Habitat use of Chinese horseshoe crab, Tachypleus tridentatus under the influence of simulated oyster cultch

Little is known about the ecological impacts of oyster culture structures on intertidal communities. In the present study, distribution and movement patterns of juvenile Chinese horseshoe crab, Tachypleus tridentatus were assessed on a mudflat at Ha Pak Nai in Deep Bay, Hong Kong. As the traditional bottom-laying method using concrete tiles or concrete posts as cultch for collecting spat is a common practice in Hong Kong, we used the structurally similar bricks to simulate the potential effects of cultch on intertidal flats. Over the two-month experimental period, all the tested sediment physio-chemical characteristics, including median particle size and total organic content, remained unchanged among the treatment areas. However, juvenile densities were lower and foraging trails shorter at low- and high-density bricks areas as compared to the adjacent bare areas. In addition, most juveniles (> 95%) were observed feeding along the outer boundaries of bricks areas. Such findings imply that the extensive artificial structures in oyster cultivation sites could induce physical disturbance and alter the habitat use of juvenile horseshoe crabs in the intertidal zone. Considering the high conservation value of Chinese horseshoe crab, appropriate mitigation measures, such as removal of accumulated farm debris and adoption of elevated cultivation systems with lower ecological impacts, should be implemented to buffer the detrimental effects on juvenile horseshoe crabs that utilise intertidal habitats as nursery and hatchery grounds.