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The Perils of a Young Trout

This story is featured in Montana Outdoors May-June 2016 issue

The spring spawning run of rainbow trout is over. From the Kootenai to the Bighorn, spawners have moved down from the tributaries, where they laid their eggs, to the rivers’ mainstem, where they will spend the next 10 months until the reproductive urge strikes again.

Meanwhile, up in those tributaries, the perils of newly hatched trout are just beginning.

And that’s assuming the eggs themselves survive. Some years, melting snow and rain produce tremendous runoff in the tributaries of major rivers. Torrents of rushing water destroy redds (spawning nests made in gravel) and wash eggs away.

Using water temperatures as a guide, fisheries biologists can predict when female trout will spawn and eggs will hatch. Rainbows and cutthroat are spring spawners. On Missouri River tributaries, for example, eggs deposited on March 30 will usually hatch around May 9, while eggs laid April 15 will hatch about May 18. Eggs of brown trout, which spawn in the fall, generally hatch in January or February, giving those young fish a head start on baby rainbows and cutthroat.

In years of late runoff, young fish hatched earlier in the spring (or even winter, like brown trout) have a better chance of surviving heavy flows than eggs not yet hatched.

A baby trout can escape swift current by swimming to a side channel; an egg can’t.

Drought is simple. Low, warm water, no fish. No fish, no eggs.

About 1 percent of rainbow eggs survive all that to become a one-year-old fish. Fisheries biologists refer to those fish as “recruited” into the population.

So how does an egg become a trout?

Once an egg hatches, the newborn trout is called an alevin. It lives in the stream gravel, feeding for three to five weeks off the remaining yolk attached to its body.

After it has absorbed its egg sac, the tiny trout is known as a fry and now must find a new food source. This vulnerable period is when most young trout die. The one-inch fish are too slow to escape predators—mainly larger trout—and have a tough time consuming enough microscopic crustaceans (zooplankton) to survive.

For the next one to three years, the young trout stays in the tributary, growing bigger in preparation for eventually “out-migrating” downstream to the mainstem river. (“Resident” trout stay in the stream their entire lives.) During this period, the juvenile trout begins to feed on mayfly larvae and caddis fly pupae, as well as scuds and tiny fish.

What perils further await this young fish? Let me count the ways.

Floods can dash young trout against rocks. Even worse are droughts. When flows are depleted, young fish are forced into the same waters as adult fish. And, as we all know, it’s a fish-eat-fish world out there.

When fish are concentrated, it’s also easier for other predators, whether kingfishers from above or otters from below, to find a meal.

If a trout escapes all that, it will reach about four to eight inches long by its first birthday. By age two, it’s about seven to twelve inches long. Now the fish has to start contending with human angling.

While many anglers practice catch-and-release, a certain percentage of fish die after they are let go. Like when a fish is caught and kept out of the water too long for photographs, or is allowed to fight too long on a hot day when the water is warm. Some anglers occasionally keep a fish or two for the frying pan. That’s allowed under most FWP fishing regulations, because it has no discernible effect on the overall population.

On an individual trout, of course, it’s a different story. But then again, no one ever said it was easy being a fish.Bear bullet

Bruce Auchly manages the regional Information and Education Program in Great Falls.

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