A study on factors affecting the decreases in abalone resources by comparing strategies in reproduction among different species

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The reproductive biology and early life ecology of abalone species (Haliotis discus discus, H. gigantea and H. diversicolor), which inhabit the southern area in Japan, were investigated by field observations and laboratory experiments in this study. Differences in reproductive strategy among abalone species including the northern species H. discus hannai, were examined. Maturation processes of adults, cues for their synchronized spawning, larval dispersion and transport to the settlement places, factors affecting larval settlement, post-larval survival and growth, were especially intensively studied and compared among species and habitats.Relatively high seawater temperature in winter since 1990's is suggested as the main factor increasing natural recruitment of H. discus hannai. However, young-of-the-year juveniles encountered the low winter seawater temperatures when they grew up to 3-7 mm SL several months after they were born. The annual recruitment level appears to be influenced also by earlier post-larval occurrence and mortality, which are strongly affected by the adult density. Dense patches of males and females appear to be important for fertilization success and post-larval survival in abalone species. For H. discus hannai, relatively high adult densities guaranteed larval occurrence and subsequent post-larval survival. On the other hand, adult densities are generally much lower in southern large species than in H. discus hannai and H. diversicolor, which is considered to be the major factor causing their low recruitments. The environmental deteriorations for larval settlement and post-larval survival were also suggested to be important factors affecting low natural recruitments of the southern abalone species. Laboratory experiments suggested that the small abalone H diversicolor has greater fertilization success than larger species due to differences in spawning behavior and gamete ability to fertilize.