When is a sustainable fish not a sustainable fish?

08 September 2011
Posted by Chris Smith.

If you're concerned about the plight of fish stocks, but still like to eat fish, you might tend to choose fish marked with the Marine Stewardship Council logo as having come from 'sustainable' fish stocks when you go to the supermarket.

Patagonian toothfishBut can you be sure that fish that has come from so called 'sustainable' fish stocks has really come from there? Well, a rather worrying study by Peter Marko at Clemsen University in the States and his colleagues, has shown that in the case of one species, the Chilean Sea bass, not all of the samples tested actually came from the sustainable fishery region as designated by the MSC.

The fish that is sold as 'chilean sea bas' is actually a species called the Patagonian toothfish, or Dissostichus eleginoides. These fish can only receive the MSC stamp of approval if they were caught in the South Georgia/Shag Rocks fishery, in the Southern Ocean, close to Antarctica.

So Marko and his colleagues set out to genetically test samples of Chilean sea bass bought in supermarkets to see if they really did come from this fishery by comparing their DNA to DNA results of a previous study that looked directly at fish caught in the South Georgia/Shag Rocks fishery. The population of Patagonian toothfish in this area is relatively isolated, and the fish are slow growing, so they are quite genetically distinct. They extracted DNA from the fish samples, amplified it and cut it with restriction enzymes before running it on an agarose gel. The digestion pattern of the samples were compared with patterns found in the previous study.

They found that  5 of the 33 samples were Patagonian toothfish did not match the genetic profile of the South Georgia/Shag Rocks fishery, and that 3 of the 33 samples were not Patagonian toothfish at all, but were actually tuna, mackerel or greenling.

The authors argued that this was due to mislabelling at some point in the chain from the fishermen to supermarket, and wasn't due to genetic shifts in the population, given they're so slow growing. It's a worrying finding, because consumers may believe they are making sustainable choices whereas in fact they unknowingly create demand for uncertified fisheries, which goes against the whole idea of MSC labelling.

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