Though no overall differences in numbers of corals existed between types of 

 reefs, the size (age) structure of corals on the reef types was dissimilar. Corals 

 on each type of reef were divided into five size classes at 3mm intervals. The 

 total number of corals in each size class for the combined five reefs of each type 

 are depicted in Figure 1; the general trend of similar numbers of newest (^3mm) 

 recruits and differing numbers of older corals between types of reefs is evident. 



A two-way ANOVA indicated a significant effect of type of reef treatment (blocked 

 over size classes) on the mean number of corals per reef. With this knowlege, a 

 one-way ANOVA on each size class was used to determine which size class(es) had 

 different mean numbers of corals between the reef types. The only significant dif- 

 ference was found in the >6-9mm class, with a Tukey's Studentized Range Test in- 

 dicating that the caged reefs had significantly fewer corals than the uncaged reefs 

 (Table 2). Shading by cages was insignificant, as the cage control did not differ 

 from the uncaged reefs. 



Table 2. Mean number of corals in each size class for each treatment type. 

 Five replicates in each cell, numbers in parentheses are standard deviations, 

 asterisk indicates significant difference (p<0.05). 



Caged Uncaged Cage control 



^3mm 3.8(±2.6) 2.6(±2.0) 4.4(±4.0) 



>3-6mm 3.8(±2.6) 10.6(±7.2) 8.6(±4.6) 



>6-9mm 1.8(±1.9) * 7.0(±3.7) 3.2(±2.5) 



>9-12mm 1.2(±1.3) 3.4(±4.0) 1.6(±1.1) 



>12mm 0.0(±0.5) 1.0(±1.2) 0.0(±0.5) 



Grazing fishes and invertebrates 



Diadema anti 1 larum was an uncommon echinoid on these reefs, well removed from the 

 hard substratum of the East Wall of the canyon. One complete census showed an 

 occasional Diadema on the ten uncaged and cage control reefs for a mean of 0.4 per 

 reef. Total available surface area (including 12X12cm holes uncensused for corals) 

 was 3.4m . 



The herbivorous fishes around the reefs were acanthurids, scarids and pomacen- 

 trids. Bicolor damselfish ( Eupomacentrus partitus ) were common residents on the 

 reefs, along with occasional juvenile blue tangs ( Acanthurus coeruleus ). Schools of 

 roaming doctorfish (A. chirurgus ) and ocean surgeons (A. banianus ) were common but, 

 due to their transient nature, difficult to quantify (see Wolf et aj. 1983 for the 

 results of the fish censuses). Princess parrotfish ( Scarus taeniopterus ) were seen 

 occasionally, again roaming about between different reefs. 



The cages effectively excluded these larger macrograzers , as evidenced by the 

 very noticable difference in cover of filamentous algae. The algal 'turf on the 

 exposed reefs was cropped extremely short, whereas thick, long tufts of filamentous 

 algae and some fleshy algae were present on the completely caged reefs (see Figure 

 2). 



DISCUSSION 



Recruitment to these reefs, as evidenced by numbers of the smallest corals, was 

 similar in both ungrazed and grazed situations. Though algal biomass was visibly 

 greater on ungrazed reefs, space for coral settlement was apparently adequate in 

 either type of habitat. 



Large differences in numbers of >6~9mm corals between caged and uncaged reefs 

 indicate that coral survivorship had been greater in areas subject to some grazing 

 pressure. Thus, any incidental predation on corals by the fishes (and echinoids) 

 was more than offset by the reduction in competition with the algae. Though differ- 

 ences between the types of reefs are suggestive in the >3-6mm class, the lack of 



94 



