A lack of information about the sources of seafood is one of the biggest barriers for diners trying to eat seafood sustainably, according to the analysis.
Eleven High Street restaurant chains and pubs were assessed, totalling more than 2,000 branches nationwide, on the sustainability of the seafood that features on their menus.
Cafe Rouge and Bella Italia came out top, followed by Harry Ramsden’s, Beefeater, Côte Brasserie and Carluccio’s.
But Prezzo, Wetherspoons and Itsu were at the bottom.
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Researchers from the Marine Conservation Society (MCS) and online restaurant guide Fish2fork found that, even when pubs and eateries buy seafood that is caught or produced responsibly, it can be difficult for the diner to tell which have made the effort to seek out sustainable supplies.
Transparency over the sources of king prawns and sea bass were the types of seafood used by chain restaurants giving the biggest causes of concern.
Wetherspoons and Prezzo both failed to make it clear on their menus where their king prawns derived. Itsu has prawns on its menus but fails even to say what type are used. Similarly, Prezzo served sea bass but failed to make clear if it was farmed or wild.
But Fish2fork and the MCS were pleased that chain pubs and restaurants are increasingly willing to switch to sustainable seafood.
Of the 11 chain restaurants and pubs assessed, just three failed to meet minimum standards of sustainability in the Fish2fork assessment criteria, which include the need to be transparent about sourcing.
During the ratings process six of the 11 restaurants changed their sourcing practices to be more sustainable.
These included Cafe Rouge and Bella Italia which have improved their Fish2fork rating over the last year. Cafe Rouge now gets a 3.5 blue fish rating and Bella Italia 3 blue fish, making them highly sustainable restaurants.
Tim Glover, managing director of Fish2fork, said: “It is fantastic that some of the UK’s most popular restaurants really are making an effort to seek out sustainable sources of seafood.
“But we are troubled that it is so often difficult for diners to know how sustainable a restaurant’s seafood is.
“We would urge restaurants to make a greater effort to make it clear on their menus where their seafood comes from, though we recognise that it can be difficult for them to get detailed information from suppliers.
“Sustainability is a vital ingredient in any dish, but diners shouldn’t be left to guess whether it’s there or not.”
Samuel Stone, of the MCS, added: “It’s brilliant to see so many high street restaurants now seeking more sustainable seafood, yet they need to bolster traceability and do more to tell their diners about the seafood they are serving.
“When eating out on the high street, diners need to know they aren’t going to be consuming seafood that is red rated or listed as Endangered.
“Telling diners where and how seafood has been produced and using the various seafood ecolabels available is key to this.”
Tim Doubleday, chief financial officer at Casual Dining Group which includes Cafe Rouge and Bella Italia, said: “We recognise the profound responsibility all restaurant businesses have in sourcing fish ethically and sustainably, and as such taking it extremely seriously, striving to be recognised as a leader in this area.
“We have worked hard to scrutinise our supply chain, systems and processes to categorically ensure sustainable sourcing across all our brands - Café Rouge, Bella Italia, Las Iguanas and La Tasca.”
Other restaurant chains praised for their level of seafood sustainability were Harry Ramsden’s, Côte Brasserie, Beefeater, Brewers Fayre, Carluccio’s and Eat.
The chain restaurants were rated according to the information on their online menus, websites and from what they directly told the researchers.
Fish2fork ratings are on a scale of 5 red fish, the worst, to 5 blue fish, the best.
Cafe Rouge 3.5 blue
Bella Italia 3 blue
Harry Ramsden’s 2.5 blue
Beefeater 2.5 blue
Cote Brasserie 2.5 blue
Carluccio’s 2.5 blue
Brewers Fayre 2 blue
EAT 2 blue
Wetherspoon 0.5 red
Itsu 0.5 red
Prezzo 1 red