Some species of sculpins, certainly here on the Pacific Coast, travel downstream to spawn and then go back upstream to live the rest of the year. It only takes a few years operation of a barrier to extirpate the population above it. This was first observed on Waddell Creek in CA in the 30s and seems to be continuously confirmed up here.

In one of the cases I was involved in, we knew that both coho and steelhead juveniles occasionally went upstream through a barrier culvert. It was access to great overwinter habitat. But, WDF was uninterested in having DOT fix it. In fact, DOT actually fixed it "accidentally" as they made a very passable situation that WDF was not going to force.