Blood and epidermal (outer layer of skin) samples from free-ranging bottlenose dolphins captured and released during health assessments in Sarasota Bay were evaluated for concentrations of mercury (Hg), selenium (Se), stable isotopes of carbon (C) and nitrogen (N) to address diet patterns of the dolphins, and blood glutathione peroxidase activity (antioxidant enzyme dependent on selenium) in conjunction with routine hematology (examination of blood) and serum chemistry panels (for example, measures of kidney, liver and other organ system function). We evaluated these multiple endpoints to determine if: 1) mercury could be producing adverse effects (toxicosis), 2) selenium may not be present in adequate chemical forms and amounts (deficiency), or 3) both (mercury toxicosis and selenium deficiency – a possible nutrient and chemical interaction).
Major objectives were to: 1) quantify and describe relationships among mercury, selenium, glutathione peroxidase, and stable isotopes of C and N in blood and epidermis; 2) elucidate major parameters that influence blood mercury and glutathione peroxidase activity; 3) relate measures of tissue mercury, selenium, and glutathione peroxidase to specific ecological, hematological, morphological, or life history parameters, including season, sex, age, and trophic level. This resulted in a manuscript that was recently submitted to a peer-reviewed scientific journal.
As expected, mercury in both epidermis and blood is almost exclusively methylmercury (the more bioavailable and toxic form as compared to inorganic forms). Epidermal concentrations of mercury and selenium reflect (correlate with) their respective amounts in blood, albeit at several times blood concentrations of mercury (epidermis has been proposed as a pathway for excretion of mercury as it sheds from the animal). The strong association between blood mercury and serum selenium, in conjunction with a lack of significant correlation between blood mercury and glutathione peroxidase, implies that a substantial proportion of blood mercury is affiliated with another selenium-containing moiety or is related to recent dietary intakes (for example, trophic level, intensive fish consumption). It is well known that mercury and selenium interact (selenium can make mercury less toxic) and we need to better understand how increased exposure to mercury may increase the demand for dietary selenium. Circulating blood mercury concentrations correlated with serum selenium concentration (age and the trophic level in the food web were found to be important considerations for the status of mercury and selenium). Current selenium concentrations in Sarasota Bay dolphins appear adequate for maintenance of blood glutathione peroxidase activity. However, dolphins appear to be subject to seasonal variability which might render them more vulnerable to toxic effects of mercury at some times of the year. We need to further assess the condition of these dolphins and their prey fish. This interaction could also be important for rehabilitation efforts to consider as well (for example, adequate selenium levels for dolphins). The support of Dolphin Quest for providing sample collection opportunities during health assessments is much appreciated, as is the support of NOAA’s Fisheries Service for sample processing.