When a person is old and dying, usually he/she is no longer have the desire to eat. The body wither, shrink, weaken..... sleeping most of the time.........going off slowly....
This is same as the fish, at least for the small fish. I noticed the small fish in my aquarium which has a life-span of a few months, will stop eating after a certain period, maybe three to four months. The shape of the body changed, shrinking and bended. It doesn't swim anymore and resting at the bottom of the tank most of the time.... Slowly.......going off....in a week... ending its life naturally.
I think this is called the spiritual (soul) preparation for the passage into another world..
Top: dying old fish
All pictures taken at my office aquarium.