Fish and shellfish, simply explained.

Fish and shellfish types

How to tell freshness

Smell is the most reliable test; looks help, but colour alone does not.

• Fresh fish smells mild and of the sea — not fishy, sour or of ammonia.
• Eyes are clear and bulge slightly; gills are red; flesh is firm and springs back when pressed.
• Fillets should look moist and bright, with no browning or drying at the edges.
• Buy only fish kept refrigerated or on a thick bed of fresh ice.
• Previously frozen fish can lack bright eyes or red gills but should still smell fresh and mild.
• Frozen packs: avoid ones with frost or ice crystals (stored long or thawed and refrozen) or flesh that is not hard.
• Live crabs and lobsters should move their legs; live mussels and clams should close when tapped — discard ones with cracked shells.
• If cooked seafood smells of ammonia, don't eat it.

Cleaning a whole fish

Many fishmongers will do this for you — ask. At home it takes a few minutes.

1. 1

   Scale: hold the tail and scrape from tail to head with the back of a knife or a scaler, under running water or inside a large bag to catch scales.

2. 2

   Trim the fins with kitchen scissors.

3. 3

   Gut: insert the knife tip at the vent and cut along the belly to just below the head. Pull out the insides.

4. 4

   Remove the gills (they turn bitter) by cutting them free at both ends.

5. 5

   Rinse the cavity in cold water, scraping out any dark bloodline along the spine, and pat dry.

• Work on a board set aside for raw fish, or wash well afterwards.
• Keep fish cold until you start; work quickly.

Filleting basics (round fish)

A flexible, sharp filleting or chef's knife and long, smooth strokes along the bones.

1. 1

   Lay the cleaned fish on its side. Cut behind the head and gill flap down to the backbone.

2. 2

   Turn the knife to point along the fish and cut along the top of the backbone from head to tail, keeping the blade flat against the bones.

3. 3

   Lift the fillet as you go and cut over the rib cage to free it.

4. 4

   Turn the fish and repeat on the other side.

5. 5

   Trim the belly edge; feel for pin bones and pull them out with tweezers in the direction they point.

6. 6

   To skin: lay skin-side down, hold the tail end, angle the knife slightly down and saw between flesh and skin.

• Flat fish give four fillets; round fish two.
• Use the frame (bones and head, gills removed) for fish stock.

Preparing shellfish

Live bivalves must be alive when you cook them.

1. 1

   Mussels: scrub, pull off the beard (the fibrous tuft) towards the hinge, and discard any with broken shells or that stay open when tapped.

2. 2

   Clams: soak in cold salted water for 20–30 minutes to purge sand, then lift out (leaving grit behind).

3. 3

   Prawns: twist off heads (keep for stock), peel shells, then slit along the back and lift out the dark vein.

4. 4

   Squid: pull the head from the body, cut the tentacles off just below the eyes, remove the beak and the clear quill, peel off the skin.

5. 5

   After cooking, throw away any mussels, clams or oysters that have not opened.

Sustainability pointers

Whether a fish is a good choice depends on the stock, where it was caught and how — so check a current rating rather than a species list.

• Look for the blue MSC label on wild seafood: it means the fishery was independently assessed for healthy fish stocks, its impact on other ocean life and habitats, and effective management. Certified fisheries are audited yearly and reassessed every five years.
• The MSC label covers environmental impact; it does not cover social conditions, packaging or the product's carbon footprint.
• In the UK, the Marine Conservation Society's Good Fish Guide rates seafood by species, area and catch method on a traffic-light scale — green 'Best Choice', amber, and red 'Fish to Avoid'. Ratings are updated regularly, so check the current guide.
• Ask your fishmonger where and how the fish was caught or farmed; vary the species you buy.
• Smaller, faster-growing species and well-managed farmed shellfish such as mussels are often recommended — but confirm with a current rating for your region.