Sg

There is a study I saw a few months ago that showed orca selected chinook, acoustically, when presented with similar sized salmon of a least a couple of species. Like you, I would find it surprising that an apex predator would be so food selective.

But, wolves apparently have preferred prey. When they put the wolves in yellowstone they chose animals that were elk-eaters rather than deer or moose. Certainly not 100%, but they do concentrate on them.

Also, we know that when grizzlies eat salmon their age at maturity is younger, females produce more cubs, the animals are more social and live at higher densities. When they don't have access to an abundant protein source they mature at an older age, produce fewer cubs, and are not at all social. In short, they need a lot more area to support fewer animals and their population productivity is lower. In short, they are more sensitive to the loss of any animal.

In the North Pacific it appears that thwe whaling in the 1950s is what triggered (eventually) the shift in killer whale diet to otters and all the associated problems with that.

Long-winded way of saying that the residents may well eat other fish but the result may be a less productive population. Which is more sensitive to disturbance and loss of young for whatever reason.