Whole body lift and ground effect during pectoral fin locomotion in the Northern spearnose poacher
Nowroozi, B.N., Strother, J.A., Horton, J.M., and Summers, A.P.
The northern spearnose poacher, Agonopsis vulsa, is a heavily armored, negatively buoyant fish that uses pectoral fins to propel it just above the bottom. We used high-speed video, kinematic analysis and flow visualization to study how A. vulsa overcomes negative buoyancy and generates forward thrust during pectoral fin locomotion. When slowly swimming (0.7 body lengths/second [bl/s]) more than 2 cm above the bottom the poacher maintains a high body pitch angle of 22° (mean of n = 5 individuals, 5 trials per individual). This angle of attack decreases steadily with increasing swimming speed to 5° at 1.7 bl/s. These results suggest that pitch angle is important in lift generation which allows A. vulsa to overcome its negative buoyancy. These results are remarkably similar to a previous study on the white sturgeon, also a negatively buoyant fish and another benthic species. The mean angle of attack in white sturgeon was found to be about 20° when swimming at 0.5 bl/s and 5° at 2 bl/s (Wilga and Lauder, JEB 202:2213). In contrast, when poachers swam within 2 cm of the bottom the mean body pitch angle was zero, suggesting that they are taking advantage of a ground effect. Using suspended particles to visualize the flow around the pectoral fins, and between the fish and the ground, we established that the poacher does shed vortices that allow it to take advantage of ground effect.
Ecology & Evolutionary Biology - Brown University