Echinoid bioerosion and herbivory on Kenyan coral reefs: the role of protection from fishing

During feeding, echinoids remove a large proportion of calcium carbonate in addition to the algae growing on dead coral and are consequently of import ance in estimating the turnover of organic and inorganic carbon in coral re efs. Rates of herbivory and the erosion of dead coral substratum, referred to as bioerosion, by the most abundant echinoid species in Kenyan reefs, Ec hinothrix diadema (Linnaeus), Diadema setosum (Leske), D. savignyi (Micheli n) and Echinometra mathaei (de Blainville), were compared in three differen t reef categories with different histories of fishing and its: exclusion. T hese were reefs: (i) protected within Marine National Parks, which exclude all forms of fishing, coral and shell collection for more than 25 years; (i i) one reef within a Marine Park, which has received protection from fishin g activities for 8 years (referred to as 'newly protected' reef), and (iii) unprotected reefs, which experience heavy fishing and some coral collectio n. The aim was to investigate the grazing and bioerosion activity by the ab ove echinoid species in these reef categories. We surveyed sea urchin popul ation densities and determined their rates of bioerosion and herbivory per individual and square meter. Individual rates of bioerosion and herbivory, of the species D. setosum, D. savignyi and E. diadema were estimated from l aboratory gut content analysis and gut evacuation experiments in the field, using elevated underwater cages. Individual rates of bioerosion and herbiv ory of E. mathaei were obtained from a previous field study [J. Exp. Mar. B iol. Ecol. 147 (1991) 121]. Sea urchin bioerosion was greater than herbivor y for all studied species and proportional to the body size of the sea urch in species. The large-bodied E. diadema exhibited the highest bioerosion an d herbivory rates (5.5 +/-0.9 and 2.2 +/-0.3 g individual(-1) day(-1) respe ctively) followed by D. setosum (1.8 +/-0.3 and 1.1 +/-0.2 g individual(-1) day(-1)) and D. savignyi (0.7 +/-0.2 and 0.4 +/-0.1 g individual(-1) day(- 1)). Highest sea urchin densities were recorded at unprotected reefs (6.2 /-1.5 individual m(-2)), and therefore, bioerosion and herbivory by sea urc hins were also highest in this reef category (1180 +/- 230 g CaCO3 m(-2) ye ar(-1) and 450 +/- 77 g algae m(-2) year(-1)). Protected reefs recorded 20 times lower sea urchin bioerosion and herbivory rates (50.3 +/- 25.8 g CaCO 3 m(-2) year(-1) and 20.7 +/- 10.4 g algae m(-2) year(-1)), due to the low sea urchin population densities in these reefs (0.06 +/-0.01 individual m(- 1)). The newly protected reef, with intermediate number of sea urchins (1.2 +/-0.1 individual m(-2)), had intermediate rates of sea urchin bioerosion and herbivory (711 +/- 157 g CaCO3 m(-2) and 299 +/- 63 g algae m(-2) year( -1)). These findings suggest that echinoids are important in the carbon cyc le and reef development, and that fishing can influence these ecological pr ocesses. (C) 2001 Elsevier Science B.V. All rights reserved.