Echinometra mathaei (de Blainville) population increases, attributable to removal of their predators by fishermen, results in competitive exclusion of herbivorous fish and other sea urchin species, high reef substratum bioerosion and decreased reef topographic complexity. This study explores the population regulation of E. mathaei under conditions of reduced predator abundance. Experimental density manipulations suggest some density-dependent mortality, yet populations on heavily fished reefs continue to increase despite high biomass and reduced food. On two of three studied reefs, including the site with the highest population density, adult and juvenile densities were positively correlated in 1-m2 quadrats. Individuals on the high population-density reef had smaller gonads and gonad indices (gonad wt . body wt-1), lower relative gut weights, a higher fraction of calcium carbonate in their guts, larger Aristotle's lantern indices (jaw wt . body wt-1), and lower respiration rates than individuals on the low population density reef. Comparisons with individuals starved in aquaria indicate that these are responses to reduced food availability. There was no indication that larger individuals were able to avoid resource limitations more effectively than small individuals. Additionally, there was no indication of reduced body size with increasing population density. The frequency of intraspecific agonistic behavior during experimental manipulations was lower (G test, p < 0.001) on the high than on the low-density reef. Agonistic behavior appears important for defending burrows which reduces predator-induced mortality but not in maintaining a balance between the population food requirements and food resources. In high-density reefs, where predator densities and topographic complexity are low, the lack of predators and agonistic behavior or territorial defense leads to unrestrained population increases and reduced energy availability to individuals. Regulation occurs at the level of the individual organism, allowing high population densities through decreased energy availability to all individuals.
