Salmon is an anadromous fish, that means it is born in fresh water, it spends most of its life in the sea and it returns to fresh water to spawn. I was wondering when fishermen fish salmon. I guess they aim for adults (more meat), right? But do they catch adults when they are in seawater or in freshwater (i.e. while climbing up a river to spawn)? Which is the best moment in order to have a better catch? And which is the best moment in order not to endanger the species?
Many fishermen catch them during the spawn because they know where they are going to be. They catch them as they are going upstream to spawn. As many types of salmon die after spawning, they are usually healthier going upstream.
When you catch the fish doesn't matter ecologically speaking. Limits on the number of fish caught are in place to help protect the species. Aside from overfishing, the main problem is the health of the rivers. Dams and other modifications can prevent the fish from reaching their spawning beds. Pollution also can prevent fish from successfully spawning.
which is the best moment in order not to endanger the species?
Pacific salmon only spawn once in their life. At the point when they have spawned they are generally not considered eatable. There is no moment to catch them and "not to endanger the species"
Salmon change color to attract a spawning mate. Pacific salmon use all their energy for returning to their home stream, for making eggs, and digging the nest. Most of them stop eating when they return to freshwater and have no energy left for a return trip to the ocean after spawning. After they die, other animals eat them (but people don't) or they decompose, adding nutrients to the stream. Unlike Pacific salmon, Atlantic salmon do not die after spawning, so adults can repeat the spawning cycle for several years. Source usgs.gov
As Schleis says in there answer fishing limits help protect the species.