Dissecting the steelhead

by Paul Hurtubise | April 28, 2020
a steelhead held over water

What makes steelhead such magnificent adversaries? What do they have that other fish lack? And while we’re at it, how can we catch, or at least hook, more? The best way to find out just might be to take a closer look at how they’re built — their anatomy — and how the different functions come together to make steelhead more than the sum of their parts.

Shape

Steelhead are fusiform (tapered on each end) shaped and built for speed in the water.

Tail and fins

Large and tactile pectoral fins can be spread wide to help them brake and change direction, but also fold away easily when speed is needed.

The caudal fin can quickly fanout to help the fish steer, acting as a rudder.

Pliable and flexible, the tail is able to assume a forked profile to sustain speed, but flares out wide for the initial push and for the purpose of spawning.

Comparing a steelhead’s tail to a chinook salmon’s, which is not as pliable, might help explain why steelhead “run” less when hooked, but show unequalled maneuverability compared to most other freshwater fish.

Scales, skin, and protective mucus

Steelhead scales are formed through crystallization of the enzyme guanine, which reflects light. The scales effectively cast extra glare in the eyes of prey and predators alike; like a natural cloaking device. This is why polarized sunglasses are so effective; they remove glare from the surface of the water and the fish.

In rivers, steelhead darken to blend into the bottom and become less visible to predators. Darkening also signals a readiness to spawn.

“Mucus on their bodies acts as a shield against bacteria, dirt, and any other elements that could irritate their skin,” said OFAH Fisheries Biologist Adam Weir. “It also waterproofs them.”

Lateral line

This is usually the steelhead’s first point of detection of any prey or danger.
Adapted to feel vibrations in the water, this line helps fish feel movements, like rocks falling in the water, other fish splashing, footsteps, and voices. It can also detect the slightest movement up close (i.e. insects thrashing in water, minnows kicking).

Brain

Steelhead have only three lobes: cerebellum (memory and command), optic lobe, and olfactory lobe. The biggest is the optic lobe.

Steelhead can remember where and when to find food, to find each other, and that some fishing implements mean danger. This is why you often see large pods of fish avoid a single float passing by, or a lure. It’s also why it’s good to change things up after you’ve hooked one.

Water

Considering how steelhead are made and where they live, water is a significant factor influencing their anatomy. The fish’s world is all wet, so smell and taste are essentially one and the same.

According to Google, water is 784 times denser than air, so things like smell, sound, vibration, and light act very differently in water than in air.

Eyes

Steelhead are nearsighted, partly because they only need to see things up to 30 feet away. They have blurry vision out of water. They literally have two “fisheye lenses,” giving them a wide-angle view on all sides, for great peripheral vision.

Looking up, they see the world outside the water as a wide circle, with a view to shore at about 10 degrees, called Snell’s window. They don’t have lids or an iris to regulate light, which is carried out in the retina. So it takes them longer to adjust to changing light.

Sight is usually their second point of detection of prey or danger. First they feel it then they see it.

Mouth and nostrils

Studies have concluded that trout using their sense of smell can detect things like human sweat, and hormones secreted by other fish in duress.
They can taste the difference between bugs and even have a preference.

Scientists surmise that they can actually taste the smallest amount of water chemistry from their home rivers, which guides them when returning from the oceans or Great Lakes to spawn.

Smell and taste are often the third point of detection for prey, enabling them to tell food from nonfood. But, they can also be used first, in determining vital information about things that are not yet visible, such as food, danger, or bait.

Muscles

Muscles enable steelhead to go from 0 to over 30 km/h in roughly a second. This ability to accelerate helps to explain why they can make such sudden changes of direction so effectively during a fight.

Gills

Steelhead lungs are not meant for fingers, so please don’t touch!
Having lungs in the back of their head means they don’t take up space in the chest, which is filled mostly with muscle. Gill rakes help protect the gills, but are spiked inside to keep food and prey in.

Next time you head out, keep in mind the number of amazing things steelhead anatomy enables them to do, and how knowing these fish well gives you an advantage. If, like me, you’re a catch-and-release angler, please remember their special vulnerabilities and treat steelhead with care.

Originally published in the April 2019 edition of Ontario OUT of DOORS magazine.

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