A mathematical analysis of the energetics of migration by means of tidal stream transport is presented. Tidal stream transport, in which fish are in midwater only when the tide is in their direction of motion and move to the bottom otherwise, is shown to be less costly than constant swimming. Savings in energy cost-per-unit distance of over 90% can be realised for juvenile fish, while for adult plaice or sole up to 40% less energy is required to cross a given distance. These results are in accordance with observations of the species mentioned, as well as of other migratory fish.