Environmental contaminants pose a serious threat to marine mammal populations around the world. More than 10,000 chemicals have been produced. Some of these have been documented to have adverse impacts on animal health and/or reproduction, but little direct evidence for these effects in marine mammals is available, due in large part to the difficulties of defining cause-effect relationships outside of laboratory experiments. The resident community of bottlenose dolphins in Sarasota Bay, Florida, provides unparalleled opportunities to investigate relationships between organochlorine contaminant residues and dolphin life history, health and reproductive parameters, by examining correlations between contaminant concentrations and sublethal effects. Most dolphins are identifiable, and many are of known age, sex, and maternal lineage (up to 4 generations). Observational monitoring provides data on dolphin occurrence, births and fates of calves, and birth order. Capture-release operations conducted for veterinary examinations provide biological samples for life history and genetic analyses, health assessment, and contaminant residue measurement. Organochlorine concentrations in blubber, milk, and blood are examined relative to age, sex, body condition, birth order, and health parameters. Reproductive success is evaluated through hormone measurement, tracking of paternity patterns via genetics, and tracking of individual female lifetime calving success.
We have worked with a variety of analytical labs to develop a preliminary picture of the role of environmental contaminants relative to dolphin health and reproduction, developing a weight of evidence approach. Initial analyses of PCBs, DDT and other organochlorine compounds have been performed by Wally Jarman (U. of Utah), Greg Mitchum (NOAA), and Assuncion Borrell (U. of Barcelona). Apparent seasonal variations in blood plasma contaminant concentrations suggest a pattern of deposition in lipid-filled tissues during winter, when blubber thickens, and mobilization in spring as blubber thins. Males accumulate some contaminants throughout their lives, whereas females begin to depurate with their first calf. Contaminants are transferred from mother to calf through lipid-rich milk, ultimately bringing the mother into an apparent equilibrium between contaminant intake through food and lactational loss. High rates of first-born calf mortality are correlated with higher concentrations of contaminants in blood and blubber of first-time mothers; subsequent calves have a higher survivorship. Males, unable to depurate, have a shorter life span than females, living only into their early 40s as compared to maximum female ages of 53 years. Preliminary findings demonstrated decreased male immune system function with increasing organochlorine concentrations. Though some 40-year-old males sire calves, testosterone concentrations decline after 25 years of age. Similar declines in other cetaceans have been hypothesized to be related to organochlorines.
Long-term observational monitoring and periodic biological sampling provide a powerful, non-lethal approach to understanding the correlations of organochlorine concentrations and health or reproductive parameters for coastal dolphins. Analyses of new and several hundred archived samples will be performed over the next 2-3 years by John Kucklick (National Institute of Standards and Technologies), and Derek Muir and PhD student Magali Houde (University of Guelph) will be investigating emerging contaminants of concern. Seasonal sample collection is being supported by Dolphin Quest, NOAA Fisheries, and Disney Wildlife Conservation Fund, and analytical work is being funded primarily through NOAA Fisheries.