Comparison of elements in bottlenose dolphins stranded on the beaches of Texas and Florida in the Gulf of Mexico over a one-year period

We analyzed tissue samples from bottlenose dolphins (Tursiops truncatus) th at had stranded on beaches in Texas and Florida over a 1-year period starti ng in September 1991. The concentrations of 10 elements plus methyl mercury (MeHg) were determined in brain, kidney, and liver, and we examined these results for differences based upon age, site, sex, and tissue type. A stron g inverse relationship between total mercury (Hg) and the percentage that w as MeHg was found in liver, kidney, and brain tissue, presumably due to dem ethylation of MeHg. A threshold concentration was found for total Hg in bra in tissue, indicating that most Hg was present as MeHg up to about 8 years of age. Increases in total Hg after this age were accompanied by an increas e in the ratio of total Hg to MeHg, indicating demethylation. Strong relati onships were found between total Hg in liver and age and between total Hg a nd selenium in liver, which have been observed before in many fish- and squ id-eating marine mammals. The only difference based on sex of the animals w as observed for MeHg, which was higher in females and contrary to the patte rn often observed for organic contaminants. Several elements (copper, Hg, l ead, zinc) exhibited intersite differences, which were not consistent. Bott lenose dolphin from Florida exhibited the highest levels of MeHg and total Hg, while animals from Texas exhibited the highest levels of lead, copper, and zinc. The essential elements copper and zinc were expected to be the sa me for the Texas and Florida animals; however, observed differences may ind icate population differences in basic physiological levels, dietary intake, or health status.