Killer whales / Orcas are known as "wolves of the sea". They aren't seen all that much in freshwater though, it is considered unusual. https://www.seattletimes.com/seattle-news/orcas-make-rare-trip-into-fresh-water-in-alaska/ I wouldn't say your statement is accurate though with the "only" part. Wolves can easily kill caribou calves and sick/wounded adults. I don't think they will mess with healthy adults unless they can separate one from the herd. No doubt the Orcas have the advantage when the Caribou are swimming though. https://journeynorth.org/tm/caribou/Predators.html