Predation by fish on intertidal oysters

Experiments in many parts of the world have indicated that sessile intertidal organisms are affected by fish predation. Fanning of oysters Saccostrea commercialis (Iredale & Roughley) and Crassostrea gigas (Thunberg) in New South Wales, Australia, depends on their natural recruitment and growth on intertidal structures in estuaries. We investigated the effects of predation by fish on numbers of oysters recruiting to experimental panels of 3 different sizes. We tested effects of excluding fish of different sizes using different sizes of mesh (12.5 and 50 mm). Numbers of oysters were significantly reduced on panels open to predatory fish. Numbers of oysters per unit area and subsequent predation on them did not vary, however, across 3 different sizes of panels, indicating that predation was not dependent on patch size. Mortality on panels open to predation averaged 40.0 % (+/-4.3 % SE). Fish also significantly altered the distribution of sizes of oysters. The effect of predation was almost entirely attributable to toadfish Tetractenos spp. Previous knowledge of the life history of oysters and succession in these intertidal assemblages suggests that effects of predation may not, however, have important long-term consequences on natural populations.