herbivore grazing rate. 



Macrophytes are not usually abundant on reefs. In the study areas, macrophytes 

 were most abundant in the intertidal zone of the algal ridges at Boiler Bay (e.g., 

 Gracilaria, Laurencia, Gelidiella, Dictvota and Acanthophora) and to a lesser 

 extent on the Acropora cervicornis in the sand plain in front of the forereef sites 

 (e.g., Asparaaopsis, Dictvota and Laurencia ). A scattering of macroalgae can also 

 be found at the backreef site (e.g., Acanthophora, Laurencia and Dictvota ). 



To determine grazing pressure on macrophytes, a Thalassia bioassay technique of 

 Hay (1981b, and Hay et jj. 1983) was used. For this, five centimeter long blades of 

 Thalassia were placed in clothes pins. The blades were checked every several hours 

 to determine the amount grazed by herbivores. Bite marks were interpreted as to 

 whether they were from parrotfishes (semicircular bites) or urchins (a shredded 

 appearance) (See Fig. 2 in Hay et _aj. , 1983). At each station, eight blades of 

 Thalassia were affixed to the rack of coral plates and an additional 20 were 

 scattered nearby on the bottom. 



The rate of Thalassia loss was greatest at the deepest forereef station (for both 

 those scattered on the bottom and attached to the rack; Table 2). The next highest 

 rate of loss at the Tague 3ay stations was in the backreef. In this case however, 

 only those scattered on the bottom were heavily grazed. The deep wall-reef sites 

 showed a consistent decrease in Thalassia grazing with depth. None of the suspended 

 Thalassia blades were grazed. 



Nearly all grazing marks were attributable to parrotfishes. No urchin marks were 

 identified on Thalassia blades attached to the racks of coral plates. At benthic 

 stations, however, a few urchin marks were observed. The nighest proportion of 

 urchin marks occurred in the shallow forereef stations of the Tague Bay locations 

 (where urchin densities were highest). The 10X urchin bites recorded for the 120' 

 station represents only one urchin-looking bite out of 10 recognizable bites. It is 

 probably an error since no urchins were found below 90'. Other shredding herbivores 

 such as crabs could have been responsible for the marks. 



The pattern of Thalassia loss across the Tague Bay reef is opposite that of all 

 other measurements of herbivory. There is no indication from other measures (Table 

 2) that the deep forereef, and to a lesser extent the backreef, receive as high a 

 rate of predation as the Thalassia bioassay suggests. Conversely, all other 

 techniques indicate that the two snallow forereef stations are most heavily grazed 

 by all herbivore groups rather than mininally grazed as the Thalassia bioassay 

 indicates. There is no question, however, that wnatever the Thalassia bioassay 

 measures, it does so consistently and repeatably. It is possible that fishes with a 

 search image for Thalassia , or other conspicuous macrophytes of sand flats or 

 lagoons, are attracted to the Thalassia of this experiment and consume it at a rate 

 unrepresentative of overall grazing rates on the reef. 



Relatively few herbivores are capable of excavating crustose coralline crusts 

 (discussed in detail in Steneck 1983). Since crusts can "erase" most graze marks as 

 they grow undisturbed for 10 to 30 days (depending on the depth of the injury and 

 the growth rate of the coralline), marks on corallines are a rough indication of the 

 rate of grazing by excavating herbivores. In addition, the bite marks of the major 

 groups of excavating herbivores (i.e., parrotfishes, urchins, limpets and chitons) 

 are readily identifiable. The pattern of graze marks among excavating herbivores 

 indicates that the greatest grazing pressure occurs in the shallow forereef. Data 

 were not collected for the deep wall -reef site. Overall urchin grazing was most 

 conspicuous on the crusts, with the proportion of graze marks generally corresponding 

 with the abundance of urchins in the area (Table 2). 



109 



