when the Florence STEP group helped relocate stream critters as part of the 5Mile-Bell creek project (Tahkenitch), there were lots of minnows- rainbow, cutthroat and coho obviously plus lots and lots of small feeder minnows called sticklebacks. there were also lots of lamprey eels, crawdads, and awesome giant pacific salamanders (
Pacific Giant Salamander | Oregon Wild), and at least one type of invasive toad.
however, I think you will find that we get almost no holdover trout in these coastal lakes. they are, for the most part, shallow (Siltcoos max depth is 25') and they get very warm in the summer (almost no inlet flow), often with large algae blooms. the coho and cutthroat are out in the ocean during that time and the planter rainbows, which I think they stop planting in May, get caught up by fishermen or eaten by birds. Last time I checked, Siltcoos and Tahkenitch were getting almost no planter fish now, the goal is to discourage normal trout fishing/trolling that ends up damaging lots of the coho smolts in the lakes.
basically here on the coast, rather than fish for holdover planter rainbows, most of us tend to fish for species that 'holdover' in the ocean.