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When a Rainbow is a Rainbow

April 21, 2011 | by Bruce Auchly

Cam Dole of Helena grips a rainbow he
caught recently in Holter Reservoir.

The rainbow trout catch is on at Holter Reservoir on the Missouri River, yet most anglers know little about the fish they seek.

Or worse, believe a couple of common myths: The rainbows now congregating around the boat ramps are there because they imprinted on a hatchery truck and keeping spawning rainbows will hurt the reservoir’s population.

The vast majority of rainbows in the three reservoirs on the Missouri between Helena and Great Falls – Canyon Ferry, Hauser and Holter – come from the state’s hatchery system. Natural reproduction is insufficient for a self-sustaining population.

There are at least a dozen types or strains of rainbow trout in existence. Although all the same species, the different strains come from different locations: some wild, some from hatcheries. As a result the fish display different breeding behavior, spawning behavior, size, appearance, habitat preference and environmental tolerances.

The two mainstays of Montana’s hatchery