Consumers are being encouraged to help conserve fish stocks by being aware of species at risk when shopping at their local fishmonger or supermarket.

The Good Fish Guide from the Marine Conservation Society lists 36 fish to avoid and 37 that it believes can be safely eaten.

The society wants people to think before they eat but stresses there is a way to enjoy fish with a clear conscience. A spokeswoman explained: "There's always an alternative. We are not saying to people, 'Give up fish'."

Some of the fish to avoid may surprise people; they are common species - or were common species and, although their numbers have been drastically reduced in the wild, their capture and sale continues.

So, the guide says one should refrain from eating hake, haddock and Atlantic cod. Particularly vulnerable to over-fishing are the bottom-dwelling, long-lived fish which are slow to reproduce. The MSC says skate, rays, plaice and turbot should be avoided, along with monkfish. Tuna should be off the menu, unless it is "dolphin friendly" pole -and-line or troll-caught yellowfin and skipjack.

On the Marine Conservation Society's "fish to eat" list are clams, cockles, mussels, trout, coley, herring and whiting. Pacific cod and Pacific halibut are acceptable. Farmed organic salmon gets the thumbs-up. Other lesser-known species for the dinner table are red gurnard and flounder.

In supermarkets the decision confronting the shopper has been eased. A pack carrying the small blue logo of the Marine Stewardship Council (MSC) means the fishery has been subjected to a demanding certification process. Examples of MSC fisheries are New Zealand hoki and Alaskan pollock, South African hake and Thames herring. Many more are seeking certification.