Population fluctuations of three sympatric species of Haliotis (Mollusca: Gastropoda) in artificial habitats at Kominato, central Japan

Abstract

Population fluctuations in artificial habitats were studied for three co-occurring species of Haliotis, H. discus discus, H. gigantea and H. diversicolor aquatilis, in the shallow areas of a small cove at Kominato, on the Pacific coast of central Japan, for more than 5 years. For information on environmental conditions, detailed maps are given with substrata, depths, and floral characteristics in and around the study areas. The artificial habitats were made by laying concrete blocks in three areas where different macroalgae dominated, i.e., Sargassum serratifolium, S. ringgoldianum and Ecklonia cava. The three species of haliotids showed different use of artificial habitats, possibly related to differences in their habits and micro-habitats. The population of H. discus discus, particularly individuals of middle and large sizes, showed considerable seasonal fluctuation after rapid colonization. H. gigantea and H. diversicolor aquatilis gradually increased in the early period after the laying of the concrete blocks, thereafter becoming rather constant in number. However, few large individuals of H. gigantea used the artificial habitats throughout the study period, while a high proportion of large, older H. diversicolor aquatilis individuals was observed. Based on the results of these observations, locomotory activity and co-existence are discussed. A comparison of population density was made between artificial and natural habitats; the artificial habitats supported much higher densities than natural habitats. In addition, the practical significance in managing the Haliotis fishery by making artificial reefs is also discussed.