RESEARCHERS AT the Massachusetts Institute of Technology’s Center for Ocean Engineering have discovered that Atlantic herring, which gather to spawn in groups, or shoals, of millions of fish, are triggered to do so by nightfall. As sunlight fades, scattered groups of fish start swimming much closer to one another, eventually becoming one large shoal. These shoals, which can be 25 miles across, protect the fish from predators and create an ideal situation for mass spawning. —Hannah Schardt