Although the health benefits of eating fish are clear, with over fishing, depleting stocks and environmentally degrading fishing methods, it is difficult to know what species to eat with a clear conscience.  The Marine Conservation Society have produced the Good Fish Guide to advise which species to eat, which to avoid, including the reasons not to eat them.

Some that are to be avoided are either from unsustainable stocks; others are long-lived, slow growing, late to mature and vulnerable to overfishing; some are caught by dredging the sea bed, damaging natural marine habitat and disturbing sealife. 

Those that are recommended are from sustainable stocks, certified organic sources, line caught or have healthy stock numbers.

The Marine Conservation Society have also produced guides on certified sustainable suppliers, cafes and restaurants and a league table on British supermarkets.  To find out more, go to:
http://www.fishonline.org/