THE LOWER FISHES 141 



2. Tiger Shark, Galeocerdo ciivier. This common species is more nocturnal 

 than most sharks. It reaches large size, has large, trenchant teeth, and is 

 especially dangerous when in the "feeding mood." Tiger sharks commonly 

 congregate about areas where refuse is dumped at sea. 



3. Hammerhead Sharks, Sphyrna species. This maneuverable species grows 

 to large size and is possibly guided to food more quickly than other sharks 

 because of the widely separated nostrils. The teeth are rather small. 



4. Mako Sharks, hums species. These are the fastest sharks that swim and 

 are among the most aggressive. They may be dangerous under certain 

 circumstances even though they are almost exclusively fish-eaters. 



5. Lemon Shark, Hy-poprion hrevirostris. It seems that this is a disagreeable 

 and unpredictable species that may, when large, be dangerous. 



6. The Ground Sharks, Black-Tipped Sharks, etc., Carcharinus species. Any 

 large members of this genus may be dangerous. The teeth are large and 

 trenchant. 



7. Sand Shark, Carchnrias taunis. There are some doubtful records of persons 

 being bitten by this shark. 



8. Blue Shark, Prionace glauca. This is a pelagic species, long reputed to be 

 dangerous. There are few records of attack.' 



9. Leopard Shark, Triakis semifasciata. This is a small shark which has 

 attacked divers on the Pacific coast. 



The conditions under which shark attack is likely are not very well known. 

 Springer (1943), in the best work yet published on this subject, has thoroughly 

 studied and compiled a list of well-authorized cases of attack. The following 

 analysis is largely adapted from him and from Budker (1947): 



1. Influence of Time of Day. There is an increase of frequency of attack with 

 increasing darkness. The bulk of attacks occur at dusk, but records of attack 

 are known for almost every time of day and night. Increased frequency 

 of attack at dusk is probably due to decreased visibility with darkness and 

 increased shark activity then. 



2. Influence of Water Clarity. As is the case with dusk, decreased visibility 

 means that sharks increase dependency on their lateral line for feeding. 



3. Influence of Water Depth. The danger seems to be slightly greater at or 

 near the surface. Sharks are in shallower water at night rather than during 

 the day. Nevertheless, there seems to be little or no danger in shallow 

 waters inshore of surf or inside of coral barriers. Sharks must at least have 

 access to deeper waters through channels or inlets. Sharks as a whole are 

 most abundant in fairly shallow waters not far offshore. 



4. Influence of Temperature and Geographical Location. Springer has mapped 

 the cases of shark attack on a world-wide basis, and it is apparent that 

 sharks are most dangerous in tropical and subtropical waters in temperatures 

 of over 66° Fahrenheit. The incidence of attack is far higher in southeastern 

 Australia than anywhere else in the world. The beaches of Sydney are 

 fenced in, so great is the danger of attack thought to be. Otherwise, 

 shark attack does not seem to be concentrated at any one place. 



