Rainbow Trout, Man’s (Aquatic) Best Friend: Part 1 of 2

Rainbow trout have been all around the world, making them Man's (Aquatic) Best Friend. There are as many hatchery strains of rainbows as there are dog breeds.

Colorful Rainbow Trout from the Pueblo Tailwater, Colorado
Rainbow Trout from the Pueblo Tailwater, Colorado

June 2019

It’s well known that states raise and stock rainbow trout for angling opportunities, but did you know that rainbow trout raised in the Montana hatchery system are different than those raised in, say, the Colorado hatchery system? I was only vaguely familiar with some of the various rainbow trout strains that swim throughout the western United States, but I wanted to know if I could really tell the difference between each strain.

After asking the state fishery biologist here in Colorado, the short answer is no. He told me that it’s actually difficult to identify strains based on colorations or physical characteristics. The differences between rainbow trout strains are genetic, and there are too many variables (like diet, think mysis shrimp) that can make different strains appear similar. Each strain has been bred in a specific hatchery to enhance desirable traits in a sportfish, such as the ability to grow quickly, live long, or mature early. And the list of rainbow trout strains is staggering. As I dug into trying to figure out all the different rainbow trout strains, I quickly realized that unique strains exist for many different hatcheries that raise rainbow trout.

leopard spotted rainbow trout from Fremont Canyon, Wyoming

Some common strains include Arlee, Bellaire, Colorado River, Eagle Lake, Erwin, Gunnison River, Hofer, West Virginia, Kamloops, Harrison Lake, Fish Lake, McConaughy, and the list goes on. Some notable strains include the Colorado River strain known as a highly successful wild trout strain. Though slow to grow, they live long and exhibit natural spawning behavior. Another wild strain comes from Eagle Lake in the high Sierras, also known to grow large. There are also highly domesticated strains, like the Hofer strain. Bred for hundreds of years in Bavaria as food, this strain grows fast, and matures quickly, but doesn’t behave like a wild fish. The variety of rainbow trout is reminiscent of the variety of different dog breeds, each developed to perform certain tasks.

But why so many rainbow strains? One answer might be that trout and salmon are so good at finding their way back to their natal streams that very few individuals end up in other spawning grounds. As a result, there’s not a lot of genetic mixing from different lineages. Hypothetically, wild fish that run out of a reservoir to spawn in multiple creeks might be different from each other despite spending much of their lives in the same reservoir. As a result, each watershed can evolve into unique strains, taking on their own characteristics. Many of these naturally unique types of rainbow trout developed into the modern types of stocked rainbows we have now. Some hatcheries still collect eggs from wild spawning rainbows, while other rainbows are raised completely within a hatchery. In both cases, trout are being bred and raised from limited genetic pools, creating unique variations within the rainbow trout species. They are essentially bred like dogs. Pick a desirable trait, and select the offspring which most exhibit that trait, generation after generation.

rainbow trout hooked on the Arkansas River, Pueblo, Colorado

When whirling disease rolled into Colorado in 1987, it quickly killed many rainbow trout statewide. Whirling disease, originally from Europe, had coevolved with european-derived brown trout that have built up a resistance to the disease, but the rainbow trout native west of the continental divide had no safeguards. Only 10% of the historical densities and biomass of rainbow trout survived. By 1997, 10 of the 14 state hatcheries were infected. But the hatcheries managed to engineer new rainbow trout by combining the natural resistance of the European Hofer strain with local wild strains. Now it is common to find “Hofer-infused” trout in streams, where fisheries managers hope that acclimated trout with some resistance to whirling disease will spawn with wild fish to integrate resistance into future generations. And so, new breeds of trout are being born.

The rebound of rainbow trout following the introduction of whirling disease has largely been a success, and additional resistant strains are constantly being optimized. Without the stocking of resistant trout, the rebound may have taken far longer, as survival rates of wild eggs are only 1/100, while hatchery eggs fare much better with 76/100 surviving. But still, some despise stocked rainbows, claiming that they dumb down the fish population. Freshly stocked rainbow struggle for at least a week to acclimate to wild habitat, leaving them naive to the particular and risk-averse standards of wild trout. But I’d remind you to not over-exaggerate the intelligence of trout. Recently interviewed by Tom Rosenbauer, Dr. Ross Carpenter, a rainbow trout neurobiologist, reminds us, “trout are kinda dumb”.

But that might be why we love rainbows. They are generally more active and willing to take flies than the elusive browns, and much more common than the native cutthroats. Finally, with the way humans have expanded our ability to manipulate species through breeding, rainbow trout and dogs aren’t really that different. Except when you need to tell rainbows apart, that’s more complicated.