One of their key findings is that size matters. Once brown trout reach 13 inches in length, they alter their eating habits. Up to that length they lie in pools, feeding on fish and insects that drift by. But at 13 inches they become active hunters searching for piscine prey. During the day the big fish would hole up and rest in one of several "home sites" that they would return to after a night's hunt. Usually, the fish pinned themselves tight against debris or other cover.