If you’ve recently opened a tin of mackerel and thought there was something fishy going on, you may have been right. An overfishing crisis means that instead of traditional mackerel, some now contain a different fish with a similar name, prompting concerns that shoppers are being short-changed.

Over the past year big brands such as Princes, as well as some supermarket own-labels, have replaced traditional tinned mackerel fillets with jack mackerel fillets.

Although clearly labelled, food experts say that jack mackerel is a poor replacement for the oily fish, because it is a different species with a different flavour and different nutritional values. They also say shoppers may not realise a product has changed until they get it home.

The new products are being sold at a similar price despite the fact that it is a much cheaper fish, leading the consumer group Which? to describe it as another example of “skimpflation”.

Tinned fish is an “affordable and convenient way for households to add important nutrients to their diets”, said Sue Davies, head of food policy at Which?.

“While manufacturers face genuine supply chain pressures and sustainability challenges, they have a responsibility to shoppers to be completely upfront when they change a product’s recipe.

“Skimpflation can feel particularly sneaky as shoppers may not realise a product has changed until they get it home and eat it,” she said.

Lidl is the latest retailer to switch and, in a recent video by the viral canned seafood reviewer Marcus Ansell, he rates its original mackerel fillets eight out of 10 while the Marine Stewardship Council (MSC)-certified replacement scores a lowly 3.5 out of 10.

“Jack mackerel is of significantly lower quality,” said Ansell. “It is quite a bad tinned fish really. I’ve had a lot of messages and some comments on my videos from people who thought they were buying real mackerel and were very disappointed when they tried it.”

Ansell said: “It’s not a substitute at all,” adding, “ It’s a completely different species and is nothing like mackerel in its taste, texture, or smell … and there’s the health aspect, which is important to some people.”

Compared with traditional mackerel, jack mackerel has roughly half as much omega-3, a fatty acid that it is recommended people have twice a week as part of a healthy balanced diet.

“So they’re scamming us on mackerel now,” said one poster under the review. “Can’t even get some tinned fish without being legged over.”

Chilean jack mackerel costs about a-third of Atlantic true mackerel, according to the Financial Times, which first reported the story.

Last year the labelling rules were changed by the Department for Environment, Food and Rural Affairs to allow the species to be marketed as “jack mackerel”. Previously it could only be sold as “horse mackerel”, “jack”, “scad” or “trevally”.

The Marine Conservation Society (MCS) warned last year that mackerel stocks were at breaking point owing to overfishing. Its Good Fish Guide, which ranks choices using a traffic-light system rates northeast Atlantic mackerel “red” making it a “fish to avoid”.

Princes said it had taken the decision to move its branded range away from North-East Atlantic mackerel following years of concern over unsustainable catch levels.

It added that the “species is clearly identified on our products, and we would consider sourcing North-East Atlantic mackerel again if sustainable management is restored and MSC certification regained”.

Lidl said it was moving its tinned mackerel over to MSC-certified Chilean Jack mackerel. “Before our next purchasing window for smoked mackerel, we will review if northeast Atlantic mackerel continues to comply with our sourcing policies and scientific advice.”

Earlier this year Waitrose suspended the sale of mackerel because of overfishing and instead directed its customers to buy herring and other species.