Indirect benefits of marine protected areas for juvenile abalone

Marine protected areas (MPAs) designed to provide harvest refugia for red sea urchins (Strongylocentrotus franciscanus) offer a unique opportunity to study the indirect effects of urchin fishing on subtidal communities. Sea urchins may provide important cryptic microhabitat for juvenile abalone sheltering beneath urchin spines in shallow habitats worldwide. We investigated the abundance of juvenile (3-90 mm) red abalone, (Haliotis rufescens) and the rare flat (<90 mm) abalone (H. walallensis) on protected and fished rocky reefs in California. Abalone abundance surveys were conducted inside 24 x 30 m plots on three protected reefs with red sea urchins present and three fished reefs where red sea urchins were removed by commercial or experimental fishing. Significantly more juvenile abalone were found in 1996 and 1997 on protected reefs with urchins present than on fished reefs ((2) = 188, df = 1, p < 0.001). Juvenile red abalone abundance was not correlated with local adult red abalone abundance or habitat rugosity. One-third of the juveniles inside the MPAs were found under the urchins' spine canopy, as were a suite of unfished marine organisms. In the laboratory, juvenile abalone survived better ((2) = 7.31, df = 1, p < 0.01) in crab predation experiments in which red sea urchins were available as shelter. Fishing red urchins reduced structural complexity, potentially decreasing microhabitat available for juvenile abalone. This example demonstrates how MPAs designed for one fished species may help other species, illustrating their usefulness for ecosystem-based fishery management and marine conservation.