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Southern steelhead generally have longer, more streamlined bodies than their northern counterparts to facilitate passage through southern California’s characteristic low, flashy streams

In the 19th Century, the Santa Ynez probably supported the largest runs of Southern steelhead throughout their range, likely between 20,000 to 30,000 adults per year, while the other large watersheds likely supported runs of a few thousand adults per year. In the last five years, the number of adult anadromous steelhead has declined significantly, to the point that it is now rare to see them in the wild.

Southern steelhead tolerate warmer water temperatures than their northern counterparts, up to about 25˚C (77˚F), before they seek refuge in pools with cool seeps. They depend on winter rains to provide passage through seasonally opened estuaries and lagoons to reach upstream spawning tributaries. Juvenile steelhead may feed in fresh water for up to three years before migrating to the Pacific Ocean. While anadromous steelhead are only sporadically present in the current population, resident Rainbow trout occupy the upper watersheds of most rivers year-round and may be responsible for maintaining steelhead runs through their offspring when streamflow conditions are favorable. Anadromous offspring from resident parents are common enough to make protection of resident fish an integral component of Southern steelhead recovery.

The environmental and genetic factors that underlie their ocean migrations are under active investigation. Populations in the larger watersheds in the northern part of the DPS (Santa Clara, Ventura, Santa Ynez and Santa Maria rivers) have remained un-hybridized, despite decades of stocking hatchery-origin Rainbow trout. Some resident Rainbow trout populations in the southernmost portion of the DPS have hybridized with hatchery-origin fish above barriers such as dams and diversions. However, three populations of pure native Southern steelhead are present in the San Luis Rey, Santa Ana, and San Gabriel rivers today.