Harbor porpoise calves ingest toxic chemicals through their mother’s milk

The use of “forever chemicals” or persistent organic pollutants (POPs) has been banned for quite some years. They include infamous compounds such as PCBs or polychlorinated biphenyls, pesticides, and flame retardants. Yet, these chemical contaminants are still present in our marine ecosystems, and the porpoises inhabiting them. Scientists discovered that more than half of harbor porpoises from the southern North Sea had contaminant concentrations above the threshold for risks of health effects.

A female harbor porpoise can transfer the contaminants stored in her blubber to her calf. — Credit: Colin Knowles via Flickr

A persistent threat…

POPs have negative effects on the health of animals, including on the reproductive, immune, and hormonal systems. These contaminants accumulate in fatty tissues, due to their chemical structure. Unfortunately for many marine mammals, their bodies are rich in fatty tissues, also known as blubber, for insulation, hydrodynamics, and buoyancy. Additionally, these chemicals do not break down easily in ecosystems and amplify through marine food webs. This means that marine mammals, often the top predators of an ecosystem, ingest high levels of pollutants through their diet. 

Harbor porpoises in danger

When talking about the effects of contaminants on European marine mammals, one species has gotten particular attention from scientists: the harbor porpoise in the North Sea. It is one of the smallest cetaceans, but it has a relatively thick blubber layer compared to other smaller cetaceans. Due to its large body surface-to-body volume ratio, the harbor porpoise has a high metabolic demand. This means it needs to feed almost continuously. However, eating contaminated prey makes them vulnerable and could have a significant impact on their health and reproduction. 

A 2021 study investigated the levels of POPs, specifically polychlorinated biphenyls (PCBs), in harbor porpoises from the North Sea. Scientists from Utrecht University collected hundreds of stranded harbor porpoises from the Dutch coast from 2006 until 2019 for post-mortem investigations. Samples from 112 of these examined porpoises were further analyzed by Wageningen Marine Research to measure the contaminant concentrations to compare between age classes and sex. To evaluate the risks of effects on the health and functions of marine mammals, a threshold level of effect is set at 9 mg/kg of PCBs for immune system disruption which follows many previous studies.

Scientists collected samples from the stranded porpoises to analyze the chemicals in their tissue — Credit: Nynke Kouwenhoven – Utrecht University

Calves get contaminated by their mothers

Of the harbor porpoises that were assessed in this study, almost 40% had concentrations higher than the threshold. The PCB concentrations were lowest in fetuses, which they received through the umbilical cord from the mother during the pregnancy. However, the study found that newborn or neonate porpoises had higher concentrations than fetuses. The scientists explained this was the result of the mom’s transfer of contaminants through her umbilical cord and her milk. More than half of the newborns had concentrations that surpassed the threshold. The PCB concentrations in milk showed the same level of concentration as the blubber of the mother animal. Indeed, the mom needs to use her blubber reserves to make fat-rich milk for her baby. But PCBs stick to fatty tissue and end up in the milk. 

Throughout their early development, harbor porpoise calves will keep accumulating high doses of PCBs through their mothers’ milk. Juvenile harbor porpoises weaned off by their mothers showed high concentrations of PCBs. In fact, half of them had PCB concentrations above the threshold. Once they start eating normal food, like small fish, they need to consume a lot of it to fuel their growth spurts. These high-feeding phases can cause another increase in PCB concentrations, but never as high of an increase as what young porpoises get through lactation.

The firstborn calf gets the highest contaminant dose

Firstborn porpoises are also most likely victims of the biggest contaminant transfer. To get to this conclusion, scientists from the study compared the PCB levels in the milk of smaller adult female and presumable younger porpoises with the larger female porpoises. Larger female porpoises had lower levels of PCBs. That suggests that indeed the older female porpoises that have most likely already given birth, offload the majority of their PCB concentrations to their first calves. 

Males cannot transfer their contaminants to the next generation

In this study, males had the highest concentrations of PCBs. Unlike their adult female counterparts, they are not able to transfer parts of their concentrations through reproduction. Over 90% of the adult males had PCB concentrations above the immune effect threshold. This is a stark difference, as only 10.5% of the adult female porpoises exceeded the threshold levels.

porpoise calves receive chemicals through their mother's milk

The future of the population

Young porpoises absorb these toxic chemicals during pregnancy and the first months of their lives. Lactation seemed to be the biggest source of pollutant transfers and most young porpoises had contaminant levels higher than the threshold of negative immune effects. One of the most important findings was that adult male harbor porpoises had exceedingly high levels of contaminants.

These results are worrisome, especially since PCBs have been banned for almost five decades. Despite the current measures to reduce POP pollution, their presence in the marine ecosystem still poses a threat to the health and reproduction of harbor porpoises in the southern North Sea. The alarming concentrations, particularly in adult male porpoises, emphasize the urgency for more targeted conservation efforts and strategies to protect the future of these vulnerable small cetaceans.

What can we do as individuals to protect the harbor porpoises?

Protecting harbor porpoises requires collective effort but begins with individual actions. You can help by making simple switches in daily life, such as reducing single-use plastic consumption and opting for reusable alternatives, which can significantly decrease marine pollution, safeguarding the porpoises’ habitat. Additionally, advocating for sustainable fishing practices to reduce accidental bycatch in fishing gear is crucial. It is also crucial to address emerging threats, like newer contaminants such as PFAS, and actively advocate for more stringent regulations to prevent their release into marine environments. By prioritizing conscious choices, supporting eco-friendly policies, and raising awareness, we as individuals can help protect these small cetaceans and their fragile habitats.

Sources & Further Reading

  • Van den Heuvel-Greve, M.J., Van den Brink, A.M., Kotterman, M.J., Kwadijk, C.J., Geelhoed, S.C., Murphy, S., Van den Broek, J., Heesterbeek, H., Gröne, A. and IJsseldijk, L.L., 2021. Polluted porpoises: Generational transfer of organic contaminants in harbour porpoises from the southern North Sea. Science of the Total Environment, 796, p.148936.
  • Wageningen Marine Research
  • Utrecht University

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Eva Schotanus has a Bachelor's in Coastal and Marine Management from the Netherlands, where she specialized in marine biology and cetacean necropsies. She's currently doing a MSc in Applied Data Science and researching the nutritional condition of stranded harbour porpoises in the North Sea at Utrecht University.

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