EFFECT OF FLOW VELOCITY AND BODY-SIZE ON SWIMMING TRAJECTORIES OF SEASCALLOPS, 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 l aboratory flume and in a shallow (7 m depth) tidal channel in Lunenbur g 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 sw im at mean free-stream flow speeds of 0, 5, 9, 14 and 21 cm s(-1). Swi mming (active) and post-swimming (passive sinking) phases of individua l trajectories were quantified in terms of swimming height, net displa cement and direction, swimming time and swimming speed. Smaller scallo ps (less than or equal to 30 mm) were advected by the how during both phases while larger scallops (> 30 mm) were advected only during the p assive phase. Net horizontal displacement (active and passive stages c ombined) 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 down stream net swimming direction (finished their swimming phase downstrea m of the start point). For small scallops (less than or equal to 30 mm SH), the proportion swimming downstream was independent of how speed at flows of 0-5 cm s(-1) (p > 0.30), but not independent of the flow s peed at higher flows (p < 0.01). For large scallops (> 30 mm SH), the proportion swimming downstream was independent of flow speed. In the f ield study, scallops ranging from 17-72 mm were induced to swim at cur rent speeds ranging from 7-55 cm s(-1). Model II regression and circul ar-linear correlation analysis showed a positive relationship between net horizontal displacement and current speed for scallops landing dow nstream and a negative relationship for scallops landing upstream, ind icating the effect of advection. For small scallops that swam at mediu m (similar to 18 cm s(-1)) current speeds, the distribution of displac ement directions was significantly non-random and the mean direction d id not differ significantly from the mean current direction. For large scallops that swam at both medium and high (similar to 38 cm s(-1)) c urrent speeds, the distribution of displacement directions was not sig nificantly different from random. However, at high current speeds the mean size of scallops that landed upstream was significantly greater a nd the mean swimming height was significantly lower than for those tha t landed downstream. Flume and field measurements of swimming trajecto ries demonstrated that predictability of dispersion patterns of P. mag ellanicus released in bottom culture based on near-bed current data wi ll be poor, especially when scallops larger than 30 mm SH are used.