Role Of The Rostrum In Swimming Performance Of Juvenile Paddlefish (Polyodon spathula)

The Paddlefish rostrum enhances swimming due to unique construction, variable form, and large size.  We measured 5-min critical swim speed, U_crit, of 66 young-of-year (4.9-9.6 cm EFL) and 20 yearlings (21.6-28.2 cm EFL) in 100-L and 1200-L swim tunnels, correlating results with rostrum size, body shape, and caudal lobe asymmetry. U_crit of young-of-year were 18.5-58.0 cm/s, yearlings 38.5-93.8 cm/s.  U_crit, expressed as body lengths per second (BLS), showed young-of-year with shorter rostra (388-671 ‰ EFL) swam faster (2.0-7.1 BLS) than yearlings with larger rostra (613-735 ‰ EFL; 1.4-3.6 BLS).  For young-of-year, U_crit was negatively correlated with rostrum relative length and area (R²>0.17, p<0.0010).  For yearlings, U_crit was negatively correlated with caudal asymmetry (R²=0.30, p=0.0114).  Swim data were consistent with computational fluid dynamic (CFD) models.  Rostrum generates lift comparable to manufactured hydrofoils but with disproportionate drag.  Multiphysics Fluid Structure Interactions showed swimming enhancements due to vortices generated when the rostrum is at angles of 5 and 10 deg from horizontal axis.  Phenomenon is represented by increase in uplift pressure of rostrum when fish are swimming in laminar flows.  Fast swimming young-of-year minimize drag with smaller rostra.  Yearlings, with increased drag from larger rostra, compensate with more symmetrical, presumably higher-thrust, caudal fins.