Coombs, S.*; Anderson, E.J.; Braun, C.B.; Grosenbaugh, M.A.: How fish body parts alter local hydrodynamic stimuli to the lateral line.

Mottled sculpin (Cottus bairdi), like many fish, use their lateral line system to form hydrodynamic images of the environment. Although these small (~10 cm long), benthic fish inhabit streams and lakes with significant ambient water motion, information about lateral linefunction comes largely from still-water conditions. We used digital particle image velocimetry to determine how uniform flow fields (2 – 8 cm/s) near the sculpin were altered along a horizontal plane at the level of the trunk lateral line canal. In the absence of the fish and at distances > 2 cm away from the fish, the mean direction and magnitude of the flow field was spatially homogenous at all flow speeds. Small scale (< 5 cm), ambient turbulence was negligible at 2 cm/s, but increasingly more pronounced at higher flow speeds. Flow around the fish's head, pointed upstream, was typical for that around a rigid body and could be predicted by a potential flow model. Flow around the large, extended pectoral fin, pointed downstream, showed separation at the edge of the fin and a trailing wake, similar to that observed for a flat plate perpendicular to the flow. A separation bubble or recirculating region (1- 2 cm wide) formed behind the pectoral fin and extended along the entire trunk of the fish. The magnitude of the mean, local flow in this region was significantly less than that further away and its direction very close to the fish was likewise very different, predominantly upstream. The shear layer dividing the separation bubble from the external flow was fairly stable at 2 cm/s (Re ~ 10²), but very unstable and wave-like at 8 cm/s (Re~10³). These results show that fish body parts like the pectoral fin can significantly alter the local hydrodynamic stimulus field to the lateral line relative to ambient water motions only a few centimeters away.