Simulating trawl herding in flatfish: the role of fish length in behaviour and swimming characteristics

Abstract Theoretical modeling indicates that the herding efficiency of flatfish by bottom-trawl sweeps is highly sensitive to subtle changes in fish behaviour. Yet the degree of variation in herding behaviour within a species, population, or individual remains poorly understood because of the difficulty observing and measuring fish behaviour in this region of the gear. The purpose of this study was to simulate herding under laboratory conditions in order to examine the behaviour and swimming characteristics of flatfish. Using a large flume equipped with a moving floor, we examined the effect of fish length on gait use, behaviour, and swimming kinematics in American plaice (Hippoglossoides platessoides). While swimming at a speed comparable to the herding speed of trawl sweeps (0.3 m s−1), smaller plaice (<30 cm) spent a large percentage of time using the kick-swim gait, while larger fish (≥30 cm) preferred cruising. In total, 65% of plaice exhibited settling behaviour, analogous to the swim-and-settle behaviour observed in response to trawl sweeps. The frequency of this behaviour and the distance swum between settles were independent of fish length. Only the frequency of gliding changed with the duration of swimming. Like other teleost species, tailbeat frequency decreased with increasing fish length. The results from this study indicate that fish length affects gait use and swimming kinematics in flatfish, but not the frequency of gliding and settling behaviours. These observations support the hypothesis of size-selective herding and provide further insight into the herding efficiency of trawl sweeps.