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Marine litter: Don’t get carried away…

Marine litter: Don’t get carried away…
Chiarina Darrah

The recently announced findings from Cefas’s survey of marine plastics on the seabed, which suggests that incidents of plastic bags on the seabed have fallen since the plastic bag charge was introduced, can only be good news. The questions are, how good? and what lessons can we draw from it?

Monitoring what’s going on at the bottom of the sea is far from easy, and Cefas’s work provides a valuable time-series of data in a less than crowded field of study. It’s carried out in a careful and consistent way, and is clear about its methodology. It is particularly valuable because it joins only a handful of other papers that demonstrate a significant correlation between marine litter levels and policy measures. But it’s important not to over-interpret its findings.  

The study is based on around 90-100 trawls a year, which is not a tiny number, but represents a pretty miniscule sample of the sea bed. By its nature, trawling will only reveal relatively large pieces of plastic, not material that has broken into smaller pieces – and can tell us very little about microplastics. The risk of ‘noise’ overwhelming ‘signal’ when it comes to identifying trends is therefore significant, although it’s also a problem of which the researchers are aware.

The study’s methodology involves looking at the percentage of trawls that contain a particular item – carrier bags, plastic bottles, fishing nets and so on. It doesn’t matter whether one carrier bag is found in a particular trawl, or fifty – it counts as one “carrier bag” instance. It’s therefore quite rough and ready as a measure, and while the researchers do look at the number of items of each kind found, the time-series wasn’t evaluated this way.  

Carrier bag charges have been appearing on the British Isles since around 2010, but only reached England in 2015, so Cefas so far has only two years of data since the majority of the UK population started to have their behaviour influenced. Future years will tell us with greater clarity whether Cefas is identifying a real trend – although obviously it is encouraging to see the data so far moving in the right direction, and the proposals to bring more shops within the scope of the levy in England, to bring it more into line with the rest of the UK, might produce a more decisive change in the numbers.

Important though monitoring at sea is, it’s not the first place we should expect to see policy impacts emerge. The uncertainty introduced by factors such as the ‘transit time’ for litter moving from land to sea and the operation of marine currents mean that Cefas’s findings will always be a delayed and attenuated result of changes in waste policy.

The results of new policies, such as a deposit return scheme, a charge on disposable cups, or restrictions on plastic straws, would be far more quickly and directly evident from terrestrial monitoring, like the Ireland national litter monitoring program. However, even there, the litter baseline, estimated prior to the introduction of the national litter monitoring program itself, is of questionable value – making it harder to evaluate the impact of policy.

Additionally, this typical ‘percentage target item by count’ monitoring method produces relative, not absolute numbers. This can make it nigh on impossible to tell whether a fall in the percentage of carrier bags found in litter is because fewer carrier bags are being found, or because while carrier bags numbers have increased, the number of crisp packets (for instance) being littered has increased faster.

The rising prominence of concern about plastic litter in the environment is welcome, as is the drive for action to address it. No-one would wish to see action unduly delayed. But if we’re to really understand the benefits those policies are delivering, better litter monitoring – particularly on land, although marine studies are also useful – will be needed. Without it, we risk taking actions that aren’t effective, or deceiving ourselves into thinking we’ve solved problems through policies that don’t work in practice.


Chiarina Darrah is a Senior Consultant at Eunomia Research & Consulting, an independent environmental consultancy with a focus on waste, energy and resource efficiency.

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