A) Three axes of rotation: Roll, Pitch (nose-up/nose-down) and Yaw (left/right turns). B) Forces in the pitch axis: the buoyant force acts at the center of volume (green circle) to elevate the fish; gravity acts at the center of mass (magenta circle) to pull the fish down. The center of mass sits forward of the center of gravity which leads to a nose-down torque that will rotate a passive fish nose-down. C) During forward translation, or in flow, the fish will rotate (shown here in the pitch axis) to align with the direction of drag (blue lines). The center of pressure (blue circle) is displaced caudally to the center of mass (pink circle) about which the fish rotates. This displacement acts as a moment arm, schematized in the corner, that generates stabilizing torque (black arrow) to align the body.