Why Biology Matters: Speedy Steelhead vs Lazy Coho Salmon

Article and photos by: John R. McMillan

John McMillan with a fine Pacific Northwest steelhead.

Do you ever wonder why fly anglers fish for steelhead in riffles? Or why we fish pools and other slow-water habitats for coho salmon?

Most anglers would just say, “because that’s where they are, duh.”

Not man fly fishers spend time rummaging through biology papers to better understand the fish. But perhaps we should: After all, becoming a better angler is as much about improving knowledge about the fish as it is about improving technique and using the right equipment.

The coho salmon’s deep body affects its ability to hold in fast water.

Anglers are a keen bunch and many are aware that different species of salmon and trout have slightly different preferences in habitat. For example, steelhead prefer riffles, runs, and other habitats with faster water and larger gravel and cobble. On the other hand, coho salmon have a stronger affinity for pools and slow water habitats, especially those with large wood for cover. And unlike steelhead, cohos don’t mind sandy bottoms or pebbles, because substrate size is not as important as the speed of the water.

I’m a fishery scientist, in addition to an angler, so I really enjoy trying to use biological knowledge about species to better understand the fish and if possible, improve my angling efficiency.

Steelhead are like torpedoes, tapered and thin, to move through heavy currents.

So what’s the biological mechanism underpinning the differences in habitat use between steelhead and coho salmon? It all starts with body shape. Out of the egg, both species look fairly similar, but within six months there are noticeable differences. Juvenile cohos have taller bodies – from belly to the back – and very large anal and dorsal fins.  Juvenile steelhead are more fusiform, spindly and tapered at both ends,  with smaller dorsal and anal fins, but relatively larger pectoral fins. 

These differences are key. The less fusiform body of the coho makes it more difficult to hold in faster water. They need to expend more energy to do so than the more streamlined shaped steelhead. However, there is no such cost to holding in slow-water habitats, such as pools, and in those instances, their taller bodies and larger anal and dorsal fins allow them to quickly maneuver side-to-side in search of food.  They are supremely adapted to feeding in pools with lots of competitors. Steelhead are not as good at actively feeding as the coho salmon. Instead, their body is best suited to holding in fast water, where they hide behind rocks and wait for the food to be delivered to them. One is a quick-turning dart, the other a submarine.

A juvenile steelhead begins to take on its fusiform shape.

In this context, juvenile cohos use pools because their body is better adapted to feeding in those habitats, while steelhead are more adapted to feeding in riffles and runs.  It also means that as adults, each species is relying on habitats that they used when they were juveniles, even though they are not feeding. It’s hard for a leopard to change its spots.

Although body shape is the primary determinant, there are other biological factors at play. Coho salmon tend to aggregate into schools more frequently than steelhead, with a clear exception being summer steelhead, which also often school and stage in pools. This is also particularly true when they are juveniles. At that life stage, coho salmon are typically found in large schools, while juvenile steelhead are not. 

Juvenile cohoes already look thicker-bodied than the young steelhead above.

Ultimately then, the habitat steelhead and coho salmon use the most as adults is the same habitat they relied on as juveniles. The reasons include body shape, feeding capability, and social behaviors. And that is why understanding their biology is so important to anglers: because it provides a mechanism for explaining how fish have adapted to certain stream habitats, and in turn, why we fish different types of habitat for different species.

A lifelong steelhead angler, John McMillan has spent hundreds of days snorkeling steelhead rivers, observing the fish in their native habitat, and at one time he was fishing about 345 days a year. John has worked for the US Forest Service, the Hoh Indian Tribe, the Wild Salmon Center, and recently for NOAA on the Elwha Dam removal project. 

One thought on “Why Biology Matters: Speedy Steelhead vs Lazy Coho Salmon”

  1. Nice article been a lifelong fisher as well a lot of changes in the last 40 years but still remnants of the glorious past…

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