Takeaways have yet to stop trying to cod customers with cheaper kinds of fish, researchers have found.
Scientists who exposed mislabelling and food fraud in supermarkets, fish and chip shops and fishmongers four years ago say some fast-food businesses are still at it.
Professor Stefano Mariani revisited his original genetic study and found 41.7% of fish being sold in takeaways in Dublin are incorrectly labelled as cod compared to 50% in 2010.
Supermarkets, however, have completely eradicated the practice of mislabelling in the wake of the expose.
"The timeline of events suggests that the media coverage highlighting the findings of the first cod mislabelling study put the large supermarkets under intense scrutiny, which acted as a positive catalyst for change," he said.
"This hasn't happened to the same degree in the takeaway industry, probably due to the less systematic custom and the convenient nature of the food. Supermarket chains constantly compete to attract and maintain a faithful custom, and their size and financial power may allow them to exert greater control over the supply chain.
"Despite the impact of media coverage in this instance, it remains necessary that the authorities commit to adequate effective enforcement, in order to eradicate illegal practices in the sector."
Prof Mariani, of the University of Salford, led the original research while he was at University College Dublin, revisited the same shops and takeaways where he bought fish for genetic testing.
Four years ago his study, the first of its kind in Ireland, uncovered the widespread mislabelling of cod in Dublin's supermarkets and takeaways and found that 28% of the products belonged to a different species of fish.
The much-loved whitefish was being replaced with some exotic sounding but foul tasting varieties without customers' knowledge.
In his revised study Prof Mariani examined the impact the expose and media scrutiny had on the market.
The research, supported by staff at UCD and the University of British Columbia, has been published in the journal Conservation Letters.
The report claims it may be possible for mass media to influence fisheries and environmental management and policy so long as research is correctly reported and government follows up with appropriate enforcement.