Effect of variations in flow velocity and phytoplankton concentration on sea scallop (Placopecten magellanicus) grazing rates

Abstract

Abstract A laboratory flume study was conducted to determine the effects of a variable food supply on sea scallop ( Placopecten magellanicus ) clearance rates ( CR s ). Time course measurements of CR s were made while manipulating either individually, or simultaneously, flow speed and phytoplankton concentration. Variables were altered bi-hourly during 14-h experiments to produce a symmetrical increase (ascending phase), then decrease (descending phase) in food supply; flow speed varied between 5–25 cm s −1 and concentrations of the diatom Thalassiosira weissflogii between 0.5×10 3 cells ml −1 . In all treatments, CR s in the ascending phase differed from those in the descending phase, indicating an effect of feeding history. Differences between phases were attributed to treatment effects since scallop CR s did not vary significantly when exposed to a constant flow and phytoplankton concentration. In variable flow and constant phytoplankton concentration, CR s in the ascending phase (5–15 cm s −1 ) of the flow cycle were 30% lower than in the descending phase (15–5 cm s −1 ). For both the ascending and decending phase CR s were independent of flow speed. The elevated CR s in the descending phase may have compensated for the period of feeding inhibition which occurred at peak flows (25 cm s −1 ). Increasing phytoplankton concentrations in constant flow caused a significant decline in CR s , but in the descending phase CR s were reduced by 50% and independent of concentration. The cause of the reduced CR s in the descending phase is unknown, but it may be related to satiation. When flow speed and phytoplankton concentration were altered simultaneously, there was an interactive effect during the descending phase but not the ascending phase. In the ascending phase (increasing flow and phytoplankton concentration) CR s declined at the same rate as in the constant flow/variable phytoplankton treatment, i.e. rates were independent of flow speed. However, in the descending phase, flow speeds of 15–10 cm s −1 inhibited feeding. Results emphasize that short-term (hours) feeding history is an important component of scallop feeding behaviour, altering the response to changes in food supply. Therefore, the predictive value of functional relationships maybe improved by the incorporation of data on feeding history and the interactive effects of flow speed.