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Courtesy Fungus Guy, via Wikimedia Commons

Anglers know that aren’t quite like rainbow trout—they’re enormous, reaching up to 55 pounds! But in fact, these ocean-running, rod-debilitating, aquatic gorillas of the Northwest are indeed rainbows, Oncorhynchus mykiss. That some members of this species rove the sea for years and become giants before returning to their birth streams to spawn, while others live quieter, local lives in freshwater, is an enigma. Depending on environmental cues like water temperature and food availability (factors we don’t fully understand), it seems any combination of parents—two trout, two steelhead, or a steelhead and a trout—can produce either form.

Now researchers at Oregon State University have added further intrigue to this mystery by discovering that steelhead and their smaller counterparts mingle (wink, wink) quite a lot: On , a tributary to the mighty Columbia, 40 percent of returning steelheads’ genes are derived from rainbow trout, not steelhead parents. “What’s particularly remarkable is the extent to which they interbreed,” says Mark Christie, a fish geneticist and coauthor of .

To spawn, steelhead dig “reds,” small nests in the gravel of the riverbed, where the females lay 3,000 or 4,000 eggs. Then, males fertilize the nest and stand guard, chasing off other steelhead. But some “sneaker” rainbow trout males can slip in under the watch of a massive male steelhead, and surreptitiously leave milt. (In fact, steelhead may not always recognize trout as potential competitors, Christie says.) Much of wild trouts' genetic contribution to steelhead likely occurs this way.

For their study, the OSU researchers took advantage of a problem Hood River steelhead confronted: , but it was also a convenient checkpoint for a quick genetic test. , and so, with a tiny clip of the fin, each and every steelhead could be sampled. Back in the lab, DNA analysis showed if a particular steelhead’s parents were already in the database, or if they were unknown—i.e. a wild trout upstream. The study examined more than 12,000 steelhead over the course of 15 years.

Another notable finding was that just 1 percent of Hood River steelheads’ genes come from “residualized” hatchery fish—that is, fish born from steelhead in a hatchery that, oddly enough, don’t turn out to be steelhead once released, but rather remain in freshwater and stay small. Fish biologists and managers worry about strays from hatcheries mating with wild strains, because it reduces the ecological fitness of native fish: wild steelhead need to be equipped to meet the challenging conditions of a river, not concrete pools (where they’re fed by machines, and so on). You may have several years ago. But Christie’s team found “there’s very little genes being contribute by these hatchery fish, which highlights that these wild, resident trout are really important as a genetic buffer for the entire steelhead population.”

The implications? Well, to start, the Northwest’s iconic salmon, also embattled ocean-going lunkers, have no such buffer. What’s more, up and down the West Coast —but listed just as “steelhead.” “In some populations,” Christie says, “it may be prudent to grant some protection to rainbow trout as well.”