5.5. Zones in the Estuary An estuary has several zones with differing proportions of fresh water mixed with salt. At the head or upper part of the estuary, where rivers flow into the sea, freshwater is abundant and saltwater is scarce. At the mouth, where the estuary joins the sea, tides are strong and there is more salt water than fresh. Different fish and animals inhabit these various parts of the estuary as illustrated in the next section. Many fish and animals have found ways to make the best use of the different zones of an estuary. - Herring ride the flow of bottom water until they reach the end (head) of the estuary where they surface to feed on the rich plankton as they drift out again. - Flounder don't seem to mind low levels of salt and are often abundant in estuaries. Flounders bury themselves in the bottom as the tide goes out and then use the incoming tides to ride up into the small creeks in salt marshes where food is abundant. - Shrimp are especially adapted to estuaries. Young shrimp travel up the estuary in the bottom water and then surface to feed. Later they settle to the bottom to be carried back up the estuary. - Young cod, haddock, herring and other fish come into estuaries to feed. - Estuaries are often ideal growing areas for oysters and mussels because the food is abundant. They can close up when the salt content of the water is not to their liking. - Atlantic salmon, sea trout, alewife, stripped bass, and shad use the estuary to adjust to changes in water while on their way upstream to spawn, or when they're young and going to sea for the first time. - Sea trout feed on the abundant food sources in the estuaries and costal areas.