Fast turns are not sped up slow turns: How bluegill sunfish change their kinematics with turn speed

Fish turn extremely often, but this behavior is relatively understudied due to how challenging it can be to get fish to perform such an unsteady maneuver repeatedly. Specifically, little is known about whether fish control turns differently at different turning rates. Here we address the challenge of studying turning by developing a device that elicits turns repeatedly at specific speeds. Using this device, we compare the swimming kinematics of bluegill sunfish (Lepomis macrochirus) during fast and slow turns through 180 degrees. We find that the fish behave differently when turning quickly than when turning slowly: a fast turn is not a sped up slow turn, but is kinematically distinct. In particular, during fast turns, bluegill bend their bodies to minimize moment of inertia before they maximize torque, while in slow turns they maximize torque first, before they bend their bodies. In fast turns, they also beat their pectoral fins at a higher frequency, but take fewer pectoral fin strokes. Differences between the two speed turns may be due to initial momentum and how momentum is conserved throughout the turn, because fast turns have higher initial linear momentum that they can convert into angular momentum to turn around. Summary statementBluegill change how they handle torque and moment of inertia when turning rapidly than when turning slowly. Tradeoffs between linear and angular momentum may play a key role.