Maybe you’ve noticed that wild steelhead tend to be tougher to catch than their hatchery-raised counterparts. According to the study linked to below, there’s a simple reason for that: gravel. Researchers found that developing hatchery steelhead show increased growth of some parts of the brain when small stones are scattered on the bottom of their tank. This gravel more closely resembles the rearing conditions of wild steelhead, and could be added to current hatchery tanks to rear smarter–more “wild”–fish.