A United Kingdom charity campaigning for healthy seas and sustainable fishing, abbreviated MCS. It works to protect marine wildlife, reduce pollution including plastics and sewage, and promote responsible seafood consumption through its Good Fish Guide, which rates fish species by sustainability. The society organises beach cleans, campaigns against overfishing, and advocates for well-managed marine protected areas. Its seafood ratings influence retailers, restaurants, and consumers choosing fish. By combining science, campaigning, and public engagement, the organisation has become a leading voice for ocean conservation in Britain, translating marine research into practical guidance for shoppers and policymakers.