Like salmon, steelhead (Oncorhynchus mykiss) are anadromous, meaning they travel to the ocean as “smolts” and return to their natal streams to spawn. Hatcheries raise eggs and juvenile fish for about a year and then release them to go to sea.

It is well established that hatchery fish make better brood stock than wild fish, producing more fish that return for harvest, Blouin said. On the other hand, hatchery fish produce fewer returning offspring when both spawn in the wild.

This tradeoff appears to happen because hatcheries are inadvertently favoring genes that promote growth in the hatchery environment at a cost to survival in the wild, he said.