The local Times Colonist has a story regarding the puzzling appearance of two dead coho found recently in Bowker Creek.
The chum salmon that were incubated in Bowker Creek for the past three years are expected to provide returning spawners this year. Chum salmon tend to spawn near the mouth of a river or creek, and don’t go as far upstream as other salmon species, as a rule. The chum salmon spawn in the Goldstream River is a good example of this.
It’s as yet unknown whether the dead coho made their own way into Bowker Creek, or were dumped there as a prank. Coho salmon as well as other salmon species, as is pointed out in the article, do not necessarily all return to their natal stream. Some small percentage of a returning stock will stray into other streams and watersheds, perhaps part of an evolutionary survival strategy given the risk of landslides and river blockages on the west coast of North America.
You can read about the effects of toxic tire dust on young salmon in this 2020 article from the Seattle Times.