There are more than 20,000 living kinds of fish known to man, and new species are discovered every year. This is more than all other kinds of backboned animals combined. Fish live in most natural aquatic regions on Earth except very salty water like the Dead Sea and the Great Salt Lake of Utah. Pollution has been a problem for many species of fish. Waste products from industries, agriculture, and urban areas create poor oxygen conditions that add toxic chemicals to the water or alter pH conditions to be too acidic or basic.
Fish are found from the sunny surface of the ocean all the way down to the darkest depths where light never penetrates. These deep ocean living fish have special adaptations to the darkness and the enormous pressures of their environment. Most fish continue to grow as long as they live. In fact, many old fish become very large. The exceptions to this rule are fish such as salmon and carp. The salmon has a definite period of growth before spawning and after spawning they die. Carp are said to have a life span of 100 years. It is believed that many fish reach the age of 50 to 100 years old, but scientific documentation for fish reaching ages of more than 15 to 20 years in natural environments is rare.
