FISHERY BULLETIN: VOL. 79, NO. 3 



the day. This difference in behavior would seem to 

 make the blue crab more accessible and would 

 probably tend to decrease rather than increase 

 CPUE during the night. 



Differences in the visual attractiveness of the 

 bait used to catch sharks may have existed, but 

 the limited visibility (<1 m) in the turbid waters 

 of the area suggests that vision was probably 

 of little importance in feeding behavior. Experi- 

 ments conducted on various species (Hobson 1963; 

 Hodgson and Mathewson 1978; Kalmijn 1978) 

 have shown the ability of sharks to locate food by 

 means other than vision and that feeding behavior 

 is directed towards almost any object present in 

 a high concentration of olfactory material. These 

 experiments indicate that the importance of 

 vision may be only to direct the final act of feeding. 

 It appears doubtful that day-night differences 

 in the visual attractiveness of the bait were re- 

 sponsible for increased CPUE during the night. 



Finally, the night hours may simply be a period 

 of increased feeding activity for young sandbar 

 sharks in this area. The existence of such a diel 

 pattern would not be surprising in that similar 

 rhythmicity for various activities (including 

 feeding) has been reported for several sharks 

 (Springer 1963, see footnote 5; Randall 1967; Hob- 

 son 1968; Nelson and Johnson 1970; Myrberg and 

 Gruber 1974; Finstad and Nelson 1975; Sciarrotta 

 and Nelson 1977). 



Numerous catches at surface and middepth 

 (Table 6) suggest that the young of this species 

 occur throughout the vertical range of the water 

 column. This would appear to be in conflict with 

 Springer's (1960) statement that sandbar sharks 

 are properly considered "ground sharks," rarely 

 seen at or near the surface. However, most of 

 Springer's observations were apparently made on 

 adult sandbar sharks. The conflict may simply 

 indicate differences between the young and adults 

 of this species. The increase in CPUE at surface 

 and middepth at night (Table 7) suggests that the 

 tendency of sandbar sharks to move into these 

 areas may have been greater at night and possibly 

 related to the movements of blue crabs discussed 

 earlier 



In general, food items taken by this shark 

 (Table 8) agree with those reported by other 

 authors (Bigelow and Schroeder 1948; Springer 



1960; Clark and von Schmidt 1965; Bass et al. 

 1973; Lawler 1977). The fact that the blue crab 

 was by far the major prey item may simply reflect 

 its abundance in the area rather than a specific 

 food preference. Wass (1973) indicated that food is 

 retained within the stomachs of sandbar sharks 

 for periods of 2-4 d or more, depending on 

 the consistency of the item. The relatively large 

 proportion of empty stomachs observed in this 

 study would then indicate that the young of this 

 species may frequently go at least several days 

 without feeding. Whether this is by choice or im- 

 posed by difficulties in capturing food is uncertain. 

 The higher proportion of empty stomachs ob- 

 served for sandbar sharks caught during the night 

 than for those caught during the day may relate 

 to the possible day-night difference in feeding 

 activity previously mentioned. 



Selectivity in the fishing method employed may 

 account for the observed yearly differences in sex 

 ratio (Table 9). Assuming equal numbers of both 

 sexes in the area, the method must have been 

 selective for females in 1977 but not in 1978 and 

 1979. Considering that the same methods were 

 employed in all years, it appears that the propor- 

 tion of male and female sharks utilizing the area 

 may have varied among years. 



It seems that sandbar sharks >112.5 cm TL 

 rarely occurred in the study area (Figure 2). The 

 failure to capture larger sharks was not felt to be 

 related to the fishing method because this method 

 has yielded numerous large sandbar sharks in 

 offshore areas (pers. obs.). A period of approxi- 

 mately 9 mo separates successive year classes 

 of sandbar sharks (Springer 1960), and age deter- 

 minations using vertebral annuli indicate rela- 

 tively rapid rates of growth for the first several 

 year classes (Lawler 1977). Lawler 's growth rates 

 suggest that the relatively distinct size classes 

 apparent in this study were probably a reflection 

 of the various year classes of sandbar sharks pres- 

 ent in the area. The impression of the existence of 

 3 to 5 different age-classes given by the size distri- 

 butions also agrees with Casey's (1976) contention 

 that young sandbar sharks may occupy nursery 

 ground areas for up to 5 yr before moving farther 

 offshore. 



ACKNOWLEDGMENTS 



•''Springer, S. 1943. Sharks and their behavior. Special 

 report to the Coordinator of Research and Development, U.S.N. 

 Emergency Rescue Equipment Section. 



We would like to acknowledge the management 

 and staff of The Marine Science Consortium for 

 their cooperation during this project. Special 



446 



