Schooling improves critical swimming performance in zebrafish (Danio rerio)

The critical swimming performance (Ucrit) of fish has typically been measured on individuals given a step-based test that ends in fatigue. Many of the fish given this test naturally travel in schools; a concern is that the Ucrit test underestimates the natural performance of schooling fish, as there are hydrodynamic benefits to schooling. We addressed whether Ucrit was improved by schooling by giving zebrafish Ucrit tests individually or in groups of three, five or ten. We found that fish swam faster in schools and that schools grew more cohesive as flow speed increased. The increased Ucrit owed to a greater number of fish swimming longer at lower speeds. In support of this, fish with the greatest Ucrit tended to have lower lactate and greater expression of citrate synthase. Our results indicate that Ucrit may increase when fish are tested in groups and that this may relate to energy savings, as well as differences in individual physiology.