They can survive for over a day like this because they use very little oxygen at this stage. Each female salmon can have between 1,500 and 10,000 eggs. Many man-made objects hurt salmon by blocking their migration route or making it much more difficult. Dams on rivers of course are the main problem to salmon migration. Other man-made objects are designed to help salmon, like fish ladders.
Fish ladders help the adult salmon migrate up a river and work very well.
Dams are more of a problem for small salmon migrating downstream because they end up going through the turbines if they can't find the passages placed in the dam to help them. Roads across and alongside streams can be a problem if dirt from the road clogs up the gravel in the stream. Salmon change color to attract a spawning mate. Salmon use all their energy for returning to their home stream, for making eggs and digging the nest. Most salmon stop eating when they return to freshwater and have no energy left for a return trip to the ocean after spawning.