THE EFFECTS OF TIME, HABITAT, AND FISHERIES MANAGEMENT ON KENYAN CORAL-REEF-ASSOCIATED GASTROPODS

Abstract

Populations of live snails were studied over a 13-yr period in 18 Kenyan reef locations. Sites were divided among three levels of management (old, new, and un- managed) and two types of habitat (reef lagoon and reef flat). Management prohibits the extraction of snails and predatory fishes, while both snails and their predatory fishes were removed in unmanaged sites. In reef lagoons I hypothesized that (1) fishing would result in increased total snail numbers in unmanaged reefs due to losses of their predators but no changes would occur in fully managed reefs, (2) the responses would be variable for commercial species depending on the balance between predatory fish effects and shell collecting; while on reef flats (3) commercial snails would be most abundant on managed reefs, and (4) there would be no population changes due to either high predation or stressful physical conditions. Most snail populations were stable with only 18% of the populations displaying statistically significant population fluctuations, and commercial species were more stable than noncommercial species. As predicted, reef lagoons had the greatest range of population densities with the fully managed lagoons having the lowest numbers. Numbers of noncommercial snails increased with time in all reef lagoons in support of the fishing effect, but against the stability prediction for managed reefs. Unmanaged reef lagoons did, however, have the largest recorded population increase. Protected reef flats did not have greater numbers of commercial snails compared to unmanaged reefs. They did, however, have a greater abundance of six commercial species while having the lowest noncommercial snail population numbers. Total reef flat snail abundance was stable except during the final sampling period in 1999 when the noncommercial snails increased in the new and unman- aged sites. This might be due to a chance or periodic oceanographic event, such as the 1997-1998 El Niiio, but could also suggest that predation on reef flats is more important than I hypothesized. The snail assemblage appears to be stable but some large-bodied species such as Cassis cornuta, Charonia tritonis, Lambis truncata, and Turbo marmoratus were found in insufficient numbers to evaluate their status.