Effect of flow velocity and body size on swimming trajectories of sea scallops, Placopecten magellanicus (Gmelin): A comparison of laboratory and field measurements

The effect of flow velocity and body size on swimming trajectories of Placopecten magellanicus (Gmelin) was studied in a 5 m, flow through laboratory flume and in a shallow (7 m depth) tidal channel in Lunenburg Bay, Nova Scotia, Canada. In the flume, scallops of 4 size classes, 11-20, 21-30, 31-50 and 51-80 mm shell height, (SH) were induced to swim at mean free- stream flow speeds of 0, 5, 9, 14 and 21 cm s^-1. Swimming (active) and post-swimming (passive sinking) phases of individual trajectories were quantified in terms of swimming height, net displacement and direction, swimming time and swimming speed. Smaller scallops (≤ 30 mm) were advected by the flow during both phases while larger scallops (> 30 mm) were advected only during the passive phase. Net horizontal displacement (active and passive stages combined) was positively related to flow speed for all size classes but the proportion of the variance explained by flow speed decreased with increasing scallop size. Most scallops swimming under flow had a downstream net swimming direction (finished their swimming phase downstream of the start point). For small scallops (≤ 30 mm SH), the proportion swimming downstream was independent of flow speed at flows of 0-5 cm s^-1 (p > 0.30), but not independent of the flow speed at higher flows (p < 0.01). For large scallops (> 30 mm SH), the proportion swimming downstream was independent of flow speed. In the field study, scallops ranging from 17-72 mm were induced to swim at current speeds ranging from 7-55 cm s^-1. Model II regression and circular linear correlation analysis showed a positive relationship between net horizontal displacement and current speed for scallops landing downstream and a negative relationship for scallops landing upstream, indicating the effect of advection. For small scallops that swam at medium (~ 18 cm s^-1) current speeds, the distribution of displacement directions was significantly non random and the mean direction did not differ significantly from the mean current direction. For large scallops that swam at both medium and high (~38 cm s^-1) current speeds, the distribution of displacement directions was not significantly different from random. However, at high current speeds the mean size of scallops that landed upstream was significantly greater and the mean swimming height was significantly lower than for those that landed downstream. Flume and field measurements of swimming trajectories demonstrated that predictability of dispersion patterns of P. magellanicus released in bottom culture based on near-bed current data will be poor, especially when scallops larger than 30 mm SH are used.