A new book from the chef Hugh Fearnley-Whittingstall called Edible Seashore – River Cottage Handbook 5 talks about foraging for shellfish, seaweed and other splendid foodstuffs on British seashores. His article in the Guardian introduces the book, and includes some decent recipes.
He is talking about a rare activity in the UK. In France, on the other hand, seeking out food on the low tide beaches is pretty much a standard practice. Here in Normandy, la Peche a Pied is hugely popular on the major low tides of the year. In the Bay of Mont St Michel, where the beaches slope very gently, the four lowest tides, at the equinoxes and solstices, reveal huge expanses of sand and rocks normally covered. People descend by the thousand, carrying buckets, rakes, spades, diggers, nets and very often uniquely personal implements, and spend as many hours as the tide allows digging, sieving, poking around, netting and generally foraging and fossicking. Whole families, from toddlers to great grandparents work together to gather enough for a good meal.
The beach at St Martin de Brehal, for example, is more crowded at the December solstice low tide than it is in high summer.
All the major beaches where it can be worth foraging have regular updates on water quality, wildlife
health and populations, so that everyone can know what is safe to eat, and what to leave alone to protect the species. This is if course available at the mairies, but also on signs at the beaches, usually on the life guard station. There are many guides, such as this local official site, and many books and other sources of information.
Generally, most areas have limits on the number of each species any one person can collect, and for many there are minimum sizes to ensure that the young have time to grow and reproduce. The main species are much as in the UK: crabs, shrimps, cockles, mussels, whelks and so on. Many beaches are sites of shellfish farming, and the relevant species should not be collected from there. Most common are oyster farms (ostreiculture) and mussels (conchyliculture).