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

Understanding how biological interactions vary under different water flow r egimes is important in systems that are known to exhibit fluctuations in fl ow due to storms, tidal forcing, or variation in habitat structure. We perf ormed a series of experiments to determine the migratory response of bay sc allops, 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 sw im actively and achieve unaided bursts of motion covering distances on the scale of 0.5 m, the expression of density-dependent emigration was dependen t on the presence of higher flows. In the low-flow regime, there was neglig ible emigration during 24 h at any density level (12, 25, or 62 m(-2)); how ever, 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 demons trate that flow can facilitate biological interactions, creating conditiona l density dependence.