134 



COLIN 



which he estimated was longer than the 21 -ft boat it 

 passed by. Tiger sharks are seldom seen by divers and, 

 therefore, are not as much of a hazard as some smaller, 

 dangerous species. Randall (1980) found the scutes of a 

 green turtle shell, shark vertebrae, bird feathers, digested 

 shark fins, and pieces of a porpoise in the stomachs of 

 Enewetak G. cuvier. 



There have been several instances of shark attacks at 

 Enewetak. Most have involved the gray reef shark and, in 

 some, injury occurred to the human involved. Hobson et 

 al. (1961) documented two incidents with gray reef sharks 

 in which spearfishing probably stimulated aggressive 

 behavior. Fortunately neither instance resulted in injuries. 

 Not so lucky was another individual whose head was 

 slashed by the upper jaw of C. ambl\;rh[jnchos after the 

 powerhead he was using to try to kill this shark failed to 

 detonate on impact (Randall, 1980). 



In April 1978, another attack by C. ambli/rhi;nchos 

 occurred in which a 1.5-m (5-ft) long individual severely 

 mauled the right arm of a diver and attacked his diving 

 partner (M. V. deGruy and P. Light, unpublished report). 

 In this case deGruy approached the shark, which was ex- 

 hibiting the threat posture, in an attempt to photograph it. 

 When deGruy triggered the electronic strobe of his cam- 

 era, the shark turned, rushed toward deGruy, and seized 

 his arm. Seconds later the shark tore a chunk from one 

 diving fin. As the diver's companion, Light, swam to his 

 assistance, the shark bolted toward Light and badly 

 slashed his hand. The shark disappeared. Both divers sub- 

 sequently recovered from their wounds. 



The most recent attack by C. ambli^rhimchos on 

 humans occurred in January 1982 when one of the repa- 

 triated Marshallesc. while spearfishing, had his left arm 

 mauled by what was probably a gray reef shark. Several 

 sharks were around this fisherman and his two compan- 

 ions, who were carrying a considerable quantity of dead 

 fishes. 



Randall and Helfman (1973) reported two instances of 

 C. melanopterus menacing humans at Enewetak. 



It is interesting to note that despite repeated success in 

 producing threat displays and attacks by C. ambli;rhi;nchos 

 by pursuit with small wet submersibles, similar attempts 

 have failed to produce the threat or attack by C albimar- 

 ginatus, C. melanopterus, and T. obesus (Nelson, MPRL, 

 1979; Starck, MPRL, 1971 to 1972). 



Crater Life 



Nolan et al. (1975) described bottom substrata and 

 fishes inhabiting the two small craters (Cactus and 

 Lacrosse) at the north end of Runit Island. Hard substrata 

 were restricted to the upper 4 m and the sides of both 

 craters sloped quickly to a sediment plain at about 12 m 

 deep. The bottoms of the craters were "heavily excavated 

 by several species of gobies and burrowing shrimps." 

 Other bioturbating organisms were also present. Colonies 

 of Hahmeda and Derbesia minima were abundant on the 

 sediment bottom. Hillis-Colinvaux (1980) found no 



Halimedae in Lacrosse crater but found a pure, dense 

 strand of Halimeda incrassata in the murky center of 

 Cactus crater at 11 m depth. This merged peripherally 

 with Caulerpa ad serrulata. but no loose plants were seen 

 on the sides of the crater. Halimeda incrassata was rare at 

 Enewetak (Hillis-Colinvaux, 1980), and the Cactus crater 

 population was the only dense (about 200 or more thalli 

 m~^) strand found at Enewetak. Hillis-Colinvaux (1980) 

 suggested that the extremely soft and fine sediment of 

 Cactus crater might have promoted the growth of this 

 dense strand, possibly from a limited number of vegetative 

 propagules. 



Nolan et al. (1975) found little living coral in the Runit 

 craters but reported that "molluscs, crustaceans, poly- 

 chaetes, zooplankton, algae, and phytoplankton found in 

 the craters seemed typical of the fauna and flora occurring 

 in the adjacent lagoon or upon the reef flat." Eighty-four 

 species of fish were observed or collected in the craters; 

 the number is incomplete for various cryptic families. Cen- 

 suses at near high and low tides indicate fewer individuals 

 of species at low tides than at high tides. 



The third small atomic test crater, Seminole, on Bokcn 

 (North) Island has not been examined biologically. Adjacent 

 sand flats seem to be an area with high numbers of small 

 blacktip sharks. Circulation into Seminole crater is much 

 more restricted than circulation into either Runit atomic 

 crater. 



The three large thermonuclear craters in the north 

 lagoon have not previously been described biologically. 

 During the Enewetak Submersible Project, several dives 

 were made in Oak crater using the submarine, and addi- 

 tional scuba dives were made on the crater slope. The 

 level bottom of Oak crater (Ristvet et al., 1978; Chapter 

 4, this volume) was heavily bioturbated, almost certainly 

 by callianassids, with a mound density equaling that 

 observed anywhere else at Enewetak. In addition, the 

 irregular urchin Maretia planulata occurred in high densities 

 of 10 m on the surface of sediment at the bottom of the 

 crater. Similar bioturbation was evident in Koa crater, 

 although at a lesser depth. Nelson and Noshkin (1973) did 

 not consider that biologically mediated presentation of 

 radionuclides from within the sediment column to the 

 water column was occurring in Koa (and Mike) craters but 

 that the "principal loss of activity from the deposits may 

 only be from the slow release to the overlying waters." 



Smith and Brock (MPRL, 1976) found that the Mike- 

 Koa and Oak craters have large amounts of rubble in the 

 vicinity of the craters which provide an unfavorable sub- 

 stratum for coral growth. The locations of the craters are 

 described in Chapter 3 of this volume. 



Why Are There No Sea Grasses 

 at Enewetak? 



Tsuda et al. (1977) have summarized the known distri- 

 bution of sea grasses in Micronesia. Only one species, 

 Thalassia hemprichii, is known from the Marshall Islands. 

 Records exist for it from Ailinglapalap, Jaluit, and Ujilang 



