SUBTIDAL ENVIRONMENTS AND ECOLOGY 



131 



species stationed farther from the reef had more cylindrical 

 bodies with deeply incised caudal fins than species remain- 

 ing relatively close. 



The fish communities on lagoon margin patch reefs 

 were examined in detail by Nolan (1975) for reefs between 

 Enewetak and Medren. These reefs are typical of those 

 found throughout the lagoon margin on the windward side. 

 He divided the fish community into four assemblages: 



1 . The patch reef assemblage (about 25 species) 



2. The roving fish assemblage (about 25 species) 



3. The sand assemblage 



4. The rubble assemblage (3 and 4 together about 

 50 species) 



The most numerous fishes living on the patch reefs were 

 cardinalfishes (Apogonidae) and damselfishes (Pomacentri- 

 dae) A large percentage of these individuals are mid-water 

 plankton feeders, relying on items brought by the steady 

 cross reef flat currents from ocean to lagoon. Nolan (1975) 

 found that fish species composition of lagoon margin patch 

 reefs on the windward side visually censused at about 

 100-day intervals fluctuated considerably over 2'/2 years of 

 observations. Among five "control reefs," each had 20 to 

 24 species at the end (mean 21). Individual reefs varied by 

 as many as 10 species during the study. The numbers of 

 individuals, however, varied by as much as a factor of 10 

 during the study. One reef went from about 100 individu- 

 als to 970 because of juvenile recruitment of two species 

 of apogonids and pomacentrids. If increases related to 

 juvenile recruitment were not considered (or in the case of 

 those reefs where massive juvenile recruitment did not 

 occur), numbers of individuals were much more consistent, 

 varying by less than a factor of 2. 



Nolan (1975) found considerable movement among reef 

 fishes between lagoon margin patch reefs on the windward 

 side of Enewetak. He reported various surgeonfishes, 

 wrasses, and parrot fishes as ranging freely between reefs. 



Although identifiable assemblages oi' fishes occur in a 

 particular environment, there is small-scale variation in spe- 

 cies composition. Nolan (1975) constructed artificial reefs, 

 made of cement pipe "modules," on the lagoon margin 

 between Enewetak and Medren to provide identical shelter 

 to reef fishes which inhabited those reefs. Artificial reefs 

 reached species equilibrium in 100 to 200 days, a figure 

 equivalent to defaunated natural reefs, but the colo- 

 nization pattern differed from natural reefs. About 10 spe- 

 cies occurred on the artificial reefs (versus about 20 for 

 small natural reefs), which had limited habitat diversity, 

 and variation over time was much higher for artificial reefs 

 than natural reefs. 



Gladfelter et al. (1980) examined the fish communities 

 of lagoon margin patch reefs between Enewetak and 

 Medren and near the deep channel but utilized reefs over 

 an order of magnitude larger than those studied by Nolan 

 (1975), averaging 150 to 200 m^ in area. Compared to 

 western Atlantic patch reefs of similar size, Enewetak reefs 

 are steef)€r sided with greater vertical relief and more com- 

 plex surface topography. The number of species (visually 



censused) on the Enewetak patch reef varied between 76 

 and 109, with about 500 to 900 individuals per reef. Con- 

 sidering trophic categories, diurnal planktivores were more 

 abundant on Enewetak than Virgin Island patch reefs, 

 probably because of the consistent ocean to lagoon cross- 

 reef currents. Herbivores were more diverse among 

 Enewetak reefs with fewer individuals. The Virgin Islands 

 reefs were surrounded by sea grass beds, a habitat lacking 

 at Enewetak, and had more nocturnally foraging 

 invertebrate-feeding fishes. 



The interrelationships between reef fishes on Enewetak 

 patch reefs are complex. Competition for space and 

 food can be intense between species and among conspecif- 

 ics. Factors controlling initial recruitment of juveniles and 

 their eventual growth to adults are additional controllers of 

 ultimate community composition. Nolan (1975) describes 

 numerous instances of unique interactions among fish 

 species inhabiting small patch reefs on the lagoon margin. 

 Many of these interactions were the result of experimental 

 manipulation, but others were simply the result of long- 

 term careful observation of the environment. Nolan's 

 (1975) record is, perhaps, the best such record of relation- 

 ships and occurrences among a diverse group of fishes on 

 small reefs over time. 



Allen (1972a, b) described instances where removal of 

 adult anemonefish from their host anemones was followed 

 within a few weeks or months by recruitment of large 

 numbers of juvenile Amphiphon. Anemones unoccupied by 

 Amphiprion were not encountered by Allen (1972a, b), 

 and he felt that anemone availability was one of the major 

 factors limiting anemonefish populations at Enewetak. The 

 situation has not changed since Allen's work; anemones 

 remain relatively uncommon and Amphiphon populations 

 are limited compared to other tropical Pacific areas. 



The fish communities of the outer reefs, deep lagoon, 

 and open ocean around Enewetak are not as well docu- 

 mented. Even the nearshore spur and groove is poorly 

 known because of its normally hazardous surf conditions. 

 The movement of herbivores and predators onto the reef 

 flat with rising tides is well documented (Hiatt and Stras- 

 burg, 1960; and others). Population sizes, movement along 

 the reef face, and foraging dynamics are not well known. 



FISH REPRODUCTION AND 

 RECRUITMENT 



Most reef fishes reproduce by either laying demersal 

 eggs on the substrate or releasing planktonic eggs in the 

 water column. Both hatch as planktonic larvae. Larval life 

 ranges from a minimum of 2 to 3 weeks to as much as 2 

 to 3 months. Lack of proper substrate for metamorphosis 

 may greatly extend this time. Some information exists con- 

 cerning the spawning habits of demersal- and planktonic- 

 egged species at Enewetak. 



Major families producing demersal eggs include p>oma- 

 centrids, gobiids, and blennies. Swerdloff (1970) and Allen 

 (1972a, 1975) have described various aspects of pomacen- 

 trid spawning at Enewetak. 



