Several fish and chip shops in England have been caught selling catfish as "traditional fish and chips" to cut costs, a BBC investigation has found.
The practice is difficult to police because cases of fish misrepresentation were usually intelligence-led and rely on costly DNA testing.
Catfish is safe to eat and is typically farmed and imported from Southeast Asia. However, it is significantly cheaper than traditional fish-and-chip shop staples in the UK such as cod and haddock.
Wholesale, catfish costs about £3.40 per kilogram (NZ$7.90), compared with cod or haddock at roughly £15 per kilogram (NZ$34.90) — less than a quarter of the price.
The BBC analysed dozens of online food reviews in which customers questioned what species of fish they had been served.
Many commenters said they had "no idea what type of fish it was", while others complained it was "not cod or haddock, just some cheap white fish".
Those reviews led the BBC to identify 10 businesses for testing: five in Liverpool and five in Manchester.
Among them, three chip shops listed cod on their menus, four listed haddock, and three listed only "fish". Under UK law, businesses are required to tell customers the species if asked.
Questioned by the BBC, the three shops that did not list a species described their product as "normal fish" or "white fish". In one case, a server pointed to a sign advertising "traditional fish and chips".
DNA testing carried out at Liverpool John Moores University found all three establishments had sold catfish.
Professor Stefano Mariani, who oversaw the testing alongside researcher Catherine Perfect, said DNA barcoding confirmed three of the 10 sampled shops were selling pangasius.
"In my experience with fish and chips, three out of 10 is quite a lot — I don't recall seeing this level of catfish," Mariani said.
The catfish portions were sold for £3.80, £4.20 and £5 — around NZ$8.80, NZ$9.80 and NZ$11.60.
By comparison, portions advertised as cod or haddock were typically priced between £4 and £6 — about NZ$9.30 to NZ$14.
Mariani said it would be extremely difficult for customers to tell the difference.
"It is very hard for a member of the public who is not a trained fish biologist to identify one fillet from another," he said.
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