Non-lethal monitoring of trace elements in Sarasota Bay bottlenose dolphins
By Colleen Bryan, MSc Candidate, College of Charleston, Steven Christopher, PhD, and W. Clay Davis, PhD, National Institute of Standards and Technology

The main focus of this project was to establish trace element baseline levels in the non-lethal sampling compartments of blood and skin for the Sarasota Bay population of bottlenose dolphins. Trace elements enter the environment naturally and as a result of man’s expanding anthropogenic activities. Essential trace elements such as copper, selenium and zinc are being measured along with known toxic trace elements such as mercury, lead, and cadmium. Whether essential or non-essential, excessive trace element exposure levels can potentially have toxic effects. Toxicity depends on both concentration and chemical form (speciation), which controls bioavailability. Increased human activity in recent decades has accelerated the input of many heavy metals into the marine environment and these potential stressors can disproportionately impact waterways and wildlife in the coastal zone. The impact of trace elements on living bottlenose dolphins is relatively unknown. This lack of fundamental information warrants the collection of accurate baseline information on the type and level of various trace metals in tissue to establish nutritive and toxicological benchmarks for bottlenose dolphins. This will allow concentration levels to be compared across regional populations and mortality cases to be referenced to a living population. Excepting mercury, measurement of trace elements in clinical samples such as blood has been unexplored in bottlenose dolphins. The resident community of bottlenose dolphins in Sarasota Bay has been studied for more than 36 years and presents a unique opportunity to investigate relationships between life history, health, and trace element concentration data. Whole blood and skin samples were collected from November 2002-June 2004 during health assessment live capture/release events in Sarasota Bay using collection protocols developed by the National Institute of Standards and Technology (NIST) specifically for dolphin health assessments. Samples were analyzed for aluminum, vanadium, chromium, manganese, copper, zinc, arsenic, selenium, rubidium, strontium, molybdenum, cadmium, lead, total mercury, and methylmercury. Statistically significant blood-skin correlations were found for several trace elements indicating that these are valid non-lethal monitoring tissues. The strongest correlation was established for total mercury and levels in blood and skin. These levels were above the threshold at which detrimental effects are observed in other vertebrate species.

There are several anthropogenic inputs of mercury to the environment, including mining, fossil fuel combustion (e.g., coal-fired power plants), byproducts from paper manufacturing, chor-alkali production, and medical waste incineration. Once deposited in the coastal zone, mercury is methylated by microorganisms in sediment and soil into its more bioavailable and toxic form (methylmercury) allowing it to propagate through the marine food chain to apex predators such as dolphins. This process occurs very efficiently along Florida’s coastal regions that are rich in marshes and mangroves. Bottlenose dolphins obtain mercury primarily from fish prey. Mercury is known to have neurological and immunological toxic effects at low concentrations. U.S. Environmental Protection Agency (US EPA) reference doses for total mercury and methyl mercury in edible fish tissue are 300 ng/g and 100 ng/g (units of parts-per-billion), respectively. Consumption of fish is the main route of human exposure to mercury and this element is often the reason for issuance of most of the fish consumption advisories in the U.S. These action levels are used as benchmarks for human risk, especially for pregnant females or children. It is interesting to note that blood total mercury concentrations for most of the older animals exceed 300 ng/g, the EPA action limit for total mercury in edible fish tissue. Total mercury levels in dolphin blood were related to age class and sex, as shown in Figure 1. Males and females accumulate mercury through the calf and juvenile stages. It seems that fish consumption plays a stronger role in mercury bioaccumulation relative to gestational or milk transfer to young. Total mercury concentrations in reproductive-age females are significantly higher than adult males and higher than the younger age classes of both sexes. This increased mercury bioaccumulation may be due to higher female feeding rates needed to keep up with the energy demands of lactation. Methylmercury was measured in a subset of samples. The chemical form of mercury in blood is predominantly toxic methylmercury (90% of total mercury signature) and in milk comprises greater than 50% of the total mercury signature.

The trace element concentrations established in this study can serve as a baseline index for future monitoring of this population and as a benchmark for comparisons to other coastal bottlenose dolphin populations currently under study. The methods developed at NIST are universal and will be transferred to all dolphin live capture health assessment projects performed in the U.S. Several manuscripts addressing the analytical methods developed for the project and the utility of employing blood and skin as non-lethal indicators for trace element status are currently in preparation. Future research will be incorporated into Ms. Bryan’s PhD project at the Medical University of South Carolina. One of the goals will be to put concentration data in a physiological context in order to qualify sublethal health impacts, using a combination of biological and physiological endpoints to aid in evaluating the effects of specific trace element contaminants. Other future goals are to conduct more trace element speciation experiments, examine trace element protein coupling and mechanisms, develop biomarkers for bottlenose dolphins and apply these methods to urine diagnostics. Support for this project has been provided by NOAA Fisheries, Dolphin Quest, and Disney’s Animal Programs.