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Fast starts are crucial in the survival of aquatic swimmers to capture prey or avoid predators. Currently, it is widely accepted that during C-starts: (1) the caudal fin generates a considerable hydrodynamic force; and (2) anal/dorsal fins are erected to significantly increase the hydrodynamic force. In this work, the above hypotheses on the role of fins during C-starts are studied using experimentally guided numerical simulations of four bluegill sunfish, whose fins are removed or erected. The amount of force created by the body and fins at each time instant was not constant and varied during the C-start. Nevertheless, in agreement with hypothesis (1), up to 70% of the instantaneous hydrodynamic force was generated by the tail during Stage 2 of the C-start, when the sunfish rapidly bends out of the C-shape. Additionally, the contribution in Stage 1, when the sunfish bends into a C-shape, is less than 20% at each instant. Most of the force in Stage 1 was produced by the body of the sunfish. In contrast to hypothesis (2), the effect of erection/removal of the fins was less than 5% of the instantaneous force in both Stages 1 and 2, except for a short period of time (2 ms) just before Stage 2. However, it is known that the anal/dorsal fins are actively controlled during the C-start from muscle activity measurements. Based on the results presented here, it is suggested that the active control of the anal/dorsal fins can be related to retaining the stability of the sunfish against roll and pitch movements during the C-start. Furthermore, the erection of the fins increases the moment of inertia to make the roll and pitch movements more difficult.

The functional role of caudal and anal/dorsal fins during the C-start of a bluegill sunfish