We’ve known for years that waste from humans can be detected in the creatures around us, from sea critters in the depths of the oceans to birds perched atop towering trees. Such repercussions are especially true for bottlenose dolphins, whose skin and blubber accumulate industrial fluids and mercury as they glide through contaminated waters and eat prey. Now, scientists say they’ve measured some of the highest concentrations of mercury observed in the species.

Bottlenose dolphins have the ignoble honor of being a prime species for investigating contamination trends in coastal areas due to their fondness for estuaries and nearshore waters. Their long lives, habitat, and high trophic position in the marine food web combine to create the perfect storm for pollutants to build up in their thick layer of fat.

To explore how widespread this contamination may be, a team led by Krishna Das from the University of Liège, Belgium, took skin and blubber biopsies from 82 English Channel dolphins living in the Normanno-Breton Gulf during boat surveys between 2010 and 2012. The mercury levels in the skin of the English Channel dolphins surpassed those found in the same species from two other notoriously contaminated locations: the Florida Everglades and Mediterranean Sea.

"We looked for mercury and several persistant organic pollutants (e.g. PCBs, DDT and other chlorinated pesticides, dioxin-like compounds, some flame retardants…) and PCBs were found at the highest concentrations," said Das to IFLScience.

"The PCBs were used mainly as dielectric fluids in industrial transformers and capacitors. The manufacture, use and commercialization of PCBs have been prohibited in France since 1987. The Seine is a hotspot for PCBs, and the Normanno-Breton Gulf, which is close to the Seine estuary, is an environment with high industrial, agricultural and urban activities."

The primary pollutant they found was made up of chlorine-containing compounds, likely from industrial fluids. Compounds called ?DDX from organochlorine pesticides comprised between 86 and 99 percent of the total pesticides, with males harboring a greater concentration than females. The gender divide could be due to females offloading “a large proportion of PCBs to their young during gestation and more specifically during lactation, while males continue to bioaccumulate PCBs throughout life,” write the researchers in Scientific Reports.

"Despite their ban, PCBs still persist in the marine environment," said Das. "They are very persistent, poorly degrade and accumulate efficiently in marine food webs up to dolphin. They might remain for several decades, some scientists even say for centuries!"

The influence of these toxic pollutants is likely having a knock-on effect, including a reduced annual growth rate in those with PCBs, according to one study. Another paper recently linked population decline in killer whales in Europe to PCB levels, while the current study’s results suggest females may pass PCBs on to their young. Such transference is likely magnified in fetuses and juveniles whose immune systems are less developed, making them more sensitive to adverse effects. 

"PCBs can act as endocrine disrupters," said Das. "It means that they can mimic the biochemical structure of the hormones, without their beneficial effects. We suspect that elevated concentrations of PCBs can alter the reproduction of marine mammals leading to a decrease of the number of newborns, affecting the renewal of the population (low calf survival)."

The authors recommend making the Normanno-Breton Gulf a conservation area to preserve one of Europe’s largest coastal populations of bottlenose dolphins.

"As emphasized by my colleague Paul Jepson and Robin Law in 2016 (published in the journal Science), in 2004 the Stockholm Convention committed more than 90 signatory countries to phasing out or eliminating large stocks or other sources of POPs, including PCBs," added Das. "Yet, PCBs continue to threaten the survival of marine predators."