Some tinned mackerel sold in the UK no longer contains traditional mackerel but has been quietly substituted with jack mackerel, a different, cheaper species, prompting criticism from consumer group Which? that shoppers are being short-changed. Brands including Princes and retailers such as Lidl have made the switch over the past year in response to an overfishing crisis, and while the change is labelled on packaging, experts say shoppers often only notice once they've bought and eaten the product, and it is still being sold at a similar price despite being a far cheaper fish.

Jack mackerel differs significantly in taste, texture and nutrition, containing roughly half the omega-3 of true mackerel, and reviewer Marcus Ansell rated Lidl's new MSC-certified version just 3.5 out of 10 compared with 8 out of 10 for the original. Chilean jack mackerel costs about a third of the price of Atlantic mackerel, and the switch follows a Defra rule change last year permitting the "jack mackerel" label, alongside Marine Conservation Society warnings that north-east Atlantic mackerel stocks are at breaking point and rated "red" on its Good Fish Guide.