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First published online January 8, 2007
Journal of Experimental Biology 210, 325-339 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02661

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Hydrodynamic function of dorsal and anal fins in brook trout (Salvelinus fontinalis)

E. M. Standen^* and G. V. Lauder

Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA

View larger version (46K): [in this window] [in a new window]   Fig. 1. Experimental apparatus. (A) Fish swam in a multi-speed flow tank with two horizontal light sheets projected simultaneously to illuminate the dorsal and anal fin wakes. High-speed cameras captured simultaneous dorsal and ventral views of the swimming fish. (B) Image of a trout swimming between the two light sheets.

View larger version (24K): [in this window] [in a new window]   Fig. 2. Kinematic and hydrodynamic function of dorsal and anal fins during steady swimming at 0.5 L s-1. Red and blue represent dorsal and anal fins, respectively. Solid lines represent fin kinematic oscillations over time. Red and blue arrows represent the direction and magnitude of fluid jets produced by fins. Green boxes on the fish indicate where the hydrodynamic data were sampled during fin oscillation.

View larger version (14K): [in this window] [in a new window]   Fig. 3. Velocity of dorsal and anal fin tips during steady swimming at 0.5 and 1.0 L s-1. Red and blue represent the dorsal and anal fins respectively. At 0.5 L s-1 the anal fin maintains its smooth velocity sinusoid but the dorsal fin shows comparatively increased acceleration and deceleration and maintains fin maximum velocity for a longer proportion of the stroke cycle. The result is a plateau on the dorsal fin velocity graph containing a series of smaller peaks and troughs at high velocities during the cycle.

View larger version (87K): [in this window] [in a new window]   Fig. 4. Simultaneous development of dorsal and anal fin jets in brook trout swimming at 0.5 L s-1. (A) Dorsal fin at peak amplitude. Due to its location forward on the body the dorsal fin reaches maximum excursion before the anal fin. (B) Dorsal fin peak jet velocity. Peak jet formation by the dorsal fin occurs just after maximum kinematic excursion. (C) Anal fin at peak jet velocity, and (D) anal fin at peak amplitude. Peak jet formation by the anal fin occurs prior to maximum anal fin excursion resulting in a shorter maximum jet phase lag than would be expected from the kinematic phase lag seen between dorsal and anal fins. Note that x and y axes represent the scale of the video image.

View larger version (117K): [in this window] [in a new window]   Fig. 5. Dorsal and anal fin vorticity and vector plots during steady swimming at 0.5 L s-1. Top panel shows the dorsal fin with formation of trailing vortices during fin oscillation. Flow between the vortex centers shows jets to either side of the fish during a complete tail beat cycle. Bottom panel is the ventral view showing the anal fin producing shear flow on either side of the fin during the stroke. These shearing regions roll up and form elongated vortex cores. Both images reveal that shear build up along the side of fins develops long before the vortex is actually shed from the fin. Vectors have been removed from the image where they were disrupted by the shadow of the fish. Note that x and y axes represent the scale of the video image.

View larger version (21K): [in this window] [in a new window]   Fig. 6. Kinematic and hydrodynamic function of dorsal and anal fins during manoeuvring at 0.5 L s-1. Red and blue represent dorsal and anal fins, respectively. Solid lines represent fin kinematic oscillations over time. Red and blue arrows represent the direction and magnitude of fluid jets produced by fin.

View larger version (111K): [in this window] [in a new window]   Fig. 7. Simultaneous dorsal and anal fin jet development in brook trout manoeuvring at 0.5 L s-1. The fish is manoeuvring away from the stimulus located beyond the top of each image. (A) Dorsal fin at peak amplitude. (B) Dorsal fin peak jet velocity. The peak jet formation by the dorsal fin occurs just after maximum kinematic excursion with less phase lag than that seen in steady swimming. (C) Anal fin at peak amplitude and jet velocity. There is no phase lag between peak amplitude and peak jet velocity. Note that the pelvic fins appear as paired bright circular regions in the dorsal view because they intersect the ventral light sheet and reflect light into the dorsal camera. Similar pelvic fin reflections can be seen in the dorsal views shown in Fig. 6. Note that x and y axes represent the scale of the video image.

View larger version (111K): [in this window] [in a new window]   Fig. 8. Vorticity and vector plots during a manoeuvre at 0.5 L s-1. The fish is manoeuvring away from the stimulus located beyond the top of each image. Top panel, dorsal fin during pause at maximum excursion as is commonly seen in lateral yawing manoeuvres. Bottom panel: anal fin returning to midline after the pause at maximum excursion. Shear layers can be seen on dorsal and anal fins. Formation of the second counterclockwise vortex appears early causing the formation of an imbalance in contralateral jet magnitude. Note that x and y axes represent the scale of the video image.