Fish is good for us. So good that we should eat a lot more of it. According to the independent food safety watchdog, the Food Standards Agency, we should eat at least two portions of fish (fresh, frozen or canned) a week, at least one of which should be an oily fish. 'Oily' fish include mackerel, salmon, pilchards, herring, trout, sardines and fresh (but not tinned) tuna.

All fish are excellent sources of protein, vitamins and minerals, but oily fish is particularly nutritious because it contains high levels of the fatty acid, omega-3. Various studies have shown that omega-3 can help prevent heart disease, improve IQ, improve immune function, may help improve symptoms of arthritis, and can help with certain skin problems.

But oily fish tend to contain higher levels of contaminants such as mercury than white fish so The FSA has advised a limit to the quantity of oily fish that we should eat. It recommends that girls and women who might have a baby one day, and women who are pregnant or breastfeeding, can eat up to two portions of oily fish a week. Other women, men and boys, can eat up to four portions of oily fish a week. In addition, marlin, shark and swordfish are not recommended for consumption by boys or girls under 16 or by pregnant women or women who may become pregnant in future.

High street fishmongers aren't as common as they once were, so, if you're lucky enough to have a good one near you, make the effort to support them. Farmers' markets and reputable street markets are also good places to buy fresh fish. It's worth going to the trouble of finding a good fishmonger in your area, even if it's a bit of a trek, as there's no substitute for really fresh fish.

Most of us are limited to the often paltry supply of fresh fish found at all but the biggest supermarkets where fish counters can be hit-and-miss affairs. Some supermarkets, however, have made the effort to employ knowledgeable fishmongers who'll be happy to tell you about the fish you're buying, where it comes from and how to cook it.

Good fishmongers will be happy to fillet a fish for you; remember to ask for the bones, head and skin as they make great stock.

Frozen fish can be just as good, if not better, than all but the very freshest fish. Because it's often frozen quickly, it has less time to deteriorate than 'fresh' fish, which may be several days old by the time you buy it.

When buying fresh fish there are a few pointers to look for. With whole fish the eyes should be bright and clear, not sunken or dry. Inside the gills should be vivid pink or red, not a dull brown. The skin should be vibrant and shiny and the flesh firm. Fillets or steaks of fish should be firm and translucent, without discolouration. Smell is also an excellent indicator of the freshness of a fish - it should smell of nothing but the sea. A fishy smell is a clear sign that the fish is past its best.

Mitch Tonks explains more about what to look for when buying fish in his video buyers guide.

Sustainability' and 'overfished' are important buzz words for the consumer with a conscience. The world's oceans are being seriously overfished, leaving some species in danger of extinction. The plight of cod and haddock, in particular, has been well documented in the UK press, leaving many consumers wary of eating these species, lest they contribute to the fish's demise.

The good news is that consumers can make a difference - and that doesn't mean giving up fish. Buy your fish from reputable fishmongers and keep asking the question: "Is this fish responsibly fished and from a sustainable source?". If you're buying fish from a supermarket, look out for the blue Marine Conservation Society (MSC) logo that guarantees that the fish comes from a sustainable source.

If you want to know which fish are 'safe' to eat and which to avoid, the MSC website is an excellent, up-to-date resource.

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Cod's still a popular fish despite being overfished. You can get it all year round, but the fresher the fish, the better the flavour. Frozen cod's good but is often not as flavourful as fresh. When choosing cod, be careful to avoid fish with blotches of pink or yellow. A very versatile fish, its flaky flesh works well poached, baked, fried, or grilled. Cod is always good in fish pie and, because it has a strong flavour, it works well with rich sauces.

Haddock is a member of the cod family and very similar in flavour, it tends to be a little more expensive than cod. It has firm flaky flesh and is best bought in winter or early spring. You can, however, buy haddock throughout the year, usually as fillets. Like cod, this is a versatile fish and can be cooked in similar ways. Smoked haddock is tasty, too.

The various fish that come under the banner 'hake' are deep-sea members of the cod family. Hake's quite a mild fish, having a more subtle flavour than cod. Hake's easy to prepare, as it has few bones. You'll usually see it for sale whole, or as fillets and steaks. It's a reliable fish fried, poached or used in fish soup.

Whiting's a small member of the cod family, and should be bought very fresh because it loses its flavour over time. Like cod, this fish is best in winter, but is on sale all year. While it's not as exciting as some of the other white fish, it's useful for turning into fishcakes or fish mousse.

