Here is some info that I have read. I didn't know about the salt gland to extract the salt. Learnt something new today! Very interesting!
"Salmon stop feeding as they enter fresh water. Their stomach is no longer needed and it begins to disintegrate internally - leaving more room for the developing eggs and sperm. They begin living off the stored fat in their tissues that has been accumulating during their life in the ocean. When this starts the flesh of the salmon begins to lack flavor, becomes pale in color and is mushy in texture. Most salmon used for food are caught in the ocean where they have the best flavor, color and texture before they start upstream. Salmon change their physiology before entering fresh water. In the ocean their bodies tend to dehydrate in the seawater. They constantly drink seawater, using a salt gland to extract the salt, so that they do not get dehydrated. The opposite is true in fresh water, their bodies tend to take on water and become bloated. So, in fresh water salmon stop drinking and begin to process large amounts of urine to rid their bodies of the extra water."
I have found that generally, when the males start developing their humps they start to thin. Maybe it is the weight transferring to different areas in the body? I caught a male Sockeye (I know not a pink) last year on Stamp River on the fly that was spawned out. I was shocked that the thing still fought even though it was ready to die. It had a massive hump on the back but was SO thin!