Kenyan coral reef-associated gastropod fauna: a comparison between protected and unprotected reefs

Abstract

A comparative survey of prosobranch gastropods was undertaken within 3 marine parks and 3 unprotected reefs to compare faunal differences attributable to management and nonmanagement policies. The total gastropod fauna was typified by low densities and high diversity and variability in species composition. Protected reefs had a higher diversity for combined reef locations and higher species richness on reef edges and lagoons but not reef flats. Differences in reef edges are attributable to physical differences in reef aspect and wave energy but lagoon differences are attributable to human influences. Total gastropod densities were similar for comparisons between reef flat and edges but unprotected reef lagoons had higher (p < 0.001) densities than protected lagoons. Overall, most commercially collected species were not significantly denser in protected sites than unprotected sites. Some species such as Lambis truncata and L. chiragra were clearly more dense within protected sites, which may be attributable to shell collection in unprotected reefs. Many important commercial species including Charonia tritonis, Cassis cornuta, Turbo marmarotus, Chicoreus ramosus and Cypraecassis rufa were found at very low densities regardless of location and may naturally occur at such low densities. An experimental study of predation on the gastropod Morula granulata suggested that predation is low but may affect gastropod distribution patterns as the total gastropod density was well predicted (r = -0.96, p < 0.01) by predation rates on M. granulata. Unprotected reef lagoons had lower predation rates (p < 0.06) than protected lagoons. Reduced predation rates and increased densities may be caused by finfish reductions due to human fishing activities. Shell collecting appears to be affecting a few but not most populations. Less direct changes in the gastropod fauna appear to be caused by overfishng of finfish and a reduction in their predators.