Bass includes silver bass, sea bass and striped bass and all have delicious flavours and are great barbecued. Buy as steaks or fillets at any time of the year. This fish is wonderful when grilled over charcoal, and can be steamed, poached or baked. Bass goes well with strong-flavoured ingredients such as garlic, herbs, and rich tomato sauce.

There are a number of fish that come under the banner of 'bream'. These include black bream, red bream from Europe, porgies and scups from America, and the Mediterranean gilt-head which is perhaps the tastiest of the lot. Rather confusingly, the Norway haddock, or redfish, is sometimes known as 'bream', but its flavour and texture isn't as good as sea bream. All have a coarse, succulent and choice flesh. Buy whole or as fillets, and ask the fishmonger to take the scales off. Grill or bake marinated sea bream in foil. American porgies are delicious when rolled in corn meal, fried in bacon fat and served with parsley and lemon.

Other white fish include coley, pollack, torsk (also known as cusk) and pouting. These fish are less flavoursome than their more famous cousins, but make good, cheap additions to soup or pies. Grouper is a notable white fish with good texture and flavour that features as a delicacy in many Mediterranean dishes. Available whole, as steaks or as fillets, you can cook these as you would sea bass.

Dover sole has a delicate flavour and its flesh is lightly textured but firm. Available all year in Europe, this great fish can be bought whole or in fillets. If you ask for it filleted, keep the bones and head, to make fish stock. It's delicious grilled, fried or served a la meuniere (a sauce of browned butter, parsley and lemon juice).

Lemon sole is a misnomer since it's neither a real sole, nor does it taste of lemon! It's in fact a variety of plaice. It's available all year as fillets or whole fish. Cook as you would a Dover sole.

Plaice has soft flesh and a delicate flavour and is recognisable by the orange spots on its upper side. It's available to buy whole or in fillets all year round. Plaice cooks very quickly - it should only need a total cooking time of 4-5 minutes, whether grilled, fried or poached.

Flounders include the European flounder or fluke, which doesn't have a good reputation for flavour. In contrast, summer and winter flounder are better regarded. Buy whole or in fillets. Cook slimmer flounders as you would a Dover sole. Thicker flounder can be cooked along the same lines as a turbot.

Turbot's considered to be the greatest fish by many chefs. It's expensive, but the flesh is flavourful and firm. Avoid turbot if the flesh has a hint of blue, which means it's stale. Available all year as a whole fish or in fillets or steaks it's good poached or grilled with a simple parsley or hollandaise sauce.

Halibut's similar-tasting to turbot. They can grow to a massive two metres (six feet) long and are available all year. Halibut can be bought as steaks, fillets or cutlets. It tends to lose texture if frozen.

Other notable flatfish include the brill, the dab, the megrim and the curiously named witch. With the exception of brill, these aren't the greatest tasting fish, but you may find they're cheaper. Cook brill as you would a halibut, sole or turbot. Dab is good grilled, or crumbed and fried. Megrim can be made into fish soup, or eaten as fried breadcrumbed fillets. Witch should be cooked similarly to sole but with more seasoning.

Sardines - baby pilchards - are fantastic when served very fresh. These long, thin, silver fish are best when cooked near where they're caught - they don't travel well. They're available throughout winter and are best in spring, and are usually sold as whole fish. They're also available frozen, and the best ones are Portuguese. Remove the scales by brushing tail to head with finger and thumb under running water. Grill the sardines or barbecue them after coating with olive oil and salt. Eat hot or cold, or shallow coating thinly with flour. Squeeze lemon over cooked fish and serve with crusty bread and tomato salad.

Pilchards - the adult sardine - are blue-green fish about the size of a herring. Best eaten from the summer catch, they're delicious fresh, grilled like sardines. However, most pilchards are processed and canned. Pilchards are a speciality in Cornwall where they swim to their northern limit.

Herrings have suffered from the effects of overfishing and are less commonly found than they once were. They're available all year but are best from spring to autumn. They're usually sold whole and the bones are soft and easily removed. Coat fillets in oatmeal and shallow-fry in butter or cook in white wine or light vinegar - the acid flavours suit the rich flesh. They're also good scored, brushed with butter and grilled. The traditional accompaniment is mustard, though horseradish is good too. Herring can be pickled, soused, marinated, salted and smoked. Herring roe is also full of flavour and worth keeping.

Mackerel's pink, firm flesh is tasty, nutritious and inexpensive. It has distinctive silver-blue skin with dark bands and a pale belly. Available all year but best in late spring and early summer. It's good grilled, dry-fried, barbecued or poached in white wine. Stuffed mackerel is also a popular option. Strong flavours like mustard, horseradish and curry are good with mackerel and it also suits being pickled, soused and marinated, salted and home-smoked.

Whitebait are young herrings or sprats and are delicious. They're quite small so serve whole - dip in milk, shake in a bag with flour and deep fry until golden brown. Serve with lemon and brown bread and butter.

Smelt - from the salmon family - are good, especially when very fresh, but do go through good and bad years. They are best in winter and spring. Bluefish is available along the East Coast of the United States and in the Mediterranean. A larger fish than other oil-rich fish, its oily, soft flesh is best with piquant flavours, and is good grilled or poached in white wine.

Fresh anchovies are best prepared and eaten similarly to sardines - they don't travel well so are best in the Mediterranean. Otherwise buy them cured or preserved for a different flavour

Poaching is a great method for cooking fish - from simple fillets to whole fish such as sea bass. By adding herbs, seasoning and cream or other thickening agent to the poaching liquid, you'll have a sauce to serve with your fish.

Steaming is a simple and healthy way to cook steaks, fillets and whole fish. Place the fish in a steamer over boiling water and cover with a tight-fitting lid until cooked.

Pan frying, shallow-frying and deep-frying all work well for fillets.

Oven-baking or roasting is good for whole fish and fillets. Cook in an ovenproof dish with herbs, a drizzle of oil or a knob of butter, seasoning and a little white wine. Cooking fish 'en papillote' is similar to roasting; the fish is cooked in the oven in sealed paper or foil, so it cooks in its own steam. This produces moist tender flesh. Adding fresh herbs, white wine, lemon or lime juice to the parcel before cooking gives a wonderful flavour to the fish.

To braise a fish, simply lay it on top of sauteed vegetables in an ovenproof dish (with a lid), add white wine and stock to the pan, pop on the lid and place in the oven for 20 minutes or so.

Stuffing a fish before baking adds flavour and texture. Round fish are most suitable for stuffing as there's more space in the body cavity. Stuffings usually contain breadcrumbs, which swell on cooking, so don't overstuff your fish. Walnut and parsley pesto makes an unusual but interesting stuffing for roast sea bass. Fish fillets can also be sandwiched or rolled with stuffing and oven baked.

Grilling or barbecuing is particularly good for cooking fillets and small whole fish. For whole fish, slash the skin on each side two or three times and brush with olive oil or melted butter. Turn the fish halfway through cooking and baste with oil so it doesn't dry out. You can barbecue the fish by wrapping it in foil with herbs, olive oil and lemon juice and placing it on top of the grill. For a real smoky flavour, use the special fish-shaped racks that hold fish tight, stopping it from breaking up when exposed to the heat.

Ceviche is a Latin American dish using lime juice flavoured with herbs and chilli to soak pieces of white fish such as snapper. This "cooks" the fish through the action of acid in the citrus juice. You can use this method with scallops and flatfish such as sole or flounder.

Try Rick Stein's recipe for Ceviche of brill

Drying, salting and smoking were used to preserve fish long before freezing came along. Drying takes place in large airy sheds. Species commonly dried include: bummaloe (for Bombay duck), cuttlefish, grey mullet, jellyfish, oyster, scallop, shark's fin and shrimp.

There are two methods of salting fish. Dry salting entails packing the fish in layers of salt and leaving it until much of the liquid has drained away; cod is the most available fish treated in this way, called salt cod or bacalao. Remember to soak it in cold water for 24 hours, changing the water several times, before use to rehydrate it and to dilute the saltiness.

The second method is brine salting (used mostly for oily fish such as herring), where the whole or filleted fish is left in a mixture of water and salt (brine).

Fish is smoked in two ways: hot smoking and cold smoking. Most fish that are cold smoked (kippers, for example) should be cooked before eating; smoked salmon is an exception. Hot smoked fish (such as trout) is sold cold and ready to eat. Types of smoked fish include: kippers, smoked salmon, smoked trout, smoked mackerel, smoked eel, smoked haddock, and Arbroath smokies (also made from haddock).

Another method of preserving fish is canning, which suits oily fish well. It's used for anchovies, which are filleted and salted before being packed in oil; sardines, tuna and pink and red salmon are also available canned, usually stored in brine, spring water, oil or a sauce such as tomato.