The physiological changes anadromous fish (fish that migrate from freshwater to saltwater and return to freshwater to reproduce) are remarkable and still not fully understood. As the fish go through the "smolting" process they change their osmotic balancing features which includes their digestive system, their swim bladder size changes, there is a decline in excretory rates, blood chemistry changes, etc. Think of it this way: in freshwater their environment is trying to flood them with water; when they go to saltwater, their environment is trying to pull water from the bodies of the fish. Consequently, anadromous fish spend considerable time in the brackish water at the mouth of the Columbia River waiting for their bodies to adjust.
As the adults move from the Pacific Ocean to the freshwater of the Columbia River, it's an immediate transition. Their ability to process nutrients/food immediately ceases. They still have a feeding behavior but can't process food, hence they still strike at eggs or other food sources from the past.
If you want more details, there is a book titled: Fish Migration and it's written by Brian A. McKeown.