Searching for plastic in the belly of a seal

Plastics are a problem facing oceans and waterways around the world. In the UK, numerous fish species in the River Thames are regularly consuming microplastics - and this is likely affecting animals further up the food chain, too.

A mature female grey seal (Halichoerus grypus) found dead on a beach in Kent in May 2018 is having its final meals inspected to see if it had ingested any fragments of the man-made material.

Plastics in the food chain

Microplastics are pieces of plastic less than five millimetres in length. At least 10 species of fish found in the River Thames are known to ingest them. 

Alex is a PhD student, working with the Museum and Royal Holloway, University of London, funded by the London NERC DTP. Her study is looking at how plastic is affecting the different levels of the food web, observing whether some are more affected than others.

'My idea, from the literature that I've read, is that smaller organisms are affected by smaller pieces of plastic. So you've got fish that could be mistaking plastic for plankton, whereas a seal isn't looking for plankton - but when it is catching fish, there might be a carrier bag floating and it catches that,' she explains.

'In theory, there should be bigger bits of plastic in bigger animals.'

A grey seal was found dead on a beach in Walpole Bay, Margate, on 21 May. It was brought to the Museum to be dissected for several scientific studies. For Alex's work, the digestive organs were removed, including the stomach and large and small intestines.

Alex will examine the contents to determine whether they contain large pieces of plastic or microplastics possibly passed on from the fish it preyed on.

How to identify microplastics

It is possible to visually identify organic and man-made materials found in animal stomachs.

'Colour is usually a really key thing,' says Alex. 'Green, for example - if it's bright, it's usually artificial. We also look for if there are any clear cellular structures, which would give you an indication that it's organic.

'Shape is another indicator. If the fibres end in a point or aren't a uniform length, it suggests that they're not plastic, but could be hair or chaetae.

'After that there is a spectroscopy technique that we use, Fourier-transform infrared spectroscopy (FTIR). The FTIR spectrometer shines a laser of light from a transmitter to a receiver. The path of the light is changed so that it is partially absorbed by the sample. Different materials absorb different wavelengths of light, creating a "fingerprint" by which we can identify the material.'

Digestion - dissolving the organic material in acid or a strong base - will help to isolate any plastics in the stomach contents of the seal.

But there are some stipulations, Alex adds: 'You can't just put it into the strongest acid you've got, because that will digest everything. We tested nitric acid and that melted the mesh we tried to filter it through. It also foams, so material ends up everywhere and the sample becomes contaminated by plastics in the lab and from the air.'

'We're trying KOH, which is potassium hydroxide and the most commonly used digestive agent, and enzymes, which seem to be going really successfully. But until we know which one works most efficiently, we can't commit to the seal with any of them.'

As there is a limited amount of seal material available, Alex is finalising what will be used by testing on fish.

'I have a general idea of what sorts of things a fish would and wouldn't eat - so if something looked like an odd shaped plastic, I might know that it's because I've melted it. But in the seal, it may just have eaten something a fish wouldn't.'

A positive impact

Until the seal became available, Alex's study has focused on fish found in the River Thames, primarily looking at species that live on and feed from the riverbed.

'I've looked at a lot of fish - I've looked at about 21 species', he says. 'Some of them there were a couple of individuals and then some there were 300.'

'It gives us an idea of what is on the riverbed that we don't see quite as much. The seal allows me to study the different levels of the food web.

'Seals feed at different depths in the water column, so we can see if there is a difference - it could be that there is a complete difference in what is in the sediment than what is in the water. One could be more exposed to plastic than the other or the type of plastics might also differ.'

From the initial superficial search of the seal's stomach contents, there was no immediate evidence of large pieces of plastic. But the material may be found through more in-depth analysis.

The study of plastics is a relatively new field. Scientists are trying to understand how far plastics can travel through animals in the environment.

'It's interesting, but it also has real-world applications,' explains Alex. 'All the news you see is that 90% of seabirds are eating plastic, and as an outsider, you might feel that there is very little you can do about it.

'But if you can do something it makes you feel more optimistic. If you find plastic, the government might see it, or policymakers, or an environmental group - so it could make a difference. It's something that I want to continue with and hopefully make a positive impact.'