SUBTIDAL ENVIRONMENTS AND ECOLOGY 



133 



ciguatera at Enewetak is related to radiation in the 

 environment. Disturbance of the marine environment 

 (dredging, construction, wrecks, etc.) has been strongly 

 implicated in producing ciguatera (Randall, 1980). 



The probable causative organism of ciguatera, a 

 dinoflagellate Gambierdiscus tox'cus, has been identified 

 and the toxin collected and purified. 



Sharks 



Several species of sharks are common at Enewetak. 

 They range from nearly harmless to extremely dangerous. 

 Some are found in only one environment, whereas others 

 are nearly ubiquitous 



The blacktip reef shark, Carcharhinus melanopterus, is 

 abundant on the reef flats all around Enewetak. Hiatt and 

 Strasburg (1960) reported C. melanopterus was the most 

 common shark on windward and leeward reefs. Hobson 

 (1963) reported blacktip sharks were most often observed 

 on sand and coral rubble flats in shallow water. It often 

 penetrates into water so shallow the dorsal fin and back 

 are well exposed. Small C melanopterus individuals are 

 most common on the reef flat. Larger individuals cruise 

 the spur and groove zone offshore and are often seen 

 around lagoon margin pinnacles. 



The whitetip reef shark, Triaenodon obesus, is f)€rhaps 

 the next most commonly seen species. It is most abundant 

 in the lagoon along the marginal sandy areas and reefs but 

 is also found on seaward reefs. Hobson (1963) found 

 T. obesus most often on patch reefs and coral ledges 

 around the margin of the lagoon. Randall (1977) reported 

 that T. obesus feeds largely on reef fishes, especially 

 scarids and acanthurids, plus octopuses. 



Also found in the lagoon is the lemon shark, Negaprion 

 brevirosths, which although large, penetrates into relatively 

 shallow water. The author once nearly stepped down onto 

 the back of a 1.5 m lemon shark while wading ashore on 

 Ikuren in knee-deep water. 



The most studied and the most dangerous shark at 

 Enewetak is the gray reef shark, Carcharhinus ambli;rh\;n- 

 chos. It is found throughout the lagoon and on the sea- 

 ward reefs. Hobson (1963) felt it was most abundant in 

 the deeper waters of the lagoon and passes. Attacks on 

 humans are discussed subsequently. Johnson and Nelson 

 (1973) described in detail the threat display of the gray 

 reef shark, which often precedes an attack. Sharks placed 

 in a situation of a diver potentially restricting its escape 

 produced the most intense displays: an exaggerated, often 

 rolling, swimming motion with back arched, pectoral fins 

 dropped, and snout lifted. Starck (MPRL, 1971 to 1972) 

 elicited attack responses on a small wet submersible by 

 pursuing C ambl\;rh\^nchos. A more detailed account is 

 presented in Anderson (1980). Subsequently, this attack 

 and its preceding threat display have been investigated by 

 Nelson (MPRL, 1978, 1979). He found that the shark usu- 

 ally attacks after displaying if the object or person contin- 

 ues to approach. The attacks are sudden, high-speed 



strikes, often with the mouth open. He believed that 

 "oriented pursuit" by the small submarine was of primary 

 importance in releasing an attack. A straight-line pass near 

 the shark never released an attack, although it did produce 

 the threat display. For more information on gray reef 

 shark behavior, see Nelson et al. (1986). 



Randall (1980) reported that C. ambliirht;nchos from 

 Enewetak and other localities fed mostly on reef fishes 

 and, to a lesser extent, on cephalofxxls. It is the most 

 common shark seen on seaward reefs. Off the southwest 

 islands and on the leeward reef face, it usually app>ears 

 before the silvertip shark, C. albimarginatus, and out- 

 numbers the latter shark two or three to one. In the 

 lagoon it is common around mid-lagoon pinnacles where it 

 seems particularly aggressive. Often when a boat stops in 

 mid-lagoon on a calm day, one or more C. ambl^rhi^nchos 

 will rise to investigate the boat from water 50 to 60 m 

 deep. 



The movements of C ambl\^rhi>nchos tagged with ultra- 

 sonic transmitters have been investigated by Nelson 

 (MPRL, 1978, 1979). He has determined that gray reef 

 sharks often move surprisingly long distances around 

 Enewetak. Deep-water gray reef sharks tagged on or near 

 the drop-off of the seaward reefs ranged as much as 

 16 km along the reef in one night. They were not as 

 predictably home ranging as lagoon gray reef sharks, and 

 Nelson (MPRL, 1979) suggested they might represent a 

 more nomadic segment of the population. Lagoon grays 

 were tracked for as long as 21 days, and although some 

 stayed in one area, others moved considerably. One 

 individual tagged at the mid-lagoon "dome" pinnacle sfsent 

 the daylight hours near that pinnacle but ranged widely at 

 night. Its home range was estimated at about 53 km . 

 McKibben and Nelson (1986) discussed movements of 

 tagged gray reef sharks at Enewetak. 



Other seaward reef sharks are the silvertip shark, 

 Carcharhinus albimarginatus, and Galapagos shark, 

 C galapagensis. The silvertip shark is found normally on 

 seaward slopes below 20 to 30 m, although Randall 

 (1980) observed one individual in the lagoon in water 2 m 

 deep. There are reports of C. albimarginatus as deep as 

 400 m (Randall, 1980). Silvertip sharks feed almost 

 exclusively on fishes, both reef and open water. Randall 

 (1980) also found at Enewetak a gray reef shark over 60 

 cm in total length in the stomach of a C. albimarginatus 

 that was 1.6 m in total length. The Galapagos shark is a 

 large, dangerous species, but fortunately it is uncommon at 

 Enewetak. Randall (1980) collected only a single specimen, 

 but little is known of its habits beyond feeding on fishes 

 (including sharks) and cephalopods. 



Probably the largest dangerous shark in Enewetak 

 waters is the tiger shark, Galecerdo cuvier. Randall (1980) 

 examined two specimens from Enewetak, 1.7 and 2.4 m 

 precaudal length (length minus the caudal fin), of 72 and 

 174 kg, respectively. McNair (1975), an accurate and 

 experienced shark observer, while diving on the leeward 

 seaward reef, observed a huge tiger shark pass above him 



