Importance of rock lobster size–structure for trophic interactions: choice of soft-sediment bivalve prey

Ecologists are becoming increasingly interested in how variation in predator demographics influences prey communities. In northeastern New Zealand, the contrasting populations of previously exploited predators in highly protected marine reserves and fished areas have been used to investigate the effects of predation in soft-sediment habitats. However, these experiments have been unable to separate the role of predator size from that of density. This study provides evidence to support the model that foraging by different sizes of the rock lobster Jasus edwardsii affects soft-sediment bivalve populations in different ways. Feeding trials were conducted to investigate whether rock lobsters of different sizes vary in their choice of taxa and size of their bivalve prey. Trials with two morphologically similar species, Dosinia subrosea and Dosinia anus, indicated that lobsters of all sizes choose D. subrosea more frequently than the heavier shelled D. anus. Further results indicated that both large (>130 mm carapace length (CL)) and small (<100 mm CL) lobsters are capable of preying on a wide size range of D. subrosea (20–60 mm). However, small lobsters more frequently chose smaller shells (<30 mm) and large lobsters more frequently chose larger shells (>40 mm). Patterns in the abundance and size class distributions of these two bivalve species at protected and fished sites supported the feeding choices observed in the laboratory. These results suggest that populations of rock lobsters with large individuals inside reserves are capable of controlling the demography of bivalve populations in adjacent soft-sediment systems.