Eating out can be something of a minefield for the ethical epicure. Is the mushroom-stuffed chicken breast organic? Did the steak au poivre come from a happy cow? Where did the spring lamb spend its short life? But by far the most challenging choice on the menu is the fish course. With cod stocks plummeting, sea bass being caught at a rate of knots and farmed fish a hot topic, many diners would be forgiven for forgoing their flounder in favour of a guilt-free nut cutlet. However, if you know what to look for, it is safe to go back in the water for your supper, thanks to a new wave of restaurants flying the flag for sustainable fish.

Chef Oliver Rowe, whose London restaurant Konstam at the Prince Albert sources more than 85 per cent of its produce from the area covered by the London Underground network, is one of the first restaurateurs to gain Marine Stewardship Council (MSC) accreditation for the fish he uses. "We use a massive amount of fish as an industry and I think that there are 20 per cent of the fish stocks there were 80 years ago," he says. "Sustainability is something we all need to focus on. We source our fish from just two fisheries, Hastings and the Thames. From Hastings we get Dover sole, mackerel and herring, and from the Thames we get herring and sprat." As a result Konstam now proudly displays the MSC logo on the menu next to its fish dishes.

While one little logo might not sound like much, the MSC's stamp of approval is a beacon to consumers who care about where their fish comes from. In order for a restaurant, factory or retailer to get the thumbs up from Britain's biggest marine charity, they have to prove that 100 per cent of the fish they use can be traced back to healthy stocks from sustainable fisheries. The at-a-glance eco-logo already appears on a large range of products in supermarkets across the country, but, until now, hasn't been featured on any menus.

Laura Stewart, food service manager at MSC, says: "A restaurant is no different from a fish finger factory or a fresh fish counter – all of them need to have chain of custody certification before they can use the MSC logo. We haven't had a huge number of restaurants entering the assessment process, and I think that's because traceability is an audit process. Factories are used to being audited for quality, but restaurants are not used to somebody coming in and asking them where their fish comes from."

But a pilot scheme, of which Konstam is part, is set to change this. "The pilot was designed as a boost to restaurants because we've had very few entering assessment, getting certified and using the logo," Stewart says. "It's a serious process, but it's quite an accolade to have it on your menu."

Ensconced in his busy kitchen at Konstam, Rowe agrees. "It's lovely to get the certification. It's very important to have something like the MSC to put on your menu for the customers. They need those kinds of signposts." Rowe's customers tend to be more adventurous than most, thanks to his insistence on local, seasonal ingredients. Forget cod and chips – on the menu this week is charcoal-grilled cuttlefish. "I love cuttlefish, it's very underused – a real unsung hero. I'm a huge fan of sprats – we've got a few salted ones left downstairs but we've nearly run out. We have been through stone after stone of salted sprats because we can't get anchovies locally."

Rowe's uncompromising approach to sourcing food is admirable, but does it ever make putting a menu together a problem? After all, he can only get MSC fish from two suppliers. "It's very exciting working around the limitations of seasonality and sustainability – that's why we do it," he explains. "And any cooking takes place within limits – whether it's the number of people you're cooking for, the amount of money you have to spend, what you know how to cook or the facilities you have. It's just what every chef has to do all the time."

And he thinks that local is the way to go. "We get all of our fish locally for two reasons," he says. "One is that whatever fish is in season, we're attuned to it and also it doesn't have to travel as far by road. It also means I know who is doing the fishing."

Sustainability is the buzzword in the fishing industry right now – not only because of increased consumer awareness about the depletion of fish stocks but also because anyone who makes their livelihood from fishing or fish products wants to make sure that the day doesn't come when the nets comes back empty. Mike Parker, deputy chief executive of Young's Seafood, one of the biggest fish businesses in the UK, with a total turnover is in excess of £300m a year, is certainly of this opinion. "You can't work in this business and not want things to be done properly," he says. "And it's corporations who can change things and help sustainability."

Young's Seafood isn't the only big business that has gained the MSC tick. Sandwich supremo Pret-a-Manger proudly displays the symbol on all of its salmon sandwiches. "I'm seeing a huge surge of interest from white-tablecloth restaurants and small chains, plus big pub operators – they're actually coming to me to find out about it," says Stewart. "We're at the beginning of something a lot bigger and having somewhere like Konstam leading the way is really important."

Stewart hopes to see more restaurants signing up for certification. Meanwhile, Rowe has his own thoughts on what the future is for eating fish. "Really, what we should all be eating is tilapia."

One of the easiest and most profitable fish to farm, tilapia are a collection of freshwater species with an omnivorous diet that grow rapidly. "Tilapia farming is exciting because it presents an option we can focus on. I've never cooked with them, but I've eaten them in Thai places – it's a good fish."

Aquaculture aside, Rowe reckons that eating with an open mind is the way to keep fish on the menu for years to come. "I think eating a variety of foods all year round is the key to sustainability – keep changing what you eat or serve and don't deplete anything."

Trying new species is essential. Rowe would encourage anyone to think beyond cod and haddock. "If you're thinking about experimenting with less familiar but sustainable fish species, just go for it. Pollack is really easy to cook, you can do the same things with it that you do with cod," he says. "They tend to be a bit smaller and you don't have the same big lushness – but pollack has a bit more flavour and a slightly finer texture. Gurnard and coley are good, too."

But Rowe's catch of the day is the sprat. "Sprat is very sexy. I'm all for the power of the sprat." It may not be trendy but this little fish is seriously tasty, and a great choice for eco-eaters looking for a fish fix.

Chef Rowe's recipes

Grilled Cuttlefish:

Cuttlefish is very much like squid to cook. It's slightly chunkier so it needs a little longer cooking time and it has a little more texture. Some people are scared it's going to be chewy, but it won't be as long as you cook it quickly on a high heat. Don't be scared of serving it medium rare.

Salted sprats:

Sprats
Coarse salt

Sprats are seasonal – it's a case of blink and you'll miss them – so we salt our own. We don't fillet them, we just give them a wash and pack them in salt. It's easy to do it at home, you just have to get a clean jar or box, put in a layer of salt, then sprats, then a layer of salt, packed down. You have to make sure that none of the sprats are touching each other. Seal the box or jar and leave it in a cool dark place – the fridge is fine – until you need them. There's probably a strict way of doing it, but we've just learnt as we go along.

Lovage and lemon dressing:

Large slug of rapeseed oil – we don't use any olive oil
Handful of chopped lovage
3 strips of lemon zest, chopped
1 clove of crushed garlic
Pinch of salt

Put this dressing on your cuttlefish. It works really well, although lovage can be a scary herb to approach because it has an intense flavour. But if you put it with two other strong flavours – the lemon and the garlic – and serve it with cuttlefish, which has quite a soft flavour, it doesn't notice as much. It works really well.

Sprat marinade for pork:

You can use sprats in a similar way to using anchovies. You can make sauces with them, and we make a Danish pork marinade using them.

1 salted sprat, chopped or crumbled
2 tablespoons brown sugar
1 teaspoon yellow prepared mustard
2 tablespoons prepared horseradish

Mix together the marinade then use it to cover the pork. Leave it for a few hours then cook as usual.

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