NATURAL HISTORY OF THE SANDBAR SHARK 



19 



a few might have been overlooked. The fins of 

 E. milberti adults (pectorals, first dorsal, and 

 lower caudal lobe) are more desirable for com- 

 mercial purposes than the fins of some other 

 species because they are thicker and have a rela- 

 tively lar>::e proportion of the material used for 

 shark-fin soup. Shark fishermen withi.' wliom I 

 tallced and who would recognize imlberti also re- 

 ported the absence of the species from their 

 catches made oif Colombia and Venezuela. Thus, 

 evidence for the occurrence of E. milberti in the 

 southwestern Caribbean, while not very satisfac- 

 tory, is negative. ~a ,n)v 



f 



MIGRATION 



It is probable that the migrations of E. mil- 

 berti are of two kinds. One is simply the gradual 

 withdrawal of the sharks from waters that be- 

 come too cold or too warm — a movement that is 

 accompanied by normal feeding acti\nties and is 

 characteristic of immature sharks. The other is 

 a movement, generally, over a greater distance 

 that may or may not be induced by temperature 

 changes. The general patterns of the major 

 movements of adult sandbar sharks suggest that 

 ocean currents greatly influence the direction and 

 extent of the movements. 



It is necessary to consider the migratory move- 

 ments of the adult male, the adult female, and 

 the immature Eid-amia milberti separately, "^'e 

 may look upon the Atlantic coast from the vi- 

 cinity of Cliarleston, South Carolina, to the 

 northern part of Florida as the core of distribu- 

 tion of the principal stock of the western North 

 Atlantic population because it is only in this area 

 that all three groups are known to be found. 

 This may mean merely that in this area there is 

 overlapping distribution. "We know tliat the 

 adult females go as far north as Long Island to 

 give birth to young in summer and in some years 

 even farther, to tlie vicinity of Cape Cod. There 

 are no data to show whether the adult males or 

 nongravid females mo\e nortliward into the por- 

 tion of the species' range lying nortii of the Caro- 

 linas. All that is known of the distribution of 

 the young is tliat the voting are born in water of 

 moderate salinity from Cape Canaveral to (\ape 

 Cod and tliat some of them move in winter into 

 the comi)aratively warmer offsliore water found 

 at depths of 50 to 75 fathoms on the Carolina 



coast. Until the young sharks reach adult size 

 they do not take part in the long southern migra- 

 tion characteristic of adults or move south of 

 Cape Canaveral. One capture of a lot of nearly 

 200 young off North Carolina indicates that the 

 young occur in schools of both sexes and of mixed 

 sizes. 



Migratory movements of adult E. milberti 

 south of Cape Canaveral are more clearly out- 

 lined from the data available from the shark 

 fishery. The annual southward movement ap- 

 pears to be coincidental with the beginning of 

 cooler weather and to be accelerated by cold 

 snaps. 



A very much larger number of adult female 

 sandbar sharks were taken than adult males by 

 the Florida shark fishermen. This and the tend- 

 ency to segregation by sex, will be discussed later 

 in connection with reproduction. But it should be 

 pointed out here that no adult males are recorded 

 from inshore nursery grounds and probably occur 

 there but rarely. In the Florida shark fishery, 

 adult males of most species brought more money 

 than adult females and were particularly sought 

 by fishermen. The fishermen were convinced by 

 observation that adult males of most species in- 

 cluding m,ilberti were usually in deeper and cooler 

 water than the females, that they usually pre- 

 ceded the females in migration, and that they 

 usually were to be found in more compact aggre- 

 gations so that fishing was best where males could 

 be found. Such figures as are available bear this 

 out. A rationale for this condition is suggested 

 by the thermal sensitivity (decreased fertility 

 with application of heat) of the male germ plasm 

 among vertebrates in general (see Cowles, 1945). 



The sandbar shark is properly considered a 

 "ground shark" and is rarely to be seen from the 

 surface except when it comes into shallow water. 

 An exception to this occurs off Salerno immedi- 

 ately following ]>eriods of especially cold winter 

 weather nortli of that area. At such times, when 

 weather and water surface conditions permitted 

 observations from a boat, large schools of mil- 

 berti were to be seen headed south, swimming at 

 about .S to 5 knots 5 to 10 feet below the surface, 

 but where water deptiis were about 20 fathoms or 

 more. Shark fishermen have tTt>kt:jHe, a«d-=fiay< 

 o^vn expedience bears this out, that it is useless 

 to trv to follow these sharks or to trv to divert 



I 



