What can a pile of seal poop tell us about plastic pollution in our oceans?

Guest Written By Aaron Bunch

Imagine that funny little emoji. Yeah, you know the one that looks like a pile of poop with a smiley face? But now think of it covered in birthday cake sprinkles! 

Yes, I am asking you to think about a pile of poop coated with small pieces of shiny colored bits.  Wouldn’t that be odd to see?  If you were a researcher studying the effects of microplastics on marine mammals, maybe not.  Microplastics are very abundant in marine ecosystems.  Animals within those ecosystems can ingest microplastics as they feed.  If this happens, animals are not capable of digesting the plastic, and it will come out with their poop or what researchers call “scat”.  So, it may look like a seal had a birthday party!   

Source: brgfx on Freepik

Despite the humor, this is a serious worldwide issue that researchers are embarking on to find the potential negative effects on animals.  One of the best types of animals to study for this type of research is large top predators.  There is a term called “biomagnification” where top level predators tend to accumulate more pollutants than smaller lower-level prey species.  This is likely the case with seals and other marine mammals which could ultimately cause harm to them.   

While looking at scat samples, researchers not only check for microplastics, but also search for their overall diet.  This can be done using DNA approaches where scat samples provide DNA leftovers from an animal’s meal from previous days.  You know, those TV shows where the Crime Scene Investigators use DNA analysis to identify a suspect from a crime?  The same can be done for scat.  The scat will hold vital information that will inform researchers of the diet. 

Source:  Nelms et al. (2018).  See reference below.

A 2019 study found six different colors of plastic in scat samples from grey seals in Skomer Island, Wales.  Blue plastic was the most prevalent in 53% of samples.  The grey seals may have picked up microplastics from eating flounder and cod fish.  Those types of fish are known to have microplastics in their stomachs.  This is where the biomagnification mentioned early comes into play—small fish eat microplastic thinking it’s a small plankton, which get eaten by a larger fish, and then the larger fish is captured and eaten by a grey seal.  It’s simple addition really, the seal may eat four fish which ate ten pieces of microplastic each, so the seal ingests forty microplastics in one feeding.  Some microplastic may accumulate and cause blockage which could ultimately lead to sickness or death of the seal.   

The lesson learned here is that research on microplastics in our ecosystems is important as these human-made materials can cause harm to animals.  Keep in mind, we as humans are the top predators on Earth.  Where might some of the microplastics be going?  We must do our part to reduce microplastics.  It is for the betterment of humans and animals alike.  


Nelms, S. E., Galloway, T. S., Godley, B. J., Jarvis, D. S., & Lindeque, P. K. (2018). Investigating microplastic trophic transfer in marine top predators. Environmental pollution, 238, 999-1007.

Nelms, S. E., Parry, H. E., Bennett, K. A., Galloway, T. S., Godley, B. J., Santillo, D., & Lindeque, P. K. (2019). What goes in, must come out: Combining scat‐based molecular diet analysis and quantification of ingested microplastics in a marine top predator. Methods in Ecology and Evolution, 2019(00), 1-11. https://doi.org/10.1111/2041-210X.13271 

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