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



373 



and offshore waters of east-central Florida. All bull 

 sharks were collected from estuarine nursery habi- 

 tats within the Indian River Lagoon system from Oak 

 Hill, Florida (approximately 28°52'N), south to the 

 Ft. Pierce Inlet area (approximately 27"28'N). All 

 blacktip, sharpnose, and bonnethead sharks were 

 collected in nearshore and offshore waters from 

 northern Cape Canaveral, Florida (approximately 

 28°55'N), south to the Sebastian InletAVabasso, 

 Florida, area ( approximately 27°45'N ). Samples were 

 collected from 1992 to 1995. 



Sharks were placed directly on ice and returned to 

 the laboratory, where species, precaudal length 

 (PCD, and sex were recorded. Stage of maturity was 

 determined by examination of internal and external 

 reproductive organs, as well as by comparison of 

 shark size with estimates of size at birth or matu- 

 rity from previous studies (Parsons, 1983, 1993; 

 Branstetter and Stiles, 1987; Castro, 1996). Individu- 

 als were classified as neonates on the basis of un- 

 healed or healing umbilical scars (Castro, 1993). To 

 allow for comparisons with other studies, total length 

 (TL), fork length (FL), and other morphometries were 

 also recorded according to Compagno (1984). With 

 the exception of embryos, all sharks sampled were 

 considered to be within the size range landed in 

 Florida recreational or commercial fisheries. Axial 

 muscle tissue samples were removed from the left 

 dorsal area anterior to the origin of the first dorsal 

 fin. White muscle tissue taken from this region is 

 representative of the portion of shark used for hu- 

 man consumption. Tissue samples were immediately 

 placed in sterile polypropylene vials, sealed, and fro- 

 zen at -20°C until analyzed. Before analysis, tissue 

 samples were digested by using standard procedures 

 (EPA, 1991; Frick'^) to convert all mercury in the 

 sample to Hg^-. The mercury in each digested sample 

 was reduced to atomic mercury by reaction with ex- 

 cess stannous chloride. This atomic mercury was 

 purged from solution in a gas-liquid separator and 

 swept into an atomic absorption spectrometer for 

 detection and quantification following standardized 

 procedures (EPA, 1991; Booeshahgi et al.^) at the 

 Florida Department of Environmental Protection's 

 Division of Technical Services by using cold vapor 

 atomic absorption spectrometry. Quality control mea- 



■^ Frick.T. 1996. Digestion of fish tissue samples for total mer- 

 cury analysis. Florida Department of Environmental Protec- 

 tion. Division of Technical Services, 2600 Blair Stone Road, 

 Tallahassee. FL 32399. Report MT-015-1. 



^ Booeshahgi, F, M. Witt, and K. Cano. 199.5. Analysis of to- 

 tal mercury in tissue by cold vapor atomic absorption. Florida 

 Department of Environmental Protection. Division of Techni- 

 cal Services, 2600 Blair Stone Road, Tallahassee, FL 

 32399. Report MT-010-1. 



sures included analysis of laboratory blanks, dupli- 

 cate or triplicate tissue samples, and standard fish 

 tissue reference material (DORM-1, obtained from 

 the National Research Cotmcil of Canada) for each 

 group of ten shark samples analyzed (EPA, 1991; 

 Frick'^; Booeshahgi et al,"*). All mercury levels are 

 reported as parts per million (ppm) wet weight. 



Data analyses 



Data regarding size and total mercury for each spe- 

 cies were tested for normality by using the Kolmo- 

 gorov-Smirnov test with Lilliefors' correction (Fox et 

 al., 1994) and for homoscedasticity by using the F,,,^,- 

 test (Sokal and Rohlf, 1981) or by computing the 

 Spearman rank correlation between the absolute 

 values of the residuals and the observed value of the 

 dependent variable (Fox et al., 1994). Linear regres- 

 sions were used to describe relationships between 

 shark size and total mercury concentration. Mercury 

 data were log transformed to meet homoscedascity 

 requirements. We used a ?-test or Mann-Whitney 

 rank sum test, as appropriate, to test for significant 

 differences in total mercury levels and sizes between 

 males and females for each species. 



Results 



Bull shark, Carcharhinus leucas 



Eighty-three percent of 53 neonate and juvenile bull 

 sharks (552-1075 mm PCD tested from the Indian 

 River Lagoon region had total mercury levels that 

 were greater than or equal to the 0.5-ppm threshold 

 level (.r=0.77 ppm; median=0.74 ppm; range 0.24- 

 1.7 ppm) (Table 1). There was no significant differ- 

 ence between lengths of males (x =735 mm PCD and 

 females (J =754.6 mm PCL; ^-test, P>0.5) nor was 

 there a significant difference between mean total 

 mercury levels between males and females (S^=0.76 

 ppm for both sexes; Mann-Whitney rank sum test, 

 P>0.05). Ranges of total mercury for both sexes were 

 also similar. 



There was a significant positive correlation be- 

 tween total mercury level and bull shark length (both 

 sexes combined) (P<0.001; Fig. 1). Total mercury lev- 

 els in juvenile bull sharks increased as individuals 

 grew larger, although some small sharks contained 

 levels as high as those in larger sharks. 



Blacktip shark, Carcharhinus limbatus 



Total mercury levels for 21 juvenile and adult black- 

 tip sharks examined ranged from 0.16 to 2.3 ppm 



