Conditional density dependence: The flow trigger to expression of density-dependent emigration in bay scallops

Abstract

Understanding how biological interactions vary under different water flow regimes is important in systems that are known to exhibit fluctuations in flow due to storms, tidal forcing, or variation in habitat structure. We performed a series of experiments to determine the migratory response of bay scallops, Argopecten irradians concentricus Say, to changes in intraspecific density under two different, tidally oscillating flow regimes in the field: 0-0.10 m s-1 and 0-0.28 m s-1. Despite the bay scallop's ability to swim actively and achieve unaided bursts of motion covering distances on the scale of 0.5 m, the expression of density-dependent emigration was dependent on the presence of higher flows. In the low-flow regime, there was negligible emigration during 24 h at any density level (12, 25, or 62 m-2); however, emigration rate increased with density (0% at 12 m-2, 60% at 25 m-2, and 71% at 62 m-2) under the higher flow regime. These results demonstrate that flow can facilitate biological interactions, creating conditional density dependence.