Habitat structure and the survival of juvenile scallops Pecten novaezelandiae: comparing predation in habitats with varying complexity

Fishing disturbances that remove epifauna and other elements of benthic habitat structure have a secondary impact on the survival of juvenile Pecten novaezelandiae, the native New Zealand scallop. We investigated habitat structure and scallop predation rates in 4 soft-sediment habitats naturally utilised by P. novaezelandiae over multiple spatial scales. We found that habitats exposed to fishing disturbance had significantly fewer elements of benthic structure and that predation rates were significantly (up to 44 %) higher compared to unfished habitats. Habitat complexity primarily differed at the spatial scale of site, which corresponds to the scale of fishing disturbance. Scallop predation was negatively correlated with the number of habitat elements such as sponges, horse mussels and ascidians in the immediate area, and positively correlated with predator density. Another factor that affected juvenile scallop survival was ambient scallop density, with higher predation rates at the site where scallop numbers had been artificially enhanced. Although size-specific predation could not be formally tested in this study, it appeared that the predation rate was higher for larger scallops (mean size of 23 cm) compared to smaller scallops (mean size of 16 cm) in at least one of the sites but this requires further investigation. The results suggest that simplification of benthic systems by bottom-towed fishing activity degrades habitats and subsequently increases the vulnerability of juvenile scallops to predation. This may have important consequences for the sustainability of wild and enhanced scallop populations.