In an era long before refrigeration, wealthy Romans built holding tanks beside their coastal villas so they could eat fresh fish whenever they wished. Sluice gates positioned with their tops eight inches above the high-water mark let seawater in and out, flushing the tanks with the natural ebb and flow of the tides. To work, they had to be built at a precise level relative to high tide."Livewell" is a modern term for the tank on fishing boats designed to keep fish alive until the crew returns to shore. Very clever, those Romans.