Swimming and posture control of common carp when penetrating mesh nets in a water tunnel

Abstract

Successful capture of actively swimming fishes by towed net is a complex process. Quantitative studies of fish behavior and swimming kinematics near and in towed nets under field conditions are almost impossible. This paper reports a laboratory study of a model system that provides insight these matters. Behavioral and swimming kinematic responses of juvenile common carp to variously oriented flat panels of netting were studied under conditions of rapid water flow in a precision water tunnel. Responses of groups of fish were recorded by digital video camera; their maneuvers and kinematic responses were analyzed. Netting panels were positioned in three directions: vertically transverse to flow, vertically and horizontally parallel to flow. Panels of diamond mesh netting (both horizontal and vertical diamonds) were mounted in rigid frames. When fish passed through the nets tail beat frequencies and amplitudes both increased above base line levels. Penetration ratios, moving speeds, yawing turns and rolling angles all varied with the positions of the netting panels or the mesh hanging ratios. The penetration process had several stages: fish initially adjusted positions using yawing and side-slip; they then made yawing turns or rolled; propulsive accelerations followed; then fish rolled and stabilized in relation to the water flow. Penetration failures were due to lack of one or more of the main maneuvers. Fish penetration and selectivity in towed fishing gear may relate to both difficulties with swimming maneuverability and to the sizes and shapes of both the fish and the netting.