CORAL RECRUITMENT AT MODERATE DEPTHS: 

 THE INFLUENCE OF GRAZING 



H. Carl Fitz 

 Institute of Ecology, University of Georgia, Athens, GA 30602 



Marjorie L. Reaka 

 Dept. of Zoology, University of Maryland, College Park, MD 20742 



El dredge Bermingham 

 Dept. of Genetics, University of Georgia, Athens, GA 30602 



Nancy G. Wolf 

 Dept. of Ecology and Systematics, Cornell University, Ithaca, NY 14853 



ABSTRACT 



The effects of grazers on the biomass of algae and coral recruitment have been 

 investigated extensively in shallow water, yet the dynamics of this interaction in 

 deeper water have received, by comparison, relatively little attention. Fifteen 

 cement artificial reefs were established at a depth of 20m in Salt River Canyon on 

 the north coast of St. Croix. One third of the reefs were exposed to all grazing 

 organisms, one third were protected from macrograzers by exclusion cages, and one 

 third had partial cages (controls). After a year-long immersion, 267 corals of two 

 genera were found, and comparisons were made between the three reef treatments. 

 Numbers of newest coral recruits (3mm diameter and less) were similar on all types 

 of reefs, indicating similar rates of settlement. However, caged reefs, with visi- 

 bly greater algal biomass, had significantly fewer of the larger juveniles than 

 those that were uncaged and exposed to (primarily fish) grazers. Though relatively 

 few herbivores occur at these depths, herbivory nevertheless appears to indirectly 

 control the survival of coral recruits, and hence determines the structure of coral 

 communities on moderately deep reefs. 



INTRODUCTION 



Coral reefs and their primary structural component, scleractinian corals, have 

 been the focus of intensive study, with numerous investigators analyzing some aspect 

 of the ecology of the corals. Yet an understanding of the factors influencing coral 

 distribution is far from complete. The present study attempts to elucidate the ef- 

 fects of fish (and echinoid) grazing on the settlement and survival of scleractinian 

 corals at intermediate (20m) depths. 



The effects of competition for space between algae and coral planulae or settled 

 corals has received some attention (e.g.,0art 1972, Vine 1974, Kaufman 1977, Potts 

 1977, and others cited below), particularly in shallow water. Sammarco (1980) man- 

 ipulated densities of the echinoid Diadema antillarum in shallow water, finding that 

 ungrazed areas of high algal biomass (free from predation or disturbance by Diadema ) 

 allowed the highest rates of coral settlement. Subsequent survival, however, was 

 highest in areas subject to moderate grazing pressures: competition from algae in 

 ungrazed areas — and predation/disturbance in heavily grazed areas — reduced coral 

 survivorship. Certain fishes also crop the algae and/or prey on small corals. 

 Brock's (1979) microcosm study revealed low rates of coral recruitment in areas 

 exposed to grazing scarids. Given adequate spatial refuges for the corals, however, 



Present address: Biology Department, Woods Hole Oceanographic Institution, 

 Woods Hole, MA 02543. 



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