I think Dr. Kristi Miller brought up the best points on that one, about how hard that is to determine. Just having stress factors within our lives and within fisheries' lives causes them to go through a period of not being able to maybe transfer from a freshwater fish to a saltwater fish.
It's something such as Beaver Creek in Stanley Park. The water levels there go up to 27° in the summer time. It's lovely to swim in, but not so good for fish. It makes it so their internal bodies don't change over and they have a difficult time changing. Instead of keeping salts in their bodies, they expel salts. They go through quite a process in smoltification. It's those kinds of struggles that they go through because of stressors that happen. Even something as simple as water quality changes can cause stress that makes them unable to go to their next stage in life.