Steelhead and Rainbow Trout Two in the Same

November 1, 2021

In 1855 scientists first described rainbow trout (Oncorhynchus mykiss) in North America by observing fish from Redwood Creek, now part of Reinhardt Redwood Regional Park! These fish may have been steelhead trout (Oncorhynchus mykiss irideus), which, confusingly, is the same species. Rainbow trout spend their whole lives in freshwater streams, rivers, and lakes. Steelhead trout, like salmon, are anadromous: they hatch in streams and rivers, spend 1-2 years living and growing in freshwater, and then swim out to the ocean to become adults. When 3-5 years old they will try to swim back up the same stream to spawn (lay and fertilize eggs). Unlike salmon who die after spawning, steelhead can spawn 3-4 different years.

What an adaptable species, transitioning between fresh and salt water, or just living in fresh water! The two varieties can also mate, with their babies becoming either variety. Fish of the original genetic strain (from 1855) can still occasionally be found swimming up Redwood Creek in winter during high water events. But how is this possible, since Redwood Creek flows into San Leandro Reservoir, and below that is Lake Chabot with no access to the ocean? After becoming landlocked, these adaptable fish now use the reservoir as their “ocean.” They are rainbow trout because they live only in fresh water, even though some of them spawn up streams.

The Central California Coastal steelhead, with much more of a steel color than rainbow trout, is a federally threatened species (even though the same species, the steelhead variety is considered threatened).

At one time they spawned in great numbers in dozens of streams between Sonoma and Santa Cruz Counties. Now many of those streams cannot support trout for a variety of human-caused reasons, and only a few streams support healthy steelhead populations. Trout require clean, oxygen-rich, and cool water shaded by trees.

The drought is having a significant impact on both steelhead and rainbow trout populations throughout California. One local example is Wildcat Creek in Tilden and Wildcat Canyon Regional Parks, which is at risk of losing its trout population. When stream levels become this low, steelhead cannot swim past human-made barriers into upper spawning grounds. Such risks make the efforts of landowners to restore streams that much timelier.

The East Bay Regional Park District is one of many agencies collaborating on a long-term effort to restore Alameda Creek, the largest watershed in the East Bay. Hopefully, someday soon, steelhead trout, and possibly even salmon, will swim up fish ladders and around other urban challenges into more protected lands to spawn. To learn more about this collaborative effort, go to: alamedacreek.org.

Hopefully, this winter will bring lots of rain and a chance for a glimpse of these incredible fish! Please be mindful of sensitive streams in parks like Reinhardt Redwood and Wildcat Canyon Regional Parks. Bring binoculars to observe the species from afar and keep children and pets out of these important fish habitats.