NOTE Adams and McMichael: Mercury level in four species of sharks from tfie Atlantic coast 



377 



high mercury concentrations. High mean methyl- 

 mercury levels (>1.0 ppm wet weight) were also de- 

 tected in 29 larger size-class bull sharks (approx. 

 1300-2800 mm TL> and six blacktip sharks (> approx. 

 1500-1800 mm TL) collected off Florida (Hueter et 

 al., 1995). Two of these bull sharks were collected off 

 east-central Florida; however, specific size or sex data 

 were not reported for these individuals. The observed 

 relation of length to total mercury in juvenile bull 

 sharks, together with results reported for larger size- 

 class bull sharks and other closely related species 

 (Lyle, 1984, 1986; Hueter et al., 1995), indicates that 

 large adult bull sharks could contain excessive mer- 

 cury levels. Blacktip sharks collected off northern 

 Australia, which were of similar mean length and 

 size range to those examined in our study, also had 

 relatively high total mercury levels (Lyle, 1984, 1986). 

 These data suggest that through bioaccumulation, 

 larger, older adults of these common species poten- 

 tially contain excessive mercury burdens. Total mer- 

 cury levels for larger size-class Atlantic sharpnose 

 sharks (>500 mm PCD that we examined were con- 

 sistently greater than 0.5 ppm. Muscle tissue from 

 the related Australian sharpnose shark, Rhizo- 

 prionodon taylori. and the milk shark, R. acutus, 

 tested from northern Australian waters (Lyle, 1986) 

 contained mercury levels that were comparable to 

 levels we detected in Atlantic sharpnose sharks. Milk 

 sharks of comparable size examined by Lyle ( 1986 ) had 

 total mercury concentrations (range=0. 16-2.00 ppm 

 wet weight; .v=1.01 ppm) that were very similar to 

 those in Atlantic sharpnose sharks in the current study. 



Bonnethead sharks had the lowest mean mercury 

 content of the four species tested. The relatively low 

 levels found in bonnethead sharks may be related to 

 the comparatively fast growth rate and short life span 

 of this species; however, diet is also an important 

 factor. Bonnethead sharks off Cape Canaveral, 

 Florida (senior author, unpubl. data), as well as off 

 southwest Florida (Cortes et al., 1996), feed princi- 

 pally on crustaceans, whereas the other three spe- 

 cies consume a larger proportion offish (Compagno, 

 1984; Snelson et al., 1984; Castro, 1996; senior au- 

 thor, unpubl. data). Although available data are lim- 

 ited, crustaceans may contain mercury levels that are 

 lower than those found in many fish species (Gardner 

 et al.. 1975; Stickney et al., 1975; Jop et al., 1997). 



Mercury levels observed in embryos and neonates, 

 as well as comparisons between pregnant females 

 and associated embryos, indicated that transmission 

 of mercury from maternal sources may be an impor- 

 tant factor contributing to total mercury concentra- 

 tions in shark muscle tissue. Total mercury levels in 

 embryos of all species examined ranged from 8.3 to 

 60.4% of levels observed in their mothers, whose 



mercury levels were greater than the 0.5-ppm thresh- 

 old (Table 2). Mercury levels of embryos within all 

 litters were found to be similar. Mercury concentra- 

 tions in embryos were variable among species. At- 

 lantic sharpnose and bonnethead shark embryos all 

 contained total mercury levels below the 0.5-ppm 

 threshold; in contrast, all four embryos collected from 

 a 1050-mm-PCL female blacktip shark had mercury 

 levels greater than the threshold level. Variations in 

 mercury concentrations found in embryos of differ- 

 ent species could stem from differences in gestation 

 periods or related physiological differences. Blacktip 

 sharks have a two-year reproductive cycle with a 12- 

 month gestation period (Castro, 1996), whereas At- 

 lantic sharpnose sharks have a one-year reproduc- 

 tive cycle with a 10—11 month gestation period (Par- 

 sons, 1983). Bonnethead sharks have a gestation 

 period of only 4-5 months (Parsons, 1993) — the short- 

 est known gestation period of any placental vivipa- 

 rous shark species (Parsons^). This short gestation 

 period, combined with other physiological factors and 

 maternal diet may explain why bonnethead shark 

 embryos contained the lowest mean mercury levels 

 in this study. Walker ( 1976) suggested that transfer 

 of mercury to developing ova and embryos may re- 

 duce mercury levels in mature females; however, 

 little is currently known about maternal transmis- 

 sion of mercury in fishes. Mercury levels were rela- 

 tively high in large females despite transfer of mer- 

 cury to developing ova and embryos. 



Results of this study indicate that the majority of 

 sharks examined accumulated levels of total mercury 

 that were greater than or equal to the threshold level 

 of 0.5 ppm determined by the state of Florida. Mercury 

 concentrations in adults and larger juveniles were t^-pi- 

 cally elevated for all species; however, relatively high 

 concentrations were also fi-equently observed in smaller 

 juvenile bull and blacktip sharks. These data support 

 the current health advisory in Florida urging limited 

 consumption of sharks because of elevated mercury 

 concentrations. Our data, in conjunction with the re- 

 sults of other studies conducted in the region ( Gardner 

 etal., 1975; Hueter etal., 1995; FHRS^), illustrate that 

 mercury concentrations in several commonly landed 

 shark species from the southeastern United States of- 

 ten exceed state and federal regulatory levels. 



Acknowledgments 



We thank J. Arrecis, J. Brooks, W. Coppenger, and T. 

 Fitzpatrick of FDEP Division of Technical Services 



^ Parsons, G. R. 1997. Department of Biology, University of 

 Mississippi, University. MS 38677. Personal commun. 



