“Assessing the potential for recovery of a Sargassum siliquastrum community in Hong Kong” Tung Ping Chau overgrazed by sea urchins

Tung Ping Chau Marine Park in Hong Kong was designated in 2001 with aims to protect its highly-diverse marine habitats from destructive human activities. Sargassum spp. (mainly Sargassum siliquastrum) formed a dense canopy in Lung Lok Shiu (LLS) in southwestern side of the Marine Park. This Sargassum bed, however, disappeared and a barren ground developed 7 years after the marine park designation. The increase in density of the short-spined sea urchin Anthocidaris crassispina, and thus the grazing pressure, as a result of restriction on its commercial fishery within the Marine Park was hypothesized as the immediate cause of the barren ground formation. In this study, we tested this hypothesis and manipulated the density of A. crassispina in exclusion cages underwater in LLS, where S. siliquastrum canopy originally existed. Transplantation of S. siliquastrum thalli and ceramic tiles with their recruits was carried out in these cages in February–May 2012 and May 2013. The results showed that the transplanted Sargassum thalli could not survive outside exclusion cages, whereas the induced recruits were almost wiped out in all tiles even inside exclusion cages. This provides evidence that the sea urchin A. crassispina could have overgrazed S. siliquastrum thalli and decimated the Sargassum canopy in LLS. It also suggested that other than A. crassispina, other grazers also contributed to the removal of S. siliquastrum recruits. Therefore, natural recovery of Sargassum bed seems unlikely if the grazing pressure remains high in this site. Proactive management strategies may need to be adapted to restore this Sargassum bed.