INTERTIDAL ECOLOGY 



153 



TABLE 8 



Major Prey Items of Panulirus 



penicillatus on Three Reef 



Platforms at Enewetak 



'Numbers in body of table are numbers of 

 prey taxa at left recovered from examination 

 of 78 P penicillatus stomachs (McCollum, 

 1981). 



UTILIZATION OF INTERTIDAL 

 HABITATS BY FISHES 



Windward platform surfaces dominated by a film or 

 thin turf of the blue-green alga Calothrix Crustacea are 

 grazed intensively by fishes. Miller (1983) observed about 

 530 fishes per hour at high tide swimming through a 3 X 

 5 m quadrant on the northern part of the Enewetak Island 

 platform. About 85% of these were herbivores, mainly par- 

 rot fishes and surgeonfishes. The predominant species are 

 Scarus frontalis. Acanthurus triostegus, and A. guttatus 

 (Hiatt and Strasburg, 1960; Bakus, 1967; Webb and 

 Wiebe, 1975; Miller, 1983). "Their teeth marks in the reef 

 rock provide evidence of the thoroughness with which they 

 crop this alga" (Wiebe, Johannes, and Webb, 1975). "The 

 most striking phenomenon about the reef flat is the innu- 

 merable toothmarks that range from the uppermost 

 reaches of the dead coral substratum to the outer edge of 

 the algal ridge, and beyond" (Bakus, 1967). 



As the incoming tide covers the platform, large schools 

 of Acanthurus triostegus "gradually browse their way to 

 the uppermost reaches of the reef flat" (Bakus, 1967). 

 Behind them arc large schools (about 300 to 400 fishes) of 

 the larger A. guttatus, then numerous schools of Scarus 

 spp., which remain in slightly deeper water, venturing 



close to shore only between mid-tide and high-tide level 

 (Bakus, 1967). Analyses of Scarus frontalis and S. gibbus 

 indicated that both species graze only on dead coral and 

 filamentous algae on reef rock. They consume considerable 

 CaC03, which is acidified in the gut and reduced in 

 particle size. These fishes commence feeding at first light 

 (about 0730 at Enewetak). Less than 4 hours later, all indi- 

 viduals (mean standard length 31 cm) had full large intes- 

 tines. Feeding continues until dusk (about 1900), at which 

 time most individuals contained food in the anterior diges- 

 tive tracts. Six hours later, all portions of all digestive 

 tracts examined were empty (Smith and Paulson, 1974). 

 These authors thus calculated transit time of food through 

 the alimentary tract of 6 hours. 



Juvenile Acanthurus (riostegus also browse only on 

 algae; stomachs of adults contain mostly algae but with a 

 few small coral fragments. Acanthurus guttatus graze "sig- 

 nificant quantities of coral fragments along with benthic 

 algae" (Bakus, 1967). Analysis of tooth scars by Bakus 

 (1967) showed that small tooth marks of A. guttatus and 

 juvenile scarids predominate in the inner 18 m; scars of 

 acanthurids and scarids intermingle over most of the plat- 

 form; and the algal ridge, surge channels, and pools of the 

 outer edge have mainly scarid scars. 



From his estimate of net primary productivity of blue- 

 green algae noted above and by estimating the biomass of 

 herbivorous fishes utilizing the platform, Bakus (1967) con- 

 cluded the Calothix Crustacea and Schizothrix cakicola syn- 

 thesize organic matter at a rate adequate to support the 

 feeding activities of the fishes. These are, however, time- 

 limited by periods of high tide and probably cannot meet 

 their entire energy requirements by feeding only on the 

 windward platform. 



Other herbivorous fishes on seaward platforms at high 

 tide are the browsing rabbitfish Siganus argentatus and, 

 near the surf-swept outer edge, the surgeonfishes 

 Acanthurus achilles and Zebrasoma veliferum (Hiatt and 

 Strasburg, 1960). 



That the limitation of algal cover on the more barren 

 regions of the platforms is due to grazing has been demon- 

 strated by Miller (1983) who reported 100% coverage of 

 the platform surface by macroscopic algae after 3 months 

 under 5-mm mesh cages that excluded grazing fishes and 

 crabs but not smaller herbivorous invertebrates. Inver- 

 tebrate abundance also increased in the exclosures. 



Algal cover grades strikingly from the thin film of 

 Calothrix and Schizothrix at the north end of the Enewetak 

 Island seaward platform to a turf of erect, macroscopic 

 Jania. Sphacelaria, and other forms about 300 m south. 

 There fish grazing intensity is only about 30% of that 

 observed farther north. The southern portion is probably 

 less accessible to herbivores because of its greater distance 

 from suitably sheltered subtidal sites required by the fishes 

 at low tide, such as the quarry on the north part of the 

 platform and the lagoon (Bakus, 1967; Kohn and Leviten, 

 1976; Miller, 1983). 



Omnivorous fishes on the seaward platforms include 

 the common blenny Istiblennius coronatus, which eats sur- 



