High density and secondary production but variable recruitment of a sea urchin in subtidal barren areas of Hong Kong

Sea urchins can cause a state shift from algal beds to barrens through grazing, but little is known how management measures may affect their population dynamics. We compared four populations of Heliocidaris crassispina inhabiting the subtropical shores of Hong Kong with different levels of protection: a no-take reserve, a marine park, and two sites with no restriction to fishing. Bi-monthly surveys revealed 1% to 36% juveniles (< 20 mm in test diameter) in the four populations, indicating variable recruitment. Density varied from 9.1 to 13.6 individuals m⁻² in the first year and 10.0 to 12.6 individuals m⁻² in the second year. Biomass varied from 8.07 to 15.69 g ash-free dry-weight (AFDW) m⁻² in the first year, and 10.33 to 16.45 g AFDW m⁻² in the second year. These values are higher than the global threshold of sea urchin biomass needed to maintain sea urchin barrens. Secondary production varied from 4.68 to 9.29 g AFDW m⁻² year ⁻¹ in the first year and 6.62 to 7.96 g AFDW m⁻² year ⁻¹ in the second year. These values are high when compared with available sea urchin production data in the literature (0.001–8.44 g AFDW m⁻² year ⁻¹). The no-take reserve population had the highest sea urchin density and biomass, but its production was not the highest in the second year of study due to low recruitment. The results were discussed with respect to the dilemma of spillover effects of marine protected areas and losing macroalgae to urchin grazing.