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A Confusing, Compelling Wild Fish: Steelhead in the Delta

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A steelhead at the Oregon Zoo. | Photo: cacophony/Wikimedia Commons/Creative Commons License
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An explanatory series focusing on one of the most complex issues facing California: water sharing. And at its core is the Sacramento-San Joaquin Bay Delta. Stay with kcet.org/baydelta for all the project's stories.

Two decades ago I stood on the banks of a forested stream in Northern California, watching salmon spawn. They were coho, sleek and strong. Three males surrounded a big female as she laid eggs in a gravel nest in just a few inches of water. The largest of the males would periodically swim over the nest, fertilizing the eggs.

A silver, 14-inch fish lurked off to the side of the pool. If it ventured out toward the female, the smaller male coho would try to chase it away. But every now and then the gray fish would get to the nest, gobble up a mouthful of salmon eggs in a hurry and dart away.

It was a steelhead, the coho's smaller cousin. I watched it feed on salmon eggs for a few hours, an honest to god wildlife passion play taking place before my eyes. The steelhead has been one of my favorite Californian wild animals since.

Steelhead are anadromous, like the coho salmon -- and like the chinook salmon with which it shares the streams that flow into the Bay Delta. They swim upstream into the cold headwaters of the Bay Delta's tributary streams, lay eggs, and fertilize them. Those eggs hatch, or some of them do, and the young steelhead run the gauntlet of the rivers until they reach the Delta, and then the Bay, and then the Pacific Ocean.

Steelhead resemble Pacific salmon in a few other ways. They can get big: adult steelhead can reach 45 inches in length and weigh in at 55 pounds. (They usually stay a lot smaller than that, truth be told.) They prefer cold water, and shift from a diet of plankton in youth to larger animals, including crustaceans and mollusks and eventually fish.

Unlike Pacific salmon, though, steelhead don't die after spawning. At least not always. About ten percent of spawning steelhead survive the ordeal to spawn again. Though males reach reproductive maturity at age two with females ready to spawn a year later, lifespans of 11 years in the wild aren't unheard of. A steelhead that old might have enjoyed a good half a dozen spawning runs, entering the Delta during winter or spring to head upstream as far as it could, mating, then presumably turning around, promising to call, and heading back to the ocean.

Steelhead, quite frankly, are confusing.

For one thing, they're rainbow trout. That's not a metaphor. Steelhead and rainbow trout are the same species: Oncorhynchus mykiss. In California waters, they're the same subpecies as the coastal rainbow trout, Oncorhynchus mykiss irideus. For some time it was assumed that close study would eventually reveal a significant genetic difference between rainbow trout and steelhead, because there's definitely a distinct behavioral difference between the two: steelhead spend much of their adult lives in the ocean, while rainbow trout stay in fresh water their whole lives.

But about 15 years ago, a relatively close genetic study couldn't find enough genetic difference between steelhead and coastal rainbow trout to justify splitting the two groups into separate species, or even to explain the difference in behavior. Steelhead and rainbow trout did show differences in their mitochondrial DNA, but whether those differences were a cause of the behavioral split between steelhead and rainbow trout is hard to say.

Steelheads' spawning behavior is complicated as well. There are "winter" and "summer" steelhead. Winter steelhead arrive from the ocean ready to spawn. Summer steelhead arrive at about the same time as winter steelhead, but must spend a few months in freshwater waiting for their gonads to mature before they can head upstream. Both lifestyles are represented in the Delta and Central Valley, though the winter-maturing type predominates, possibly due to the Central Valley's warmer waters in summer. It just gets too warm in some streams in summer for steelhead to be comfortable.

Steelheads' status as a legally protected species in the Delta and Central Valley isn't so confusing. Steelhead that spawn in any rivers and streams that eventually flow through the Delta are considered to belong to the Central Valley steelhead Distinct Population Segment (DPS), which is legally equivalent to a species as far as the U.S. Endangered Species Act is concerned.

That DPS, which includes all steelhead spawning in the Central Valley between the Shasta Dam and the Merced River, was listed as Threatened under the Endangered Species Act in 2006. A 2001 review of the steelhead's status by the National Marine Fisheries Service (NMFS) recommended that the Central Valley steelhead be assessed for downgrading to Endangered status, citing significant losses in population since 2006.

What's the problem with steelhead in the Central Valley and the Delta? The same thing that's wrong with their Chinook salmon cousins: their ecosystem has been massively altered. About 95 percent of the Central Valley steelhead's historic spawning habitat was lost in a century of dam building. Steelhead juveniles are being sucked into the pumps of the state's giant water projects, invasive clams and other factors are depleting the Delta's former fish nurseries of the plankton that forms the base of the Delta food chain, and invasive fish gobble up all the small fish they can, including juvenile steelhead.

At this point, the majority of the steelhead in the Delta and its tributaries are produced by hatcheries. And again, the hatcheries' role in keeping steelhead numbers up is complicated. Two hatcheries, the federal Coleman hatchery and the state's Feather River hatchery, raise and release steelhead that are genetically similar to wild fish in the Delta watershed. Two others, the state's Nimbus and Mokelumne hatcheries, raise fish whose distant ancestors were originally from rivers along the Northern California coast.

The first two hatcheries may well be preserving the Central Valley steelhead lineage despite massive destruction of their habitat. The second two? Well, at best, they're not helping.

During the 1950s, well after the dams went in, an estimated 40,000 Central Valley steelhead still passed through the Delta each year on their way to spawn. In 2003, the National Marine Fisheries Service estimated about 3,000 remained.

No one knows how many Central Valley Steelhead there are now, though it's worth keeping in mind that NMFS thinks the population has declined since listing. And the ongoing drought isn't helping.

A few months after I watched my first steelhead stealing salmon eggs, I swam in a deep pool in a tributary of the Klamath River. My head a few feet below the incredibly cold surface, I watched as parr-marked steelhead fry darted about. One, about an inch long, came to rest just above my outstretched palm. It regarded me with its left eye, then pivoted to watch me with its right.

Which could have been a commonplace moment in any of a hundred Central Valley tributaries a hundred years ago. Whether the Delta and the Valley that feeds it will still have room for steelhead a century hence is anyone's guess.

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