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.