Observing Herbi vorv 



Quantifying fish grazing employed two methods. The first involved watching 

 each rack of six coral plates at each station for five-rninute intervals several 

 times a day and recording which species fed on them. The second method is identical 

 to the first except that an underwater time-lapse movie camera takes the place of a 

 diver. Each movie lapses between 10 and 12 hours and generates 3600 observations 

 (frames). During the movies, divers avoided the area except to simultaneously count 

 fish bites over a few five minute intervals. Since the plate areas are known, the 

 number of bites per square meter per hour can be determined for both techniques. 



Fish grazing was greatest in shallow forereef habitats and least at depth (Table 

 2). Mean grazing frequencies of over 5,000 bites per meter square per hour were 

 recorded using visual and time-lapse techniques in 1982 and visually in 1981. At the 

 algal ridge and bank barrier-reef sites grazing was predominantly from non-denuding 

 herbivores (i.e., Eupomacentrus ) . This group of damselfishes Cfour species) 

 accounted for 72£ (+ 30) and 50% (+ 22) of the bites observed at this site visually 

 and on film, respectively. Pomacentrid grazing on the deep wall-reef was relatively 

 low (6 7i + 12). Denuding herbivores (particularly Microspathodon ) were most 

 abundant at the rnid-depth forereef site. Excavating herbivores ( Scarus and 

 Spar i soma ) were most abundant at the shallow forereef site. Juveniles of this 

 group were the primary herbivores at the deep wall-reef sites. 



It is difficult to determine why a better correspondence between visual and 

 tin.e-lapse movie techniques does not exist, since both were conducted at the same 

 time (Table 2). Time-lapse, of course, gives high resolution data over an entire 

 day whereas visual techniques give high resolution over only short intervals spread 

 over several days. The latter technique also requires the presence of a human 



observer which may suppress normal grazing frequencies during the observation period. 



Invertebrate herbivores are easier to count, but more troublesome to actually 

 observe feeding since their mouths are under their bodies. Since they have reduced 

 mobility, their range of grazing influence is relatively restricted. Thus, some 

 indication of their impact can be assessed by determining their population density 

 in a given area. 



Excavating invertebrate herbivores (i.e., echinoid Diadema and limpet Acmaea ), 

 are most abundant in shallow forereef environments (Table 2). In fact, the two 

 shallowest forereef stations have more echinoids per area than all seven other 

 stations combined. Limpets were only found at the shallowest station. 



Impact of Herbivorv : 



The impact of herbivory requires studying the plants being eaten. Because 

 different functional groups of algae have different structural and morphological 

 properties that contribute to the rate at which they are consumed (discussed above), 

 I will treat three functional groups of algae differently. 



In terms of areal coverage, minute turfs are the most abundant algal form on 

 coral reefs. They are impossible to handle without damage so all turf experiments 

 and observations were confined to the coral plates. 



To determine herbivore impact on turfs, the rate of biomass loss to herbivores 

 was studied. For this, a set of six plates at each depth (at the deep wall-reef 

 site) was suspended in the water column at the depth they had been growing in for 

 the past three years. An identical set of plates remained on the benthos. No 

 herbivores were observed grazing the suspended plates (nor was there any evidence of 

 grazing using other techniques (i.e., Thalassia bloassay technique, described 

 below). Table 2 shows that there 1s a steady (nearly linear) decline 1n the rate of 

 turf algae biomass loss to herbivores with depth. Assuming that the productivity of 

 suspended plates equals that of the benthic plates, the difference in algal biomass 

 after a period of 6 days should indicate the amount of algal biomass that is eaten 

 during that period. Negative numbers indicate that algal growth rate exceeds the 



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