Secret Lives of Steelhead and Rainbow Trout

Steelhead trout swimming. Photo by Oregons State University

Steelhead trout. Photo by Oregon State University

When I was about 7 years old I had the thrill of catching my first fish. It wasn’t in a cold wild stream, nor a calm mountain lake.  Rather, it was after putting a rented fishing line into a stocked trout pond filled with rainbow trout. The fish probably weighed about 5 lbs. and was pretty tame by any standards.  Nonetheless, it was still thrilling to me to see the beautiful bright animal on the end of my line.  I’ll never forget it.

Pacific Northwest rainbow trout are even more special.  Like their larger cousins–the wild salmon–some of these beauties slip downstream and enter the Pacific Ocean, returning as massive, 30 lb. steelhead trout. And for many years, it was assumed that these ocean-running fish were their own species, separate from their more diminutive freshwater bound cousins.  But now we know better.

These cousins are actually a lot closer than we thought! Researchers at OSU have concluded that around 40% of a steelhead trout’s genes come from wild rainbow trout. This means that both populations are interbreeding. It also means that the populations are genetically fluid–or mixing at different points in their evolution. So a steelhead enters a stream, breeds with a local rainbow trout. Some of those offspring are returning to the sea as steelhead, while others may stay local. And the rainbow trout then act as a “fail-safe” for steelhead when the ocean conditions are poor (and hence the returning adults are fewer). This is not the same case with salmon, who must have success in fresh and saltwater environments throughout their life-cycles.

Furthermore, hatchery raised trout are only contributing a very small amount of genes to steelhead–its mostly a wild fish thing.

So consider that lowly rainbow trout as a contributor to its larger, perhaps more amazing  ocean-going cousins.  Evolution is in action in these fish.  Check out more on this topic at ScienceBlog.

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