By catching, releasing, and recovering fish. In some cases, the fish is held for (say) 24 hours. The problem is that the fish are often assumed to be alive, to have survived, because they swim away. There have been some studies that look at long-term survival (months) and the Canadians did look at survival to spawning in the wild for steelhead.

Some studies on C&R'd Atlantic Salmon, though, showed that while the fish survived and spawned they ended up producing fewer smolts. That is the big black hole in most studies; we don't look at the impact of C&R on spawning success. This is especially critical in fisheries on adult salmon and steelhead as they have very little or no time to eat and recover fitness.