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Fighting fish fraud with science

March 18, 2019

Oil being brushed on herring on a tray

Herring or sprat?

Once a fish is on your plate, it can be very difficult to tell if it’s Atlantic or Pacific cod, toothfish or seabass. This uncertainty can lead to seafood mislabelling, and studies show that on average 30 percent of seafood is mislabelled around the world.

Mislabelling can be accidental or deliberate. If it is deliberate, fish are often substituted for a cheaper or lower quality species. This can sometimes have dangerous consequences and in 2007, people became ill after eating toxic pufferfish that had been labelled as monkfish.

So, how can we find out if the seafood we’re eating is what we think it is?

Tracing DNA

Scientists can tell you what fish is on your plate by looking at its DNA.

DNA is the string-like molecule that carries the genetic information of everything alive on Earth, and we inherit it from our ancestors.

You can find DNA in the nucleus of an animal cell, but there is also DNA within mitochondria, the tiny powerhouses of animal cells. Every animal species has a region of DNA that is unique to them, known as the barcode. Mitochondrial DNA is a great place to look for the barcode, as it doesn’t change much between generations.

In a method known as DNA barcoding, researchers take a food sample and extract the mitochondrial DNA. They read the sequence of a snippet of DNA that contains the barcode and compare it to a library of barcode regions for thousands of different species. If the barcode matches a sample in the library, the species can be identified. Many studies have used DNA barcoding to study fish fraud and a recent study by the MSC found that less than 1% of certified seafood is mislabelled.

As DNA has become easier and cheaper to sequence, researchers can now map more of the genome of an animal. By looking across the genome, they can see subtle differences in the DNA.

In some cases, this new technology lets researchers identify where the fish has come from. For example, it can reveal the difference between a North Sea cod and a Barents Sea cod. This technology is not only useful for traceability but can also be used to inform fisheries management advice.

For species that swim vast distances or who breed with other groups, there is a chance their DNA won’t reveal any differences. Tracing these species back to their geographical location requires different types of technology altogether.

Tracing elements

A new traceability technique that is gaining popularity is isotope mapping. Isotopes are slightly different versions of the same element. Areas of ocean water contain different and identifiable isotopes. As an animal feeds and grows, these isotopes are embedded within its body, so their body is mapped to the environment. Their isotope profile can tell us where the animal has been and what it’s eaten.

Imagine a map of ocean temperature changing from red to blue as warmer waters become colder. A team of researchers recently made a similar map, but instead of temperature, they used isotopes. To create the map, they used isotopes found in jellyfish. Jellyfish are a great option to make such a map because they live all around the world, grow fast and don’t move around much before they die. This means their isotope ratios are a good reflection of their location.

Researches can use this map to estimate where an animal lives by looking at the matches of the isotopes in its body compared with the isotope map. 

Trace elemental fingerprinting (TEF) is another technology that can tell us where an animal is from. The concentrations of elements in shells and fish otoliths (a part of the ear) is related to the concentration of elements in the environment. Researchers can find out if two animals are from the same location by looking at the elements in their bodies. This technique works very well for aquaculture, where fish or shrimp live in a pond with a particular element profile. 

Between these different approaches it should be possible to find one that gives a good indication of where a seafood product came from. Sometimes, multiple approaches might be needed and sometimes none might work. In these cases, we may need to look toward digital solutions in the supply chain.

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