Fisheries and fish supply are undergoing a fundamental structural transition, as indicated by a ten country analysis. Aquaculture now provides around half the fish for direct human consumption and is set to grow further, but capture fisheries continue to make essential contributions to food and nutrition security throughout the global South. Capture fisheries provide diverse, nutritionally valuable fish and fish products which are often culturally preferred and easily accessed by the poor. Technological changes in aquaculture have dramatically increased fish supply, lowered relative fish prices, and reigned in price volatility. Policies that recognize and safeguard the diversity and complementarity of roles played by capture fisheries and aquaculture are needed to ensure that the transition in fisheries sustainably improves food and nutrition security in the global South. •Fish make an irreplaceable contribution to food and nutrition security.•Global capture fisheries output is static, but aquaculture is growing rapidly.•Capture fisheries remain the most important source of fish for many poor people.•The two sectors' contributions to food and nutrition security are highly complementary.