On the stern of the Sharp, Mendee and I process fish diets. She slings a camouflage-green burbot over her thigh and slices its belly from tail to gills so the intestines spill out. Finding the stomach, she chops it off and splits it open, then hands it to me. It is rubbery and limp, like a popped balloon. I hold it over a petri dish and squirt water into the pink ridges so that the burbot’s last meal pours into the dish. This one had a few snails. The last one had three plump silver grayling stretching it taut, which we gutted in turn to see what they had eaten. This is how ecologists reconstruct a food web: unlocking who’s inside whom, like opening a Russian nesting doll.