In the first instance, I think predation is an issue. The key question is how the imbalance arose. Now that we have so few salmon smolts leaving the rivers.... One can almost think of this as a threshold, and that salmon must pass a certain numerical threshold before they get above that bump.
Just to be cognizant of this, the other element, as I mentioned, is that as one goes further south there have been shifts in the marine fish community structure such that there are now more fish—such as herring, sand lance, and some other things—that are almost certainly competing with young salmon smolts for zooplankton, the key one being Calanus finmarchicus. There are more competitors in the marine—