Some industrial chemicals that were banned more than 30 years ago are still being passed on to dolphins born today. A new study has found that pollutants called polychlorinated biphenyls (PCBs), banned from use in Europe since the 1980s, persist in the Mediterranean Sea and show up in high quantities in bottlenose dolphins that swim those waters.
Male dolphins were found to have significantly higher concentrations of PCBs in their blubber than females, which suggests that females offload these pollutants to their offspring. “Once a female has a calf, a lot of these fats will mobilise from their blubber into the milk, and consequently into the calf,” says Tilen Genov at the Slovenian Marine Mammal Society, who led the study. He says recent research also suggests that PCBs can be transferred through the placenta.
Genov and his colleagues tracked 32 dolphins in the Gulf of Trieste from 2011 to 2017, taking samples of skin and blubber tissue. Overall 87.5 per cent of the dolphins had PCB concentrations above 9 milligrams per kilogram of lipid weight, the threshold at which physiological problems like hormonal disruptions begin, says Genov.
Advertisement
Previous research has found that 41 milligrams per kilogram of lipid weight is the threshold of PCB contamination in ringed seals where reproduction begins to be impaired. In the current study, 65.6 per cent of the bottlenose dolphins had levels of PCBs in their blubber higher than this.
Female dolphins that had not given birth also had significantly higher toxicity levels than females with calves. The pollutants affect firstborn dolphins more, because many of the toxins the female dolphin has accumulated throughout her life will find their way into her milk when she begins feeding her first infant. Provided she has calves at regular intervals, the level of contamination in her body tissue will stay relatively low.
“PCBs are highly persistent. They are difficult to degrade. And these animals are long-lived and they are on top of the marine food web. So, these things magnify through the food chain and each level has higher concentrations of these pollutants,” Genov says.
He says PCBs on their own may not necessarily destroy dolphin populations, but combined with other stressors like shipping traffic, plastic pollution, and fishing gear that can strand or kill the animals, they could have an impact on bottlenose dolphin numbers.
“Because of the reproductive effects, if you have an oil spill or a mass mortality, the pollutant load might prevent successful recovery, even though it might not be the primary cause of population decline itself,” he says.
Persistant pollutant
With other banned pesticides, like the insecticide DDT, contamination levels dropped off after the substance was banned from use, says Paul Jepson at the Zoological Society of London, a member of the research team. “But with PCBs, you have to heat them to over 1000°C for 5 minutes. They don’t just fall away. The PCB levels in Europe have stabilised over the past 20 years,” he says.
Because PCBs are passed along in mothers’ milk, the best we can do to reduce the contamination is to clean up polluted sites so the chemicals don’t enter the food chain, says Jepson. PCBs are not water soluble, so they enter the food chain through plants and river sediment and are passed along as larger animals eat smaller ones. The US has been aggressive in cleaning up river sediment near old industrial sites that used PCBs, and Europe needs to follow suit, Jepson says.
“Everywhere in the US, levels of PCBs are going down. We see that in seals, otters, birds, killer whales, and dolphins. A lot of these populations are doing well, porpoises have returned to the San Francisco Bay after a long absence. A large part of this is because of the clean-up of PCBs,” he says.
Science of the Total Environment
