I can attempt to answer your question.
The facts that we know about pinnipeds in the Salish Sea are that the numbers have increased since the early 1970s. There may have been around a couple of thousand of harbour seals, for example, and today there may be more like 40,000 or so. There's been a very big increase.
This has happened in parallel with many of B.C.'s other marine mammals, such as killer whales, sea lions, sea otters and humpback whales. Bigg's killer whale or West Coast transients are predators of pinnipeds as well, so there's been a big increase in the numbers of those groups.
You mentioned that they prey upon salmon. We do know it is a fact that juvenile coho and chinook may comprise 1% to maybe 6% of the harbour seal's diet. There's some uncertainty there. For pinnipeds in B.C., unlike for most predators, we do have a good long-term time series on the numbers.