"There are plenty more fish in the sea!" - alas, not always true in most places anymore.

Cod FishingSadly, for many species this is old saying is no longer true. In some places cod can be fished sustainably, for example off Iceland, in the Baltic Sea, in the Atlantic off Newfoundland at the Flemish Cap fishing ground.
However off Norway, the West of Scotland, in the North Sea and off the east coast of America, cod was badly over-fished in 2011 - and at the Faroe Plateau and in the Celtic Sea fishing restraint is also needed.

Apart from some well-defined species like cod, there is also a basic problem that it is not easy for non-specialists to be able to recognise what fish they are dealing with in the market. There are certainly a lot fewer names for traded fish than there are species!

However a lot of work has been done at the University of Wales in Bangor and altogether 15 research groups across Europe and Russia have spent 3 years sequencing DNA from cod, hake, sole and herring. They have now improved the specificity of tests to the point where populations within species can be identified - and hence the origins of the caught fish.
Cod
These test are now very important in helping to monitor fish stocks to prevent over-fishing and allow threatened populations to recover. But now that the UK has withdrawn from its trading agreements with Europe to become a so-called "Third Country" the tests are going to be crucial for UK fishermen to be able to verify the origins of their catches and regain access to some of the markets they have lost.

Thrashing around the sea in gun-boats is so nineteenth century! Quick DNA tests on catches before they are landed will be a far more effective, safer and cheaper way to determine what has or has not been legitimately fished and what can be accepted for sale - and an effective deterrent against "fish piracy".