Population dynamics of the sea urchin Heliocidaris erythrogramma on the east coast of Tasmania

Formation of sea urchin 'barrens' has been documented widely in temperate regions of both northern and southern hemispheres. While the phenomenon has been reported worldwide, the mechanism(s) underpinning barren ground formation are poorly understood. Elucidating the mechanism(s) of barrens formation is important, not the least reason being because it may provide options for management to minimise the establishment and or spread of urchin barrens. One potential mechanism is through increases in urchin population density as a result of reduced predation mortality because of exploitation of predators. We examined whether fishing of rock lobster (Jasus edwardsii) and fish predators on rocky reefs in Tasmania could account for population increases of the sea urchin Heliocidaris erythrogramma to the point where overgrazing of macroalgae may lead to urchin barrens. Large-scale surveys (over > 100 km of coastline) of abundances of urchins and putative predators revealed a significant negative relationship between urchins and lobster abundances, but not between urchins and demersal fishes. At smaller scales, both large lobsters and demersal fish are significantly more abundant inside no-take marine reserves than in equivalent adjacent habitat subject to fishing. These observations are consistent with results of small-scale experiments in which the rate of urchin mortality was ea. 23 times greater inside two marine reserves than in adjacent exploited habitats. Caging experiments also showed that predation by rock lobsters on sea urchins is highly size-specific. Juvenile lobsters are not capable of predating urchins with test diameter (TD) > 60 mm, while medium-sized lobsters preyed on urchins up to 80 mm TD, and large urchins >81 mm TD were only at significant risk of predation mortality from large lobsters. Demersal fish were notably less important predators of urchins between 40-100 mm TD than were rock lobsters. Inside reserves, if urchins were tethered so that they could not seek shelter, then juvenile urchins were predated more frequently than larger ones. However, when tagged urchins were free to move, mortality of adult urchins was significantly higher than that of juveniles, because juveniles were able to shelter in small crevices. I also examined the possibility that higher urchin population densities on barrens could be the result of sporadic large recruitment events. In mark-recapture experiments, three distinct urchin populations were used to construct individual-based growth models to generate size-at-age relationships. Age frequency distributions of urchin populations in algal beds and adjacent barrens habitat were estimated using this relationship and examined for the presence of dominant cohorts which may indicate prominent recruitment events. Significant differences were found in the age frequency distributions of urchin populations in barren and algal bed habitats at two separate locations, with dominant peaks in the age structures of urchins in barrens populations unmatched by those in adjacent algal beds. Estimates of age-specific mortality rates and age frequency distributions derived for several distinct urchin populations enabled construction of matrix models of population growth. Using the empirical estimates of size-specific per capita rates of predation by lobsters on urchins, I estimate the effect of exploitation of lobsters in regulating urchin population density by comparing model projections for urchins based on lobster populations inside and outside marine reserves. The modelling suggests clearly that the reduction in biomass of legal-sized lobsters on the east coast of Tasmania due to fishing is sufficient to account for increases in populations of H. erythrogramma to the point where barrens formation may occur.