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First published online November 17, 2005
Journal of Experimental Biology 208, 4479-4494 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01922

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Locomotor function of the dorsal fin in rainbow trout: kinematic patterns and hydrodynamic forces

Eliot G. Drucker¹ and George V. Lauder^2,*

¹ Washington Trout, PO Box 402, Duvall, WA 98019, USA
² Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA

View larger version (23K): [in a new window]   Fig. 1. (A) Rainbow trout, Oncorhynchus mykiss, shown with the three laser plane positions used to image flow in the wake of the dorsal and caudal fins. Note the small adipose fin, common to salmoniform fishes, located in the midline between the dorsal fin and the tail. Unlike other median fins, the adipose fin does not possess intrinsic musculature or skeletal supports. Plane 1, located at mid-dorsal fin height, was used to image wake flow patterns produced by the dorsal fin alone. At this position, the light sheet was sufficiently distant from the dorsal surface of the trout's body that dorsal-fin wake flow patterns could be calculated without interference from body flows. Plane 2 intersected both the trailing edge of the dorsal fin and the dorsal lobe of the tail. This plane was used to quantify wake patterns from the lower portion of the dorsal fin and to quantify movement of the tail through the dorsal fin wake. This position also permitted occasional observation of adipose-fin wake flow patterns when trout moved slightly upward, bringing the adipose fin within the light sheet. Plane 3, located at the tail mid-fork position, was used to image the wake shed by the body and caudal fin alone. Laser planes 1–3 were similar in relative position to those used by Drucker and Lauder (2001a) in their study of sunfish dorsal fin function. (B) Sunfish, Lepomis macrochirus, scaled to the same body length as the trout in A, showing differences in dorsal fin morphology, placement and relative size. The spiny dorsal fin (absent in trout) is anterior to the soft dorsal fin, which is shaded gray in both species. Both the relative area of the soft dorsal fin and the portion of the fin's trailing edge that extends posteriorly free from the body (marked by asterisks) are smaller in trout than in sunfish. As a result, the trailing edge of the soft dorsal fin is considerably closer to the leading edge of the tail in sunfish than in trout.

View larger version (64K): [in a new window]   Fig. 2. Light video images of the posterior trunk and fins of rainbow trout during steady swimming at two speeds and during turning. Lateral and dorsal image pairs for each behavior were recorded simultaneously. As swimming speed increases from 1.0 to 2.0 body lengths (L) s–1, dorsal fin height decreases, while the height of the adipose fin remains unchanged (A,C). During turning, the dorsal fin is erected (E) and moves unilaterally (indicated by arrow in F) toward the stimulus, causing body rotation in the opposite direction. Ad, adipose fin; An, anal fin; D, soft dorsal fin; Pv, pelvic fin.

View larger version (38K): [in a new window]   Fig. 3. Kinematic patterns for the soft dorsal and adipose fins of trout oscillating in tandem during steady swimming at three speeds. The image at the top illustrates the points digitized from dorsal-view video (Fig. 1A, plane 2): red and blue symbols indicate the trailing edges of the dorsal and adipose fins, respectively, with body reference points at corresponding longitudinal positions represented by white symbols. At low swimming speed (0.5 L s–1), the amplitude of dorsal fin oscillation exceeds the amplitude of body bending (A), while the excursion of the adipose fin closely tracks that of the body (B). With increasing speed (1–2 L s–1) dorsal fin amplitude declines as body amplitude increases (C) until the dorsal fin's trailing edge and the body exhibit nearly identical excursion patterns (E). At higher speeds, the frequency and amplitude of adipose fin oscillation continue to match those of the body (D,F).

View larger version (138K): [in a new window]   Fig. 4. Visualization of the dorsal fin wake during steady swimming at 0.5 L s–1 by rainbow trout. The frontal-plane wake velocity field (Fig. 1A, plane 2) is shown as a matrix of yellow vectors on either side of the body; vectors overlying the body have been deleted. The trailing edge of the dorsal fin (D) is visible at the left of each panel, with white arrows indicating the direction of its movement. The adipose fin (Ad) and caudal fin (C) are also illuminated by the light sheet. Flow fields are shown at two times corresponding to the end of dorsal fin movement to the right (A) and the end of the next half-beat to the left (B). Vortical wake structures generated by the dorsal fin are numbered 1–4 in order of their appearance within the frontal plane. Structure 1 is the oldest, having been generated by the dorsal fin stroke to the right preceding that illustrated in A. The paired vortex morphology characteristic of younger wake elements is no longer evident in structure 1. Structure 2, formed during the stroke to the left preceding that shown in B, is a well-developed vortex pair with central jet flow. Structure 3, formed by the fin motions shown in A and B, has the same morphology as structure 2 but is situated on the opposite side of the body. Structure 4 is the youngest wake element, part of an incipient vortex pair and produced by the dorsal fin stroke to the left illustrated in B. Note that the dorsal fin wake at this low swimming speed is comprised of vortex centers located on each side of the body and that wake jets are oriented posterolaterally. Free-stream velocity of 8.5 cm s–1 has been subtracted from the vector fields to highlight vortices. Yellow scale arrow, 10 cm s–1; white scale bar, 1 cm.

View larger version (145K): [in a new window]   Fig. 5. Visualization of the dorsal fin wake during steady swimming at 1.0 L s–1 by rainbow trout. The frontal-plane light sheet is at position 1 (Fig. 1A), intersecting the middle of the dorsal fin, which is erected at this speed (see Fig. 2A); all other conventions follow those of Fig. 4. Note the continuous trail of vortex centers over the body, a wake morphology that stands in contrast to the discrete, bilaterally positioned vortex pairs observed at 0.5 L s–1 (Fig. 4). Free-stream velocity of 19.4 cm s–1 has been subtracted from the vector field. The white arrow shows the direction of dorsal fin movement. Yellow scale arrow, 10 cm s–1; white scale bar, 1 cm.

View larger version (109K): [in a new window]   Fig. 6. Visualization of the dorsal fin wake during steady swimming at 2.0 L s–1 by rainbow trout. The frontal-plane light sheet is at position 2 (Fig. 1A), intersecting the middle of the dorsal fin (at left), which is depressed at this speed (see Fig. 2C). The caudal fin is visible at the far right; all other conventions follow those of Fig. 4. Note the absence of vortex centers and fluid jets; background turbulence dominates the dorsal fin wake at this speed. This wake morphology contrasts sharply with the strong dorsal fin vortices and propulsive jets observed at lower swimming speeds (Figs 4, 5). Free-stream velocity of 38.4 cm s–1 has been subtracted from the vector field. Yellow scale arrow, 10 cm s–1; white scale bar, 1 cm.

View larger version (117K): [in a new window]   Fig. 7. Visualization of the dorsal fin wake during a slow turn at 0.5 L s–1 by rainbow trout. The frontal-plane light sheet is at position 2 (Fig. 1A), and vectors overlying the body have been deleted; the dorsal fin is visible to the left and the adipose fin to the right. This image shows the dorsal fin soon after it has reached maximal excursion to the right of the body and has begun the return stroke toward the midline (indicated by the white arrow pointing to the left of the body). Unilateral dorsal fin abduction results in the well-developed vortex with clockwise rotation and the strong laterally directed jet. During the return stroke, the counterclockwise member of the vortex pair develops, the wake jet is strengthened and the fish's body yaws to the left. Free-stream velocity of 8.5 cm s–1 has been subtracted from the vector field. Yellow scale arrow, 10 cm s–1; white scale bar, 1 cm.

View larger version (17K): [in a new window]   Fig. 8. Schematic summary of dorsal-fin vortex wake patterns observed in rainbow trout during steady swimming (A,B) and turning (C), compared with previously described patterns in bluegill sunfish performing similar steady swimming and turning behaviors (D,E) (sunfish data from Drucker and Lauder, 2001a). Turning behavior in both species was initiated during steady swimming at 0.5 L s–1. Line drawings of the fishes are not precisely to scale (although individuals of both species were approximately 20 cm in total length). The soft dorsal fin is shown in red and the cores of associated wake vortices are represented by curved arrows (note that the counterclockwise vortex in C was not consistently well developed within the horizontal laser plane). Blue vectors indicate both the mean orientation and magnitude of stroke-averaged force within the horizontal plane (normalized to soft dorsal fin area). In general, trout generate dorsal fin forces of lower relative magnitude and with more lateral orientation than sunfish during comparable swimming behaviors.

View larger version (85K): [in a new window]   Fig. 9. Dorsal–caudal fin kinematic phase relationship and hydrodynamic interaction in rainbow trout. (A–D) Dorsal-view video frames showing mediolateral excursion of the trailing edge of the soft dorsal fin and the leading edge of the tail during steady swimming at 1.0 L s–1. (E) Corresponding plot of dorsal and caudal fin motion versus time for approximately two fin beat cycles showing that mediolateral oscillation of the tail is approximately one-quarter cycle out of phase with that of the dorsal fin. (F) Path of the caudal fin's leading edge (red dots) plotted over the course of one tail beat cycle and shown in relation to the wake of the dorsal fin within the frontal (XY) plane (position 2, Fig. 1A) at 1.0 L s–1. At this speed, the tail passes directly through the centers of the dorsal fin vortices and experiences an incident flow whose velocity exceeds that of the free-stream by 3%. Free-stream velocity of 18.0 cm s–1 has been subtracted from the vector field.

View larger version (15K): [in a new window]   Fig. 10. Schematic illustration of laterally directed dorsal fin force (large blue arrow) generated during steady swimming at 0.5 and 1.0 L s–1 by rainbow trout, the roll and yaw torques (curved blue arrows) induced by this dorsal fin moment, and the hypothesized counteracting torques (red arrows) produced by the pectoral and anal fins to achieve a moment balance during steady locomotion. Asymmetrical movement of the caudal fin and oscillation of the pelvic fins could also assist in generating compensatory roll and yaw moments. Although the paired fins are not recruited continuously by trout during steady swimming (Drucker and Lauder, 2003), intermittent excursions of these propulsors may serve to correct moment imbalances. Steady swimming by trout involves the active use of multiple fins to maintain body stability in the face of environmental perturbations. CM, center of mass of the body.