As salmon begin to mature, they adapt for life in salt water in an intermediary stage known as smolts. This process marks the beginning of their first migration from their home stream to the ocean. Fish like salmon, that move from fresh to salt water and back again over the course of their lives, must be able to change their physiology, the way their bodies work. Most salmon species spend some time in the estuary of a river, where the fresh water mixes with the salt water. Here, they gradually get used to life in salty water in preparation for the time they will spend at sea. Very few fish have the ability to adapt from living in fresh water to salt water, and then return back to fresh water. In a process called smoltification, salmon adapt to the changes salt water causes to their bodies. In fresh water, the salmon’s body is saltier than the water in which it swims. To work properly, the body needs salt so it tries to keep the salt in. Some escapes, but the salmon gets enough from the food it eats to make up for the loss. In the ocean, the water is saltier than the salmon’s body needs to be, so it must try to keep the salt out and the water in. When salmon swim in the ocean, the salt water draws water out of the fish’s cells. Salmon adapt by drinking sea water to replace the water their cells lose. They excrete the excess salt through their gills and urine. Freshwater fish would die in salt water because they cannot replace the water in their cells. As the smolts prepare for ocean life, their appearance also changes, from the dark colours of the fry to the silvery colour of adult salmon. This helps them hide in the light conditions of the surface waters of the open ocean, where there is no dark shade from overhanging trees. In estuaries, the mineral and organic elements of a river mix with ocean nutrients brought in by tides, creating a nutrient-rich environment that supports diverse plant and animal growth. Estuaries provide salmon with a good supply of insects and crustaceans, such as tiny shrimps for food. While in the estuary, smolts can grow from 4 or 5 cm in length to as much as 9 cm. However, estuaries are home to many fish predators, including larger fish, birds, snakes, seals and even orcas. People build cities and industries on estuaries, as well as dyking and dredging them, or extending landfills into them for development projects. The loss of estuary habitat means that there is less room for salmon and other estuary animals to mature, feed and adapt. If smolts cannot live in an estuary, it is a sign that other plants and animals are at risk, also. Different species of salmon spend different amounts of time in estuaries. Some leave almost immediately, while others spend several months there. While approximately 30 fry from a redd of 2000 to 2500 eggs grow into smolts, less than four survive to become adults.