Chris Clarke writes about steelhead. It’s a subject I’m on close terms with, too; my father was an obsessive steelhead fisherman, and I grew up spending many memorable weekend mornings chilled and damp on the banks of the Green River, eyes fixed on the tip of a fiberglas rod. I also felt the year-by-year decline, as actually catching a magnificent fish became a rarer event over time. I was interested in the biology of these fish, though, and these are also things my father explained to me:
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.
It was terrible to be 12 years old and to be already confronted with the difficulties in the species concept, even before I’d learned anything about species concepts. They’re both delicious. They look the same, although the lateral stripe is less prominent in large steelhead…but it’s also less prominent in large rainbows, and the difference seems to be simply that steelhead grow larger during their time at sea. They’re the same animal, genetically and morphologically — their only significant differences are their choices of environments.
Actually, maybe it was a good thing to study biology while already primed to be suspicious of rigid classification schemes.