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Fishery Bulletin 97(2), 1999 



lantic sharpnose sharks (>500 mm PCD were always 

 greater than 0.5 ppm. 



Bonnethead shark, Sphyrna tiburo 



Total mercury levels for the 95 juvenile and adult 

 bonnethead sharks sampled (297-1081 mm PCD 

 ranged from 0.13 to 1.5 ppm (3c =0.50 ppm; me- 

 dian=0.29 ppm) (Table 1). Forty percent of all juve- 

 niles and adults tested had mercury levels that were 

 greater than or equal to the 0.5-ppm threshold level. 

 Mean lengths of males (.v=564.6 mm PCD and fe- 

 males (x=619 mm PCL) were not significantly dif- 

 ferent (Mann-Whitney rank sum test, P>0.1) and 

 there were no significant differences in mercury lev- 

 els between males (x=0.49 ppm) and females (x = 

 0.50 ppm) (Mann-Whitney rank sum test, P>0.5). 

 There was a significant positive correlation between 

 total mercury level and bonnethead shark length 

 (both sexes combined) (P<0.0001; Fig. 4). 



Total mercury levels for the 41 embryos examined 

 (206-255 mm PCL) ranged from 0.08 to 0.35 ppm 

 (.r=0.16 ppm; median=0.13 ppm). Mean length for 

 male and female embryos were both 232-mm PCL. 

 Total mercury levels of male (x=0.15 ppm) and fe- 

 male (x=0.18 ppm) embryos were similar (Table 2). 

 Mercury levels for embryos within each litter were 

 similar (+ 0.03 SDXTable 2), as were overall mer- 

 cury levels among litters (mean mercury level/litter 

 - 0.1-0.28 ppm). Total mercury levels in embryos of 

 this species equaled 9.1-60.4% of levels observed in 

 their respective mothers. 



Discussion 



Results of this study indicated that most individu- 

 als in the four species of sharks tested had accumu- 

 lated levels of total mercury that were as high as or 

 higher than the regulatory threshold levels. Approxi- 

 mately 60% of the juvenile and adult sharks tested 

 in this study had total mercury levels that were 

 greater than or equal to the 0.5-ppm threshold level, 

 and 12% had levels greater than 1.5 ppm. Informa- 

 tion regarding mercury in sharks from other areas 

 is limited; however, elevated levels of mercury in 

 various shark species have been documented in sev- 

 eral regions, including waters off Florida and the 

 southeastern United States (Gardner et al., 1975; 

 Hueter et al., 1995; FHRS-), Canada (Forrester et 

 al., 1972), Great Britain (Vas, 1991), and Austraha 

 (Lyle, 1984, 1986). Although the life-history patterns 

 of different shark species are diverse, most sharks 

 grow slowly and have long lifespans. These general 

 life-history characteristics, together with the high 

 trophic status of many shark species, may contrib- 

 ute significantly to the accumulation of high concen- 

 trations of mercury (Lyle, 1984). 



Mercury levels were directly related to shark size 

 in all species tested. Length differences accounted 

 for 78%' of the total variation in mercury levels ob- 

 served in bonnethead sharks, nearly 70% in Atlantic 

 sharpnose sharks, and approximately 54% in blacktip 

 sharks. The relation between mercury levels and 

 length of juvenile bull sharks was less clear; length 

 differences accounted for only 24% of the variation 

 in mercury levels. The comparatively weak relation 

 between mercury level and size in this species may 

 stem from the fact that only neonates and small ju- 

 veniles were sampled. Growth rates of bull sharks 

 at the juvenile stage are thought to be quite variable 

 (Dodrill, 1977;Thorson and Lacey 1982; Branstetter 

 and Stiles, 1987), which may explain some of the 

 observed variability of mercury levels in juveniles of 

 this species. For species we examined, the relations 

 between total mercury and shark size demonstrated 

 that larger, presumably older, individuals accumu- 

 lated higher levels of mercury. The accumulation of 

 mercury with increasing size and age is likely to be 

 a result of the slow and inefficient elimination of 

 mercury from fish tissue in relation to the rate of 

 accumulation (Rodgers and Beamish, 1982; Bryan, 

 1984). 



Because mercury levels increase as individuals 

 grow larger, levels in juveniles can potentially be 

 viewed as minima for the overall population. Al- 

 though all bull sharks tested were either neonates 

 or juveniles and the majority of blacktip sharks were 

 juveniles or young adults, both species typically had 



