Marine scientists have a hard time figuring out who eats who. On land, you can directly observe predation or study the scat that animals leave behind. In the ocean, it isn’t so simple. Fish poop immediately mixes with seawater and is difficult to collect, and fish often feed out of sight. Historically, scientists have turned to lethal methods like dissection and gut content analysis, or, for a non-lethal alternative, stomach lavage (essentially a stomach pump) to study fish diet. Over time, less invasive techniques have been developed.