HORTON, J.M.*; DRUCKER, E.G.; SUMMERS, A.P.; Univ. of Massachusetts, Amherst; Univ. of California, Irvine; Univ. of California, Irvine: Swiftly swimming fish show evidence of stiff spines

Fast swimming fishes typically have low amplitude and high frequency tail beats, an indication of a stiff body. Previous work has shown that cartilaginous fishes increase intramuscular myotomal pressure with increasing speed, and this is thought to increase stiffness. We tested the hypothesis that myotomal pressure increases with swimming speed in a bony fish�the coho salmon (Oncorhynchus kisutch). We simultaneously recorded the axial kinematics, and myotomal pressure from two bilaterally inserted pressure transducers located near the dorsal fin of adult coho salmon during steady swimming at a variety of speeds. Our analysis showed that (1) the axial kinematics and Strouhal number of coho salmon were similar to those of other fast fish during steady swimming, (2) myotomal pressure varied from sub-ambient to super-ambient with body bending, attaining peaks at the extremes of body curvature, and (3) baseline pressure, or average pressure inside the myotome did not increase with increasing speed (r²=0.20, P=0.10). This last finding differs from the results of previous studies of a cartilaginous fish (Squalus acanthias). We propose that bony fishes attain stiffness primarily through a stiff spine rather than increased intramuscular pressure during high-speed swimming.