### Meeting Abstract

**37.5** Saturday, Jan. 5 **Pressure across the head of a freely-swimming rainbow trout (***Onchorynchus mykiss*) in uniform flow *LIAO, J.C.*; CHAMBERS, L. M.; AKANYETI, O.; The Whitney Lab for Marine Bioscience, University of Florida Gainesville; Robotics Laboratory, University of Bristol, UK; The Whitney Lab for Marine Bioscience, University of Florida Gainesville* jliao@whitney.ufl.edu

The mathematician Sir James Lighthill indicated that in order to reduce drag swimming fish should move in a manner that reduces the pressure difference across the head. According to his theory, this pressure difference is equal to *C*_{1}A - C_{2}UΩ , where the coefficients *C*_{1} and *C*_{2} are defined by the head morphology, *U* is the swimming speed of the animal and *A * and *Ω * are the lateral acceleration and the angular velocity of the head, respectively. The maximum drag reduction is predicted to occur when *A * and *UΩ * are in-phase and their ratio is *C*_{2}/C_{1}. Passive undulating body does not naturally achieve this indicating that active head control is likely required. In this study, we provide the first direct pressure measurements on a free swimming trout and use these measurements to experimentally validate Lighthill’s equation. We swam four rainbow trout of total body length (*L*) 18.5±1.0 cm (mean ± standard deviation) at 3, 4 and 5 *L*/s. We simultaneously measured the swimming kinematics and pressure along the head using a high speed camera and miniature pressure catheters. Pressure measurements from all speeds closely matched values estimated by Lighthill; the Pearson’s linear correlation coefficient was 0.89±0.04 (p<0.05). In contrast, the ratio (0.64±0.09) and the phase difference (43.8±6.4°) between *A * and *UΩ * differed significantly from the theoretical optimums (0.95±0.08 and 0°, respectively), resulting in an average pressure difference 17.0±7.2 Pa. However, this value is still 49% less than the expected pressure difference for a head rotating passively, indicating that active head control is correlated to the reduced pressure difference.