it alludes to the cross that Christ was crucified on. For Christians, the cross is symbolic of pain and suffering for a greater good. Keep in mind that Santiago suffers three wounds while beneath this mast in his skiff. Those wounds could reference the three wounds that Christ would have received as he was nailed to the cross. Additionally, Santiago is left to carry the mast to his shack in the same way that Christ was forced to carry his cross.

If you need a third symbol, I would go with the marlin. It symbolizes nature and nature's power. In some ways, the marlin and the battle that Santiago has with it reminds me of the Naturalism literary period. Authors like Stephen Crane often wrote about heroic battles against nature that ended in failure. Nature simply doesn't care how hard a human fights or how badly a human needs the win. It is the marlin and all of the other creatures that prevent Santiago from bringing the catch home. Nature doesn't care about Santiago's desires and pride.

Santiago and the fish are alter egos for one another, connected by the fishing line, and are both Christ figures. The book borrows biblical imagery to illustrate this symbolism. For example the story opens and closes with Santiago carrying his mast to and from the boat just as Christ carries a cross in the Bible. The side of the fish is pierced in the novel just as Christ's side is pierced in the Bible. Throughout the novel Hemingway will spend a paragraph describing something happening to Santiago and then the next paragraph will describe a similar situation with the fish.

Hemingway said:

No good book has ever been written that has in it symbols arrived at beforehand and stuck in. ... I tried to make a real old man, a real boy, a real sea and a real fish and real sharks. But if I made them good and true enough they would mean many things.

In light of this, it might be said that if The Old Man and the Sea is read according to the mode in which Hemingway wrote it, there are no symbols. However current critical theory emphasizes reading literature from a perspective contemporaneous with the reader's experience: reading literature from one's immediate current experience. Therefore in a reading in accordance with this critical view, there are some significant symbols in the novella The Old Man and the Sea. Also in accordance with this critical view, there is more than one approach to recognizing symbols and each approach is tied to the theme it emphasizes.

For instance in one critical opinion the symbols support a theme of ultimate victory in the face of the human condition in which Santiago symbolizes a Christ-like figure who is struggling to fulfill the injunction found in Genesis to have dominion over nature, yet is hindered by humanity's suffering, which was introduced into the world by the original sin propagated in the Garden of Eden. Manolin is the devoted and loving disciple of Santiago, while the blue marlin symbolizes the natural world that Santiago is meant to have dominance over, and the sharks symbolize the suffering unleashed by original sin.

Another interpretation builds different symbology around the theme of humankind's noble struggle against encroaching defeat and death. In this interpretation, Santiago symbolizes the conquering warrior, a Beowulf-type figure engaged in a continual combat against opposing forces. Manolin symbolizes his loyal squire and friend. The marlin symbolizes the mighty Grendel-type opponent, the opponent of a lifetime. The sharks symbolize encroaching defeat and death.

There are many symbols used in the course of telling Santiago's story. Santiago himself could be seen as one symbol, a representative of the human spirit that maintains its pride and dignity despite hardships and the decline in ability that comes with old age.

The memories of Africa and especially the dreams of the lions are symbolic of his youth, the days when he was free and capable of controlling his destiny, and of his dream of returning to those carefree days.

The sea represents everything that challenges any person in life. It provides his livelihood, reveals beauty and danger, challenges him to great exertion but also rewards him with the great catch of his lifetime. In the end, however, the old man needs to struggle on, alone and disappointed but never defeated.

One good example of symbolism is the sharks that attack the fish, strapped to the side of Santiago's skiff. Santiago, fighting them off, knows that they are not even hungry, but are simply responding to the smell of blood in the water:

They were hateful sharks, bad smelling, scavengers as well as, killers and when they were hungry they would bite at an oar or the rudder of a boat... they would hit a man in the water, if they were hungry, even if the man had no smell of fish blood nor of fish slime on him.(Hemingway, The Old Man and the Sea, Google Books)

Santiago considers the sharks to be sub-creatures, with no purpose beyond killing and destroying. He has done an amazing thing, caught an enormous fish, and these sharks care nothing for his achievement except that it easily feeds their own hunger. In the sharks, Hemingway symbolizes people who have no ambition or drive to succeed, but wish only to tear down people who have achieved something. Those who fall under this category live in endless envy of ambition and creativity, never willing to create or achieve for themselves but only desiring to destroy any works that are popular, visionary, or creative. The sharks, in their mindless drive for destruction, showcase this anti-ambition.

In Old Man and the Sea Hemingway, above all else, “tried to make a real old man, a real boy, a real sea and a real fish and real sharks. But if I made them good and true enough they would mean many things.”  Which is to say he did not build in much overt symbolism.

Many read the novella as a biblical parable, and it is styled as such.  Enotes' section on "Symbolism" does a wonderful job developing this:

Santiago, for example, has often been compared to Christ in the way he suffers. His bleeding hands, the way he carries the boat mast like a cross, and the way he lies on his bed with his arms outstretched, all have clear parallels in the story of Christ’s crucifixion. In this interpretation of the story, Manolin is seen as a disciple who respects and loves Santiago as his teacher.

As far as the sea, it is among the most universal, naturalistic settings in all of literature.  There are three levels of symbolism that I use to teach the novel: below the sea, on its surface, and above (in the air).  The obvious correlations are fish (below), man (surface), and birds (above).  I think Hemingway says there is much for man to learn from all creatures big and small: from below (the secrets of the deep) and from above (the freedom to see all).

Santiago's hands are another major symbol.  He speaks to them often when they cramp.  They are like Wilson, his only companion, in the film "Cast Away."  (He often speaks to the boy, not present, as well: another instance of apostrophe).  Santiago's hands represent both his strength (the arm wrestling match) and his weakness (old age).

The marlin is symbolic of the novella itself.  Hemingway is trying to bring it so shore for all to read, but inevitably he can only bring back a skeleton (a limited version of the idea he intended to create).

Inside the story, the marlin also represents the grandeur of all creatures and the brotherhood between man and beast in terms of survival.  Santiago calls him "brother" and "my fortune" (economically, and it terms of reputation among other fishermen).

Sharks, however, are not brothers, and Santiago uses his knife to stab them in the brain.  They are scavengers (like literary critics).  They only take, without creating.  Hemingway has little respect for them.