Station Holding of Trout: Behavior, Physiology and Hydrodynamics

Trout commonly experience unsteady flows such as those caused by a stationary object exposed to running water. Instead of avoiding these flows, trout often use flow fluctuations for station holding. The behaviors associated with station holding are entraining, Kármán gaiting and bow wake swimming. We investigated the swimming behavior of trout in the vicinity of a stationary or moving 2-D shaped cylinder. To uncover the sensory modalities used for station holding, we studied the behavior of intact trout and of trout whose lateral line system was partially or totally impaired in the light or under infrared illumination. We also studied the activity of the axial red swimming muscles of entraining, Kármán gaiting and bow wake swimming trout and the neuronal processing of vortex information in the hindbrain of fish. Further studies showed that small motions of the caudal and/or pectoral fins are necessary to stay in preferred areas irrespective of the unsteadiness imposed by the wake of an object. Computational Fluid Dynamics simulations were carried out to uncover the forces that allow trout station holding with a minimum of energy expenditure.