The part you're talking about occurs at the end of chapter 8 on pages 157 and 158 during an exchange between Randy, Florence, Alice, and Lib.

Randy enters the house to find Florence crying at the table. Between sobs she tells him that a cat called Sir Percy ate a canary bird called Anthony and most of the fish in the aquarium. As Alice explains, Florence can't really blame Sir Percy since they have been able to give him very little food over the past few weeks. It is that point Randy realises that life has become about the "survival of the fittest... The strong survive. The frail die."

The exotic fish die because the aquarium isn't heated. The common guppy lives. So does the tough catfish. The house cat turns hunter and eats the pet bird. If he didn't, he'd starve. That's the way it is and that's the way it's going to be.

In this post-apocalyptic world this means that humans will have to find any means possible to keep themselves alive. Randy jokes that one day they may even come to see tinned dog food as a delicacy, but Florence knows that it means much more than that. As she says they may have to turn into hunters themselves just to survive. She tells Randy: "I don't want to live in that kind of world."

Written by Pat Frank in 1959, Alas, Babylon is one of the first science fiction novels to tackle the threat of a nuclear war between the USA and the Soviet Union.