Effects of fine sediments on settlement and survival of the sea urchin Evechinus chloroticus in northeastern New Zealand

Populations of the sea urchin Evechinus chloroticus were monitored at wave-exposed low-sediment reefs and wave-sheltered high-sediment reefs in the Hauraki Gulf, northeastern New Zealand. Urchin populations on both wave-exposed and sheltered reefs were dominated by adults. Wave-exposed reefs supported higher densities (2.7 to 9.6 ind. m(-2)) than did wave-sheltered reefs (0.1 to 6.6 ind. m(-2)). Juvenile urchins (< 30 mm test diameter [TD]) were 25 times more abundant on wave-exposed reefs. Urchin settlement of 0.2 to 0.6 ind. 100 cm(-2) was recorded at wave-exposed reefs but was undetectable at wave-sheltered reefs. Laboratory experiments were conducted to assess whether the volumes of sediment loads observed on sheltered reefs could affect the survivorship of early life stages of urchins in the absence of other environmental factors. Results indicated that fine sediments at one-third and two-thirds of concentrations found on wave-sheltered reefs inhibited the settlement of larvae and reduced the survival of urchin recruits (2 to 5 mm TD) and juveniles (10 to 30 mm TD). Survival rates of juveniles were higher than for more recent recruits, suggesting size-dependent mortality. A lack of significant differences between high and low treatments in all experiments suggests that urchin settlement and survival can be affected negatively at low levels of sediment that are common in natural environments.