MECHANICS OF THE STARTLE RESPONSE IN THE NORTHERN PIKE, ESOX-LUCIUS

The thrust produced during an escape response (C-start) of a northern pike, Esox lucius (0.41 m total length, 0.398 kg), an acceleration specialist, is estimated using the Weihs model for fish fast-starts. High-speed cine film records (250 Hz) of pike C-starts were digitized and the angle of attack and displacement of all body sections determined. These parameters were incorporated into the model and thrust was calculated. A comparison of model predictions with values predicted from required forces found six of eight performance parameters to be within 22% for three fast-starts. A lack of significant difference between model estimates and required forces suggests that lift and acceleration forces are the major components of total thrust during C-type fast-starts. The caudal region, including the caudal, anal, and dorsal fins, contributes the most (> 90%) to positive total thrust during the propulsive stage (stage 2). High velocities, large angles of attack, and large surface areas of these sections account for this. Approximately 77% of the positive total thrust during the propulsive stage is generated by the caudal fin and 28% by the anal and dorsal fins. Lift forces averaged over the fast-start are greater than average acceleration forces, but when positive acceleration forces of the propulsive stage only are considered, average lift and acceleration forces are similar (60 and 43% of total positive forces, respectively). Also, acceleration forces peak earlier than lift forces and contribute to an earlier escape response.