Fast-start performance of eight groups of rainbow trout with various finrays removed was measured to test the hydromechanical theory (Weihs, 1973) that large fins are required for acceleration. A trend towards decreasing performance was found for the following sequence of fin-ray amputations: control (pelvic rays amputated); dorsal fin; anal fin; dorsal lobe of caudal fin and ventral lobe of caudal fin; ventral lobe of caudal fin and anal fin; dorsal and ventral lobes of caudal fin; both caudal-fin lobes and anal fin. The series represents progressive reduction in fin and body area, as well as reduction in these areas where lateral movements are largest. Effects of fin amputation on performance were statistically significant (P > 0.05) in comparison with controls only for the last three groups lacking the caudal fin. The results confirm the hydromechanical theory, and lead to the conclusion that mechanically optimum lateral body profiles for faststart and for steady (cruising and sprint) performance are mutually exclusive. This mechanical restriction can be circumvented only in bony fish, with the evolution of flexible, collapsible fins that permit major variation in lateral body profile.

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