Effects of mercury on Sarasota’s bottlenose dolphins

Jan 15, 2009 By , , and

 

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.

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Health and Physiology

About the author

Dr. Todd O’Hara (DVM, PhD) studies “Wildlife Toxicology” at the University of Alaska Fairbanks with an emphasis on animals of high latitudes and in marine systems. He lives with his wife Carla (a real “vet”); and son Lars and daughter Anne. Dr. O’Hara came to Alaska for the “opportunity of a lifetime” to work and live “in the field” with amazing people and wildlife and started out in Barrow, Alaska. His family is hooked on Alaska, literally since they fish year-round. His major interests are environmental/ wildlife toxicology, assessments of subsistence foods, and wildlife conservation and medicine. Our laboratory and colleagues are interested in the physiology and adverse effects (toxicology) of Hg on the Sarasota Bay dolphins and how it relates to nutrients and other markers of health/disease. This is similar to our work on polar bears, arctic fox, ice seals, harbor seals, Steller sea lions, and humans!

About the author

University of Alaska, Fairbanks

About the author

University of Alaska, Fairbanks

About the author

University of Alaska, Fairbanks