It is true that many salmon do in fact die after spawning. There are many various species of Pacific salmon spawning semelparously. The word Semelparous refers to the reproductive strategy of an organism. A species is considered semelparous if it is characterized by a single reproductive episode before death. In truly semelparous species, death after reproduction is simply a way of life and is part of an overall strategy that includes putting all available resources into maximizing reproduction at the expense of future life. About 90 to 95% of all Atlantic salmon also die following their first spawning, however some survive to spawn two or three more times. As many as four spawning have been reported in some cases. The surviving salmon, predominantly female, return to sea to feed between spawning.
However this does not indicate why the survivors were able to make it, whether they somehow are better able to withstand the freshwater than their cohorts, or if they just manage to have enough food reserves to get them back to sea and back to full health is still in question. It is suggested however that most that die do so as a result of possibly using all of their energy to return to their home stream for making eggs and digging up the nest. During this time most salmon stop eating when they return to freshwater and as a result have little no energy left for a return trip to the ocean after spawning.
After these salmon die, other animals have been known to eat them. However humans do not. If the dead salmon are not consumed after dying, they decompose and add nutrients to the stream. Some common species of salmon that have been known to die often after spawning include the Chinook and Coho salmon. They may be less palatable since they do not feed during their spawning runs, which cause their muscle tissues to begin to break down. Fish that usually do not die after spawning and are generally found to be in much better condition during spawning include fish such as the steelhead, brown trout and brook trout.