Is tilting behaviour at low swimming speeds unique to negatively buoyant fish? Observations on steelhead trout, Oncorhynchus mykiss, and bluegill, Lepomis macrochirus

When swimming at low speeds, steelhead trout and bluegill sunfish tilted the body at an angle to the mean swimming direction. Trout swam using continuous body/caudal fin undulation, with a positive (head-up) tilt angle (0, degrees) that decreased with swimming speed (u, cm s−1) according to: 0=(164±96).u(−1.14±0.41) (regression coefficients; mean±2s.e.). Bluegill swimming gaits were more diverse and negative (head down) tilt angles were usual. Tilt angle was −3·0 ± 0.9° in pectoral fin swimming at speeds of approximately 0.2–1.7 body length s−1 (Ls−1; 3–24 cm s−1), −4.5 ±2.6° during pectoral fin plus body/caudal fin swimming at 1·2–1·7 L s−1 (17–24cm s−1), and −5.0± 1.0° during continuous body/caudal fin swimming at 1.6 and 2.5 L s−1 (22 and 35cm s−1). At higher speeds, bluegill used burst-and-coast swimming for which the tilt angle was 0.1±0.6°. These observations suggest that tilting is a general phenomenon of low speed swimming at which stabilizers lose their effectiveness. Tilting is interpreted as an active compensatory mechanism associated with increased drag and concomitant increased propulsor velocities to provide better stabilizing forces. Increased drag associated with trimming also explains the well-known observation that the relationship between tail-beat frequency and swimming speed does not pass through the origin. Energy dissipated because of the drag increases at low swimming speeds is presumably smaller than that which would occur with unstable swimming.