So far they haven't been a direct threat to killer whales, but they do modify habitat. There are predatory fish that are introduced into aquatic systems that prey on juvenile salmon. That of course reduces in-river survivorship. We have a problem with green crabs in the state of Washington now that are modifying the near-shore environment, which is critical habitat for salmon.
A lot of these invasive species come through ballast water. They can be attached to hulls of ships. Then there are deliberate introductions because somebody wants an ornamental plant or exotic pet or something like that. Those can carry diseases. Because killer whales have high levels of PCBs, their immune systems are impaired. Because southern residents have been in a genetic bottleneck for a long time, they're losing the genetic diversity that allows the population to survive an introduced disease.