Body size and swimming types in carp larvae : effects of being small

The viscosity of water is a major hydrodynamic force and especially important for tiny objects (on or below a mm scale) moving at low speeds. Fish larvae between 4.8 and 8.1 mm total length (TL) are of intermediate size. During their growth the flow while swimming becomes dominated by inertial instead of viscosity forces. The present paper presents changes in beat frequency, wave speed, absolute speed, distance covered per swimming cycle and the distribution of body curvature along the trunk during larval growth. Just hatched larvae show only one type of swimming but further types develop during growth. Early wriggling (resistive) swimming is rapidly replaced by beating with tail and tail fin suggesting the increasing dominance of inertial hydrodynamic forces. Similar movements as in the 4.8 mm larvae are found in the first body wave of bigger larvae thus strengthening the hypothesis of the changing relationship between viscosity effects and swimming motion with size.