SUBTIDAL ENVIRONMENTS AND ECOLOGY 



129 



tunnels below. Burrow systems linking the down holes with 

 the volcanoes are complex, often consisting of a series of 

 interconnected horizontal tunnels (as much as 4 to 5 cm in 

 diameter), and sloping to vertical tunnels connecting differ- 

 ent levels. As much as 1300 g of sediments were ejected 

 per day from each volcano. Callianassids alter grain size 

 distribution of processed sediment to produce a very con- 

 sistent sediment size fraction which is depleted (compared 

 to some other Enewetak sediments) in both coarse (>2 

 mm) and fine (<90 microns) sediments (Suchanek and 

 Colin, 1985). As much as 3 liters of water evolves daily 

 from volcanoes during sediment-pumping activities (Colin 

 et al., 1986). Volcano water contained suspended particu- 

 lates >0.45 microns in diameter at levels at least five 

 times that of water immediately overlying the sediment, 

 which itself has elevated particulates compared to "aver- 

 age" lagoon water (Colin et al., 1986). 



"Tagged" (painted with fluorescent paint) sediment 

 experiments have demonstrated that most large particles, 

 more than 1.5 to 2 mm in diameter, entering the ghost 

 shrimp burrow system are not returned to the surface 

 (Suchanek et al., 1986). Probably such particles are too 

 large to be temporarily suspended by pumping, and it is 

 believed that callianassids "store" large particulates in 

 unused portions of the burrow system. 



A constant "disturbance" effect occurs in bioturbation 

 at Enewetak. The constant grazing by herbivores, the 

 digging into the sediment by carnivores looking for prey, 

 and the ingesting and then expelling of sediment by the 

 surface-dwelling species cause the upper few centimeters 

 of sediment to be constantly disturbed (Suchanek and 

 Colin, 1986). The sediment surface, through this action, is 

 a continuous mosaic of small pits and mounds, disturbed 

 places and tracks, all from this surface reworking. In the 

 lagoon at depths shallower than about 5 to 6 m, this evi- 

 dence is quickly obliterated by wave ripples, but below 

 that depth the disturbances remain apparent for some 

 time. 



FISH COMMUNITIES 



The fish fauna of Enewetak is quite diverse, numbering 

 more than 800 species. There are, certainly, a number of 

 species yet to be recorded from the atoll. Fish species are 

 not evenly distributed around the atoll; many occur com- 

 monly in only one type of habitat. These preferences result 

 in general fish communities which are identifiable assem- 

 blages of species. Hiatt and Strasburg (1960) made the 

 first (and still best) attempt to characterize fish communi- 

 ties of various areas of Enewetak and to examine their 

 trophic relationships. Subsequent researchers have exam- 

 ined feeding by various portions of the Enewetak fish 

 fauna (Hobson and Chess, 1978; Randall, 1980; Bakus, 

 1967; Gerber and Marshall, 1974; Smith and Paulson, 

 1974; Reese, 1975, 1977; and others), but a definitive 

 study of the overall trophic dynamics of fishes has never 

 been undertaken. 



Randall has reviewed records of fishes since Schultz 

 and collaborators (1953 to 1966), and a checklist is 

 included in Chapter 27 of Volume 11, this publication. 

 Although this may be approaching a definitive list of fishes 

 for the Marshall Islands for shallow-water species, it was 

 apparent from the observations and photographs from the 

 submersible Makali'i that a significant number remain to be 

 recorded (many undoubtedly undescribed) from depths 

 greater than those usually penetrated by scuba divers. 



The species of fishes inhabiting a given location at 

 Enewetak are strongly influenced by environmental factors. 

 Primary among these are substratum types (hard or soft, 

 variations of these), depth, current, wave action, and oth- 

 ers. The food of Enewetak fishes is based on two different 

 sources: primary production from atoll bottoms and waters 

 and oceanic zooplankton and phytoplankton. The relative 

 importance of these two pathways has never been 

 rigorously compared, but the high productivity of reef flat 

 and lagoon versus the low density of phytoplankton and 

 zooplankton in oceanic water upcurrent of Enewetak imply 

 that the former is of considerable significance. Seeing the 

 immense numbers of large, herbivorous fishes on spur and 

 groove, reef flat, and shallow patch reefs impresses one 

 with the amount of fish life supported by algae growing on 

 the substratum. 



Predators of mid- and upper-water lagoon areas arc 

 varied. Hiatt and Strasburg (1960) differentiate mid- and 

 surface-water communities but p>oint out that some large 

 carnivores enter both areas. They felt that surface water 

 communities contained various sizes of zooplankton, small 

 plankton-feeding fishes (round herring and silversides), 

 larger macroplankton-feeding fishes (such as halfbeaks), 

 and piscivores (needlefish, tunas, barracuda, jacks). Randall 

 (1980) and Hiatt and Strasburg (1960) have discussed the 

 food habits and general habits of many of these. 



The Carangidae are important predators at Enewetak. 

 The members of the genus Caranx are largely fish eaters, 

 occasionally taking cephalopods or crustaceans. Caranx 

 ignobi/is, the largest species of the genus, reaches 80 kg, 

 and as Randall (1980) notes "may be encountered any- 

 where in the atoll environment including water surprisingly 

 shallow for such a large fish." Caranx melampi^gus is very 

 common on Enewetak reefs and feeds largely on reef 

 fishes, including some such as Caracanthus sp., that live 

 deep within the branches of living corals. The rainbow 

 runner, Elagatts bipinnulatus, is a mid-water feeding caran- 

 gid which occurs in schools above reefs. 



Among scombrid fishes, the dogtooth tuna, 

 G\;mnosarcla unicolor. is the only tuna commonly seen 

 around lagoon pinnacles. It also occurs on outer reefs and 

 is a predator on free-swimming fishes, including Naso 

 spp., Caesio, Ptercxaesio, and Decapterus (Randall, 1980). 

 Hiatt and Strasburg (1960) reported it and the common lit- 

 tle tunny, Euthynnus affinis, as slashing through the dense 

 schools of round herring. 



Some species of moderately large fishes are detri- 

 tivores. Mullets are common on shallow reefs, both lagoon- 

 ward and seaward of the reef flat. Crenimugil crenilabis has 



