The steelhead trout used to be a native species to the Arroyo Del Valle in Pleasanton, California. Some are anadromous fish (i.e. they spend most of their time in freshwater developing after they hatch and then spend part of their later life in the sea or ocean) but others will remain in a freshwater stream or creek their entire lives. Typically, anadromous populations swim up the creek in winter to lay their eggs and then swim back, leaving the eggs to develop on their own. In order to lay their eggs, female steelhead trout need a stream riffle with a gravel bottom. The eggs then hatch, and the newborn steelhead (alevin) soon develop into fry that consume zooplankton for subsistence. Once developed into smolts, they swim back the way the parents
