Impacts of the New Zealand screwshell Maoricolpus roseus on growth and condition of juvenile commercial scallops Pecten fumatus

The invasion of SE Australia by the exotic New Zealand screwshell Maoricolpus roseus has sparked concern over its potential impact on native soft-sediment assemblages and, in particular, on commercially harvested molluscs. Here we quantify the impact of both live and dead and empty M. roseus shells on the growth, condition and survival of juvenile commercial scallops Pecten fumatus. In a manipulative experiment, P. fumatus juveniles exhibited up to 27% slower linear shell growth, 5% lighter shells (dry weight), 12% less somatic tissue (ash-free dry weight) and 17% poorer condition when caged with live M. roseus. Dead and empty screwshells also reduced scallop growth and condition relative to controls but only at low scallop densities. Scallop mortality did not differ between treatments. The level of impact observed was also dependent on the density of P. fumatus. At low scallop densities, the effects of the invasive M. roseus are largely due to competition for space, as the results from treatments containing live screwshells and those containing dead and empty screwshells were comparable. However, at higher scallop densities, the significant difference between the treatments indicates competition for both space and food. Moreover, comparison between the different scallop densities provides evidence of a combination of both inter- and intraspecific effects. Given the widespread distribution of M. roseus and its occurrence at high densities across large areas of seafloor in SE Australia, impacts on scallop growth and condition have the potential to greatly affect both commercial and recreational scallop fisheries in SE Australia.