Once a fish is on your plate, it can be very difficult to tell whether it’s Atlantic or Pacific cod, toothfish or seabass. This uncertainty creates an environment that can lead to seafood mislabelling, and studies show that on average 30% of seafood, worldwide, is mislabelled.
Mislabelling may be accidental or deliberate. If it is deliberate, fish are often substituted for a cheaper, or lower quality, species. This may have dangerous consequences; in 2007, people became ill after eating toxic pufferfish that had been labelled monkfish.
So, how can we find out if the seafood we’re eating is what we think it is?
Tracing species through 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 in cell 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 good place to look for the barcode, as it doesn’t change much between generations.
Using 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 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, the technology lets researchers identify where the regional origin of the fish. For example, it can show 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 breed with other groups, there is a chance their DNA will not reveal the differences. Tracing these species to their geographical location involves using a different type of technology.
A new traceability technique 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 the body is mapped to the environment. The isotope profile can tell us where the animal has been and what it has 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 jellyfish isotopes. Jellyfish are a good option to make such a map because they live in all the oceans of 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.
Researchers can use this map to estimate where an animal lives by matching its isotopes 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 in fish otoliths (an ear bone) are related to concentrations of elements in the environment. Researchers can discover 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 specific element profile.
Between these different approaches it should be possible to find one that gives a good indication of the source of a seafood product. Multiple approaches may be needed and there will be cases where no technique will work. In these cases, we may need to look toward digital solutions in the supply chain. – Marine Stewardship Council