Predation by fish on intertidal oysters

Experiments in many parts of the world have indicated that sessile intertid al organisms are affected by fish predation. Fanning of oysters Saccostrea commercialis (Iredale & Roughley) and Crassostrea gigas (Thunberg) in New S outh Wales, Australia, depends on their natural recruitment and growth on i ntertidal structures in estuaries. We investigated the effects of predation by fish on numbers of oysters recruiting to experimental panels of 3 diffe rent sizes. We tested effects of excluding fish of different sizes using di fferent sizes of mesh (12.5 and 50 mm). Numbers of oysters were significant ly reduced on panels open to predatory fish. Numbers of oysters per unit ar ea and subsequent predation on them did not vary, however, across 3 differe nt sizes of panels, indicating that predation was not dependent on patch si ze. Mortality on panels open to predation averaged 40.0 % (+/-4.3 % SE). Fi sh also significantly altered the distribution of sizes of oysters. The eff ect of predation was almost entirely attributable to toadfish Tetractenos s pp. Previous knowledge of the life history of oysters and succession in the se intertidal assemblages suggests that effects of predation may not, howev er, have important long-term consequences on natural populations.