Holothurian assemblages before the harvest-boom era in inner reefs of Ishigaki Island, focusing on population dynamics of lollyfish Holothuria atra Jäger, 1833

Abstract

Distribution and abundance of holothurians before the boom of fisheries in the 1990s were analyzed in relation to types of bottom sediment and biomes in inner reefs of Ishigaki Island, Okinawa, Japan. The data had been collected continuously over three years at Yoshihara and once at three other sites. Fifteen species were found, among which the dominant Holothuria atra and Stichopus chloronotus were abundant on bottoms with sand and seagrass, and those with sand and coral cobbles, respectively. The continuous survey at Yoshihara revealed that the overall density of H. atra was 2 to 3 ind. m−2, which was the highest among values reported previously, and spatial-temporal distribution of H. atra was almost stable throughout the research period, but the distribution of St. chloronotus fluctuated irregularly. The density of H. atra fluctuated seasonally (high in winter and low in summer) and was correlated negatively with air temperature and irradiation, and positively with cloud cover. The body length of H. atra became longer in summer and shorter in winter. The frequency of asexual reproduction (fission) was estimated at less than 1% of annual recruitment, and the growth rate was estimated to be low (K=0.0845 in von Bertalanffy equation). The high density and slow growth suggested that the H. atra population had reached carrying capacity at Yoshihara. The sandcoated ratio of H. atra was positively correlated with wind speed, however, no trend was found with the spatial-temporal variation in the density and distribution of the sand-coated individuals.