Do microplastics affect cetaceans?

In November 2018, a beaked whale stranded in eastern Indonesia gave the world a photograph that has become emblematic. In its stomach, the vets found nearly six kilograms of plastic debris: 115 cups, 25 bags, 4 bottles, lengths of fishing rope, a sachet still full. What would have been almost unimaginable twenty years earlier has become an ordinary news item in marine conservation. But behind the shock image of macroplastics lies a more diffuse, more insidious and probably more serious reality: microplastics, particles invisible to the naked eye, are now present in every ocean, ingested in industrial quantities by cetaceans, and their toxicological effects are only beginning to be documented.

Three routes of exposure

Cetaceans are exposed to microplastics through three main mechanisms. The first, and the largest by volume, is direct ingestion by filtration in mysticetes. The blue whale engulfs ninety thousand litres of water with each feeding mouthful, and filters that water through its baleen. Every particle larger than the mesh of the baleen is retained. This is the route by which it ingests krill, and also every microplastic in suspension.

A study by Shirel Kahane-Rapport and her team, published in Nature Communications in 2022, quantified this ingestion for the first time from sensors placed on blue, humpback and fin whales off California. The result is startling: depending on the species, daily microplastic ingestion ranges from a few hundred thousand to more than ten million particles a day. The blue whale, the largest filter feeder, is the most exposed.

The second route is trophic accumulation. The krill, the squid and the fish that cetaceans eat have themselves ingested microplastics. At each level of the food chain, the associated chemical compounds accumulate in the tissues. Sperm whales, which feed on deep-sea squid, thus accumulate the particles that have passed through their prey, without filtering anything themselves.

The third route is accidental ingestion of macroplastics: plastic bags mistaken for cephalopods, pieces of netting lost at sea (“ghost nets”), floating packaging. It is this route that produces the dramatic cases that reach the media, such as the Indonesian whale or the Spanish sperm whale of 2018 (29 kg of plastic in the stomach, died of peritonitis).

Chemical toxicity: the real threat

The danger of microplastics does not lie first in their physical mass, since a cetacean can generally eliminate the particles through the digestive tract. The real problem is chemical. Plastics carry and release into the tissues:

• Manufacturing additives: phthalates (plasticisers), bisphenols (BPA in particular), flame retardants, antioxidants. Many are endocrine disruptors that interfere with hormonal systems (reproduction, immunity, development).
• Adsorbed pollutants: plastics attract and concentrate on their surface persistent pollutants that are present in very low concentrations in seawater (PCBs, PAHs, heavy metals). A plastic particle can concentrate these toxic compounds up to a million times relative to the surrounding water.

Once inside the cetacean’s body, these compounds migrate towards fatty tissue, in particular the subcutaneous blubber. They accumulate there over the animal’s lifetime. Females then transfer a large part of their load to the calf through milk that is extraordinarily rich in lipids (40% fat in the sperm whale). For a calf this means a massive exposure at the start of life, during the critical phases of neurological and immune development.

Translocation into the tissues

A recent publication (Romeo et al., 2024) has brought a worrying finding: microplastics have been found in the lung and liver tissue of humpback whales, that is, beyond the digestive tract. This demonstrates that the particles are not simply swallowed and excreted: some cross the intestinal barrier and spread through the whole body. It is this translocation that most concerns marine toxicologists today. Once in the tissues, the particles can cause chronic inflammation, which is beginning to be associated with various pathologies in the species studied.

The Mediterranean, a hotspot

The Mediterranean Sea is one of the seas most polluted by plastics in the world. Its semi-enclosed configuration, its densely populated coastal basin (about 200 million inhabitants, plus massive tourist numbers) and its intense shipping traffic create an exceptional concentration of debris. In some surface areas, up to 1.25 million fragments per square kilometre are measured: a density higher than that of the great ocean gyres.

Maria Cristina Fossi and her team at the University of Siena documented in 2016 (Environmental Pollution) that Mediterranean fin whales carry significantly higher phthalate concentrations in their blubber than Pacific fin whales. The same team showed that these whales filter their main feeding areas (the Pelagos Sanctuary in particular), which also happen to be areas where microplastics accumulate at the surface, a particularly unfavourable overlap.

PCBs: a historical poison

Polychlorinated biphenyls (PCBs) deserve a mention of their own. They are organochlorine compounds used on a massive scale in the twentieth century (electrical insulation, lubricants, paints), banned in most industrialised countries since the 1980s. But they are extremely persistent and they accumulate in marine food chains. The cetacean predators at the top of the chain, killer whales, sperm whales and Risso’s dolphins, carry the highest concentrations in the world.

Jean-Pierre Desforges and his colleagues published a study in Science in 2018 that caused a considerable stir: according to their models, several European killer whale populations (Gibraltar, the British Isles, coastal Norway) are headed for extinction from PCBs over the next thirty to fifty years, even in the absence of any other threat. The levels measured exceed the thresholds of reproductive toxicity. It is one of the strongest warnings addressed to the international community about the persistence of historical chemical pollution.

Sources

• Kahane-Rapport, S.R. et al. (2022). “Field measurements reveal exposure risk to microplastic ingestion by filter-feeding megafauna”. Nature Communications, 13.
• Fossi, M.C. et al. (2016). “Fin whales and microplastics: The Mediterranean Sea and the Sea of Cortez scenarios”. Environmental Pollution, 209.
• Desforges, J.-P. et al. (2018). “Predicting global killer whale population collapse from PCB pollution”. Science, 361.
• Lusher, A.L. et al. (2018). “Microplastic interactions with North Atlantic mesopelagic fish”. Marine Pollution Bulletin, 127.
• Romeo, T. et al. (2024). “Microplastics in stranded cetaceans”. Frontiers in Marine Science.

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