I think Holden is deciding if he is a fish or a duck. His biggest decision is, as the Clash say, "Should I Stay or Should I Go?"
Holden's dead brother Allie is the fish frozen forever, trapped in the ice of death and childhood innocent. He was lucky, according to Holden. Allie never had to worry about painful coming-of-age preoccupations: sex, materialism, an uncertain future, homelessness, a mental breakdown, phonies.
Holden is obsessed with things stuck, literally and figuratively: the Museum of Art never changes; the Eskimo is mummified forever; the essay about Egyptians (mummies) written for Old Spencer; Jane keeps her kings in the back row. Holden's long been a conservative, a kid who puts on a red hunting hat (an homage to Allie) in order to block out, to retreat, to stay put, and to never grow up or old.
So, if Allie is the fish and the ice is death (or the death of innocence and childhood), then what's above and who are the ducks?
The ducks are abandoned teenagers like Holden. Yes, Mother Nature should take care of them, but is she? Are Holden's parents mentioned more than twice in the whole book? They've sent him to a series of boarding schools and, at the end, to therapy in California. Mother (nature) and Father are doing a pretty crummy job. Teenagers were very alienated after the war. As witnesses to the war, holocaust, the spead of crass commercialism, and the threat of nuclear annihilation, who wouldn't be?
Should Holden fly South? Should he go beneath the surface like Allie and James Castle? This the dilemma Holden faces most in his journey: suicide.
He hates the world above the ice: everyone's a phony except James Castle (who suicided), Allie (dead), Mercutio (dead), Phoebe (a kid), and the nuns. Holden seriously wants to join them: to be the romantic hero who, like James Castle (initials "J. C."; his Christ-figure), died for a noble cause.
Another main point about the ducks in Cental Park in the novel Catcher in the Rye by J.D. Salinger is in how they relate to Holden Caulfield in terms of the title "catcher." The ducks, Holden wonders, may get "rescued." rescue is a big theme in the novel, and indeed Holden himself envisions his role as a rescuer or catcher in some ways. However, a catcher can also be a killer (note Holden's red hunting hat - and duck shooting is great sport.) Also, the ducks have no sanctuary on the lake when the weather is cold - the surface is frozen and they skid about, unable to connect with the water - unlike the fish, which are underneath. Holden has difficulty connecting with the world and the people in it - he is like a fish, hanging out and hiding at the bottom.
Although his facts are not entirely accurate, the point of what the taxi driver is saying is that "Mother Nature" takes care of the fish in the winter. He says,
"If you was a fish, Mother Nature'd take care of you, wouldn't she? Right? You don't think them fish just die when it gets to be winter, do ya?"
The taxi driver in effect is comparing Holden to the fish, and telling him that he needs to trust a little more in "Mother Nature" to take care of him, instead of worrying and obsessing all the time.
The conversation begins when Holden asks the taxi driver where the ducks who swim in the pond "in the springtime and all" go in the winter. Holden wonders if they just fly south when the weather gets cold, or if, perhaps, someone comes around to pick them up and take them to a safe place. The taxi driver says that the fish have no problem spending the winter in the lake, even when the water freezes, and although his explanation is a little off-the-wall as to how they survive, his point is that they do survive just fine, even though, according to him, "It's tougher for the fish than it is for the ducks." Holden, who becomes anxious about everything in his life and seeks to avoid difficulty, is like a duck, who takes off when the going gets rough. The taxi driver is saying that the rest of the world, like the fish, has it as bad, if not worse, than Holden does, and if Holden could just learn to trust a little more that things will be all right, he would, like the rest of the world, get along just fine (Chapter 12).