Holden is preoccupied with things that need to be rescued (the ducks) and things frozen in time (the fish beneath the ice).
Holden is like the ducks, in that he thinks they have become so domesticated in Central Park that they have lost their homing instincts and abilities to fly south for the winter. He fears their caretakers, like his parents, have abandoned him. He wonders if the ducks must be forced to move elsewhere, the same way he will be made to go to prep school and then off to college. He wants to be a catcher in the rye and rescue the ducks himself.
His great fear is what is beneath the lagoon, the fish that seem to be frozen. They are representations of Allie, who since his death, will be forever be frozen in time: young, innocent, perfect.
The desperately lonely Holden has taken the time to visit his favorite teacher at Pencey Prep, old Mr. Spencer. But Spencer's lecturing and storytelling grows old quickly for Holden, who begins to daydream. He chooses a topic that he returns to several times in The Catcher in the Rye: He wonders how the ducks who swim in the lagoon in Central Park South survive during the winter months. Does someone load them in a truck and take them to safety?
The question is more of a rhetorical one, since Holden is actually wondering about his own future. What will happen to him now that he is displaced with winter approaching? His aimless life seems similar to the ducks in park, and he wonders if someone will appear to save him as well.
This theme is repeated several times later in the novel, and during his return to New York he asks an unfriendly cabbie the same question. The flustered taxi driver gives him a less-than-accurate answer, telling him that the ducks will adapt and hibernate. In truth, the ducks do as many Northern snowbirds do during the winter months: They migrate south to warmer weather. This is a thought that later crosses Holden's mind--to escape his problems by moving to new surroundings.
In Chapter two of "The Catcher in the Rye" Holden is visiting Mr. Spencer in his room. He has been looking at him thinking he is a hoot. He especially thinks that old guy’s legs look funny. He presents his paper on Egypt and Spencer begins to read it out loud.
While he is talking to him, his mind starts to wander. Holden is discussing one thing and thinking about another. He refers to himself as shooting the bull which basically means talking junk. His mind wanders to his thinking about the ducks at a lagoon in Central Park. He wonders where the ducks always go when the lagoon freezes over. He wonders if someone picks them up and moves them by truck somewhere else for the freezing months.