Could microplastics impact sharks?

Show notes

Whale and basking sharks are some of the biggest fish in the ocean – but could these gentle giants be threatened by something so small it can’t even be seen with the naked eye? We pitch this question to microplastics and pelagic fish expert Dr Alina Wieczorek. Alina has studied microplastic pathways from the smallest to the largest creatures in the ocean and in some of the deepest and most remote places in the world. She has some really interesting, and at times deeply concerning, insights for us, but also a message of hope.

We start with our usual ice-breaker (pun intended): what is Alina’s most memorable experience in the ocean (4.40)? For Alina, her experiences are always topped every time by the next one and made all the more special because you never know what to expect when it comes to the sea. She also describes her childhood wild-swimming with her father (6.15), which sparked an early passion and curiosity for the underwater world that later developed into a career in science. We discuss her journey into the world of microplastics, which began with a year out in Uruguay at a sea turtle rescue station (7.08). There Alina witnessed the impact of plastics first-hand, observing necropsies on deceased turtles with stomachs filled with bags, straws, and other man-made debris. This motivated her to start (and persevere through) a PhD, which looked at the pathways of microplastics in pelagic environments (10.28).

So…what do we actually mean by ‘microplastics’? As Alina explains (12.05), microplastics are roughly defined as a particle of plastic smaller than 5mm. They either start out that way – for example, microbeads in cosmetics – or are formed as larger pieces of plastic break down. Our understanding of plastic pollution has come a long way in a short space of time, however, and we now have multiple definitions of particle size, including nanoparticles which are invisible to the naked eye. Plastics make their way into the sea either via landfill when they are washed out by rivers or dumped directly into the ocean, or from spillages (14.44). The latter usually involves ‘nurdles’, the names of the raw materials used to make plastic items. However, many of the microplastics Alina observes in her samples come from fibres, which make their way into the ocean differently. They rub off our clothes in the washing machine or splinter off discarded fishing gear.

We then discuss the link between microplastics and sharks – or, more specifically, filter-feeding sharks (18.16). Essentially, filter feeders take in vast quantities of seawater and extract tiny organisms, called zooplankton, from it. They have modified structures in the mouth that acts as a giant sieve, catching their food as they swim. Alina’s work looked at how such shark species may be ingesting plankton using this feeding strategy. One way would be direct consumption of seawater. The other could be indirectly through the plankton. If plankton is consuming microplastics themselves, even if it is only one plankton for every hundred, the sharks are taking in so many that it would accumulate in their system.

Alina had a few ways of investigating this for her SOSF-funded project, ‘Microplastics: a macro-disaster’ (21.04). First, she identified feeding ‘hotspots’ for whale and basking sharks and took water samples from there. Second, she analysed faecal samples from the Maldives Whale Shark Research Programme. She is still examining the water samples, but the whale shark poop has a very interesting and worrying story to tell (26.10). Microplastics, specifically fibres, were present, suggesting that whale sharks do indeed take them in. This could be good news, as it could mean that the plastics are simply passing through the shark’s system. But it could also spell trouble. Some contaminants could be left behind in the shark’s tissues, and it also means that the plastic returns to the ocean as something that is more ‘bioavailable’. In other words, it has organic matter around it, meaning that other fish will consume it. And these fish will be eaten by other marine animals, which will then die and decompose, releasing the plastics back into the ecosystem. It’s a continuous cycle. Alina’s next job is determining which plankton species are taking plastics on board.

But could ingesting tiny plastic particles actually hurt our sharks? It’s a difficult question to answer, as there are moral and ethical barriers to testing this theory (31.00). But Alina discusses that in other laboratory-raised fish, like sea bass, plastic ingestion has been found to have some pretty nasty side effects – inflamed intestines certainly don’t sound like fun.

She also explains that her other research has found microplastics are a more gargantuan problem than we might first think (33.20). She has found evidence of microplastics in 73% of mesopelagic (deep and remote) fishes and in the polar regions. Other scientists have also discovered microplastics in fishes at the Marianas Trench, the very deepest part of the ocean. Alina’s work has also identified that if the amount of plastics in the ocean continues to increase, they could slow down an extremely vital oceanic process: the biological pump (36.50). This would affect all life not only in the ocean but on the planet, as the biological pump is responsible for transporting atmospheric carbon to the deep sea.

It’s all quite alarming, but is there any hope? Alina believes we can use the plastics disaster as motivation to do better (43.50). She feels encouraged by how engaged the public, and world leaders, have become with the problem. And on an individual basis, there’s plenty to do – from recycling and re-using the plastics we already have to switch to clothing made from natural materials like wool and bamboo, participating in beach cleaning and writing to your local government (48.00). Her best advice? “Treat plastics like a precious commodity” (52.50). Essentially, don’t treat it as something to be used once and discarded – take care with it, use it for as many purposes as you can and when you can’t use it anymore, dispose of it responsibly.

About our guest

Dr Alina Wieczorek

Alina has recently joined the fisheries acoustics team at the National Institute of Water and Atmospheric Research (NIWA) in Wellington, New Zealand. She assists in the stock assessment of commercially valuable fish and is particularly interested in mesopelagic and deep sea fish. Alina obtained her BSc and PhD from the National University of Ireland, Galway, where she worked on stable isotopes to study the diet of small inshore sharks and wrote her PhD thesis on microplastic pathways in marine pelagic systems with a particular focus on heterotrophic nanoflagellates, salps and mesopelagic fish. Aside from her academic commitments, Alina is passionate about community outreach and has worked with several schools to bring science closer to society. Driven to make a change, Alina has been leading a Save Our Seas Foundation keystone project since 2019, aiming to study microplastic ingestion by whale sharks and basking sharks and bring scientific findings closer to key stakeholders and decision-makers.

Twitter: @MPLarestFish and @AlinaMWieczorek    Instagram: @MPLargestFish