Bottlenose dolphins are long-lived, fish-eating marine mammals that are at or near the top of the food web in coastal ecosystems. As a result, they are vulnerable to accumulating heavy burdens of persistent organohalogen contaminants (POCs). POCs are man-made compounds that are used in industry, agricultural, and domestic settings with uses such as electrical insulating fluids, flame retardants, and insecticides. They were released into the environment before their toxicities and environmental consequences were fully understood; however, we now know that POCs can persist in the environment for decades and can have effects ranging from cancer to effects on the immune system and reproduction. POCs found in dolphins include banned compounds such as the polychlorinated biphenyls (PCBs) and chlorinated pesticides (e.g., DDT) which were banned from production in the 1970s, as well as compounds in active use such as the polybrominated diphenyl ethers (PBDEs). Mixtures of all of these compounds can be readily detected in the bottlenose dolphins of Sarasota Bay.
POCs do not occur in dolphins as single compounds but rather as mixtures with dozens of other POCs. It is well known that contaminants interacting in mixtures may have toxicities differing significantly from those found for single compounds. Therefore, knowledge regarding mixture composition is important for understanding the link between contaminant burden and adverse health effects in marine mammals. Many factors influence how dolphins are exposed to contaminant mixtures, including diet, age, reproductive maturity and nutritional state. The Sarasota Dolphin Research Program provides an unparalleled opportunity for assessing contaminant exposure at the population level as many of these parameters are known for the resident bottlenose dolphins.
To assess the degree of POC exposure and potential health effects in the Sarasota Bay bottlenose dolphin population, approximately 195 blubber, blood, and milk samples were collected for contaminant analysis during health assessments since June 2000. In addition, the primary Sarasota dolphin prey, including pinfish, pigfish, and mullet, were analyzed for 81 POC compounds to assess the role of diet on dolphin contaminant exposure.
To date, this collective data set indicates that the mixture of POCs in a dolphin changes throughout its lifetime. Contaminant profiles in both young and older juvenile animals (ages 1.5-10 years) are highly reflective of a milk diet, despite the gradual shift to a fish-based diet that starts as early as 18 months old. In male dolphins, profiles appear to shift with increasing age to contain higher proportions of compounds that dolphins are unable to break down and excrete. In contrast, female bottlenose dolphins experience dramatic shifts in POC mixture profiles upon reaching reproductive maturity, when compounds are selectively offloaded into her milk and passed to her first-born calf. Since POC mixtures differ in toxicity, there may be health implications associated with the shifts in contaminant profiles throughout the lifetime of Sarasota bottlenose dolphins. Future plans include assessing the toxicity of these mixtures using in-vitro bioassays. This data may also be used in the future to assess potential health effects in other wild cetacean populations. Funding for this project was provided by the National Marine Fisheries Service and the National Institute of Standards and Technology.