The 2010 oil spill in the Gulf of Mexico raised everybody’s awareness of the dangers of oil and other pollutants to dolphins and other wildlife. We haven’t studied oil-effects directly, but we have been assessing contaminant loads, including hydrocarbon byproducts, in dolphins for more than 10 years.
Dolphins concentrate environmental contaminants.
Our research gives us unparalleled opportunities to examine how environmental contaminants, which may remain in the environment for decades, impact dolphin health and reproduction.
Dolphins concentrate environmental contaminants from lower levels of the food chain. Contaminants ingested by small fish and invertebrates, get concentrated in larger fish when they eat the small fish. The pollutants are concentrated further in dolphins when they eat the larger fish.
Dolphins thus build up contaminant loads over time. As a top predator on the food chain, this “bio-concentration” poses a serious threat to dolphins.
Below are brief summaries of some of our research to study the impact of contaminants on bottlenose dolphins.
Finding cause-and-effect relationships with environmental contaminants is difficult. But from observations of five generations of dolphins in Sarasota Bay, we have data on their ages, reproductive histories, and health to compare with contaminants findings.
First-time dolphin mothers transfer high levels of contaminants to their first calf.
Persistent organic pollutants are usually stored in lipid (fatty) tissues. First-time dolphin mothers have been accumulating contaminants in their lipid-rich blubber during the 8-10 years since birth. They transfer high levels of contaminants to their first calf in their lipid-rich milk.
High rates of first-born calf mortality are correlated with higher concentrations of contaminants in the blood and blubber of first-time mothers. Fortunately, subsequent calves are more likely to survive. That’s because the interval between calves is short enough (typically about 3 years) that contaminants don’t build up to such high levels. But mothers due continue to transfer low (non-toxic) levels of contaminants to their calves.
Males accumulate contaminants throughout their lives, which may explain why males have a shorter life span than females. Males can live into their early 40s, while females can live into their 50s. Though some 40-year-old males sire calves, testosterone concentrations decline after 25 years of age.
Seasonal sample collection is supported by the Chicago Zoological Society, Dolphin Quest, Georgia Aquarium, Morris Animal Foundation, NOAA Fisheries, and Disney’s Animal Programs. Analytical work has been funded primarily through NOAA Fisheries.
Geography and Contaminants
Persistent organic pollutants are man-made compounds used in industry, agriculture, and domestic settings. Uses include electrical insulating fluids, flame retardants, and insecticides. Mixtures of all of these compounds can be detected in the bottlenose dolphins of Sarasota Bay. These pollutants have been linked to effects on the immune system, altered hormone regulation, altered development, and cancer. They occur in some juvenile dolphins at concentrations believed to cause a variety of toxic effects, and that continued exposure may impair later reproduction.
The National Institute of Standards and Technology has analyzed persistent organic pollutants in blubber biopsies from 480 bottlenose dolphins sampled at 14 Atlantic and Gulf of Mexico locations. The pollutant levels in bottlenose dolphins vary from place to place. For instance:
- The average concentration of PCBs in Sarasota dolphins were near the median value (68 parts per million or ppm) of the 14 sites. Dolphins from one of our study sites near Brunswick, GA. averaged 430 ppm. The Brunswick dolphins live near four heavily polluted areas designated as national Super Fund cleanup sites. [To dive deeper, link toEcology/2010Balmer.asp)].
- Levels of PBDE (a pollutant which comes from flame retardants) were relatively low in Sarasota dolphins. This may be due to the Sarasota Bay watershed generally having older homes and few sewage outfalls into Sarasota Bay. Flame retardants are generally associated with newer homes and offices, and sewage outfalls.
- Chlordane levels (from pesticides) in dolphins from Sarasota, Tampa Bay, and northern Biscayne Bay near Miami were significantly higher compared to other locations. Homes built prior to the mid-1980s had their foundations treated with chlordane to prevent termite infestation. Miami, Sarasota, and Tampa all have a large number of older homes that were treated with chlordane, thus explaining the high proportion of this pesticide in dolphin blubber.
Support for this project was provided by the NOAA Fisheries, the National Institute of Standards and Technology, Dolphin Quest, and the Chicago Zoological Society
To assess the exposure to contaminants and potential health effects in Sarasota dolphins, we took almost 200 samples of blubber, blood, and milk during health assessments from 2000-2009. To determine where the sources of the contaminants were coming from and how much bio-concentration was occurring, we also took samples from primary dolphin prey species such as pinfish, pigfish, and mullet.
Levels of POC’s in prey fish are lower than those in dolphin blubber, providing evidence that contaminants are magnified through the food web. Dolphins, as a top predator, would be expected to show higher concentrations than lower levels on the food chain.
Results of the analysis suggest:
- Juvenile dolphins (ages 1.5-10 years) show effects of contamination from their initial milk diet.This occurs despite the gradual shift to a fish-based diet that starts as early as 18 months of age.
- In male dolphins, accumulation increases with age to contain higher proportions of compounds that dolphins are unable to break down and excrete.
- Female bottlenose dolphins experience dramatic shifts upon reaching reproductive maturity.Pollutants accumulated since birth are offloaded into the milk and passed to her first-born calf.
- Since different mixtures of pollutants differ in toxicity, there may be health implications associated with the shifts in contaminant profiles throughout the lifetime of Sarasota dolphins.
Support for this project was provided by the NOAA Fisheries, the National Institute of Standards and Technology, Dolphin Quest, and the Chicago Zoological Society.
Click here to dive deeper to learn more about Mercury and Dolphins
Blood and skin samples from Sarasota dolphins showed residues of methyl mercury, which is a more toxic form of mercury. Skin concentrations of mercury were expected, because the skin has been proposed as a pathway for excretion of mercury as it sheds from the animal. The mercury levels in the blood are related to their recent meals. Dolphins absorb mercury from their prey fish, which already contain mercury from lower trophic levels on the food chain.
This research was supported by Dolphin Quest, NOAA Fisheries, and the Chicago Zoological Society.
Dolphins near pollution sites
Dolphins eat fish that live in contaminated in the waters around four EPA Super Fund cleanup sites in Brunswick, Georgia. High contaminant levels have raised concerns about dolphin health this region, and about the movement of the contaminants through the food web.
To better understand movement patterns in the area PhD student Brian Balmer conducted a dolphin photo-identification study in 2008-2009. Preliminary results suggest that overlapping stocks of coastal and estuarine dolphins are moving in and out of the contaminated waters.
A capture-release health assessment of 28 bottlenose dolphins was conducted by NOAA in August 2009 to examine potential health impacts from pollutants.
Dive deeper: dolphins near Superfund sites
The released dolphins each carried a small radio transmitter on their dorsal fins, so their movements could be tracked along 90 kilometers of coastline. Three dolphins also carried satellite-linked transmitters to monitor longer their movements over longer distances. Satellite-linked tags transmitted location data for over 58 days.
This research was supported by the Chicago Zoological Society, NOAA Fisheries, and the Georgia Department of Natural Resources. The health assessments were conducted by researchers from the NOAA Center for Excellence for Oceans and Human Health, Hollings Marine Laboratory, the NOAA Fisheries Marine Mammal Health and Stranding Response Program, the Chicago Zoological Society’s Sarasota Dolphin Research Program, and the Georgia Department of Natural Resources.
All photos © Sarasota Dolphin Research Program under NMFS permit #522-1785