NATURAL HISTORY OF THE SANDBAR SHARK 



33 



centration of sharks occurring near the mouth of 

 tlie Mississippi River in the summer moutlis may 

 be as great as anj'where in the world. Some idea 

 of the predation that may occur is furnished by 

 the resuhs of one line of ISO shark hooks set on 

 the night of July 22, 1947, off Pass a TOutre, La. 

 This line caught 68 large sharks, whole and un- 

 damaged, but iO of the hooks had only the 

 heads of large sharks and 12 more had only 

 the heads of small sharks. Almost all of the 

 larger sharks on the line contained small sharks 

 or pieces of large ones. One tiger shark, Gat^o- 

 cerdo. had swallowed a medium-sized bull shark 

 which in turn had a major portion of a somewhat 

 smaller blacktip in its stomach. The blacktip, 

 however, had driven the head of a Mitstelu^ cants 

 al)out 4 feet long up the leader so it would be 

 reasonable to assume that the large tiger shark 

 was tlie fourth shark to be taken in that one 

 night on one hook. The bottom conditions, 

 depths, and temperatures where this line was set 

 appeared to be similar to conditions on milhcrfi 

 nursery grounds but predation would presumably 

 eliminate young 7niJberfi in the area. 



Full-grown sandbar sharks probably are rarely 

 subject to successful shark attack by other spe- 

 cies. The tiger shark (Galeocerdo), dusky shark 

 {EuJamia ohscura), and bull shark (Carcharhinus 

 leucas), have all been found occasionally with 

 pieces of full-grown sandbar shark in tlieir stom- 

 achs, but the sandbar sharks may have been on 

 shark lines when attacked. Perhaps none of 

 these species are able to catch adult sandbar 

 sharks under ordinary circumstances. Great 

 white sharks, Carcharodon carchanas. have been 

 found with adult milberfi in their stomachs and 

 it is probable that the great white shark could 

 catch them. The white shark is not common 

 enough, however, to be an important factor in 

 predation. At Salerno, captures were about 27 

 great white sharks per 100,000 of all species. 



All carcharhinids more than 6 feet long may 

 occasionally eat young E. m./'lherfi, but circum- 

 stances of seasonal and geographical distribution 

 keep most species from preying on them. There 

 are two notable exceptions: the tiger shark and 

 the bull sliaik. Tliese sharks feed on young 

 •sharks or on small species regularly, and both 

 may be found at times within the known range 

 of young inilbcrti. 



Tiger sharks, perhaps for reasons of poor speed 

 and maneuverability associated with their smaller 

 and lighter weight fins, catch young siiarks less 

 frequentlj' than do bull sharks. Furthermore, 

 tiger sharks aie primarily nocturnal in forays 

 into inshore waters and at such times newborn 

 or small sharks retreat to shoaler water. On the 

 other liand, bull sharks are not exclusively noc- 

 turnal and thej' are more fi-equent in relatively 

 shallow water. Stomach contents show that they 

 are regularly predatory on small species such as 

 Carcharhinus acronohis, C. porosus, Scoliodon 

 terra-novae, and Apnonodon isodon. The bull 

 shark also is the only species in the range of 

 E. milberfi with a preference for shark as a 

 bait. 



There is strong circumstantial evidence, derived 

 from an examination of the geographical ranges 

 and nursery-ground preferences of the various 

 species of Eidamia, that the bull shark is the 

 most important predator on j^oung sharks and 

 actually restricts the distribution of milberti. The 

 life history of the bull shark is similar to that 

 of species of Eulnmia. but the bull shark is al- 

 ways found in shallow water and its nursery 

 grounds are in bays or estuaries, even in brackish 

 and fresh water. The nursery range of the bull 

 sliark is shoaler and less saline than the nursery 

 range of milberti, but adult bull sharks inhabit 

 the depth and salinity range normal to nursery 

 grounds of milberti. Young bull sharks remain 

 in estuarine waters during the early growing 

 season, and since the females do not eat at the 

 time the young are born and generally move out 

 of the very shallow water immediately after the 

 birth of the young, the young bull sharks are 

 protected to some degree from predation by the 

 large members of their own species. 



Bull sharks occur from Long Lsland southward 

 and are migratory but their centers of abundance 

 are in the (Julf of Mexico and southward, par- 

 ticularly near the mouths of large rivers. Along 

 the Atlantic coast north of Florida, bull sharks 

 are not common, but may be subject in this area 

 to great fluctuation in abundance. Bull sharks 

 are extremely common around the mouths of the 

 Mississippi and Orinoco Rivers and between these 

 points along the inshore Continental Shelf. There 

 is practically no information about tlieir occur- 

 rence in the West Indies. 



