O. mykiss is the scientific species name for the entire population of rainbow trout and Central Valley steelhead. While steelhead are the ocean going form of O. mykiss, rainbow trout are the resident form staying in the rivers and deltas. Steelhead are anadromous, meaning they primarily live in the ocean but breed in fresh water. rainbow trout may or may not become anadromous at some point in their life-cycle.
The Stanislaus River currently has more than 20,000 wild rainbow trout. While it is impossible to determine the percentage of these fish that will remain resident rainbow trout in the River, and will instead leave the Stanislaus River and eventually find their way to the ocean, the Districts have counted an average of 100 steelhead leaving each year from the Stanislaus River to the ocean. While existing river flows have been sufficient to support the thriving rainbow trout fishery, the BO is designed to increase flows to protect the steelhead (the anadromous form of O. mykiss). The Districts studied the effects of the BO and concluded it will actually reduce flows in more years and raise water temperature to lethal levels endangering the O. mykiss.