Two-thirds of the fish consumed by UK consumers is imported from abroad, and its costs would rise, due to trade barriers resulting from a hard Brexit. In addition, the fall in the price of UK fish would cause UK fishermen to fall in profits. The cost of fish comes down to a combination of controlling fish stocks and the cost of operating a fishing fleet. The waters around the United Kingdom suffered from overfishing, which in turn led to a reduction in the size of catches.
In short, people all over the world are eating more and more fish. People in Britain eat an average of 20 kg of fish a year, which means that imported fish is needed to meet demand; if the UK were to rely only on fish caught domestically each year, fish supplies would run out in July (. The problem is even more pronounced in other countries, such as Spain and Portugal, where people eat on average between twice and three times the amount of fish consumed by the British. At the simplest level, demand for fish is increasing, but fish stocks are declining, which means prices are rising around the world.
Why is fish so expensive in the UK? Two-thirds of the fish consumed by UK consumers is imported from abroad, and its costs would rise, due to trade barriers resulting from a hard Brexit. Except for the diesel you need to power your trawler and you need a lot of diesel to get to where the fish are and diesel costs A LOT more than fish food. Previously, illegally caught fish entered the market, as did blackfish (over-quota fish that has been caught and has not been reported; see the Scottish Blackfish Scandal). The increase in the price of fish can be seen as a positive development or as a damage to fish stocks, depending on how the situation is handled.
And despite what I've read about selling UK fish in the EU, there isn't a single fishmonger or supermarket I've visited in Spain that sells fish from the UK. Sunday Times columnist and author of The End of the Line Charles Clover says that effective measures to stop illegal fishing have been one of the causes of rising fish prices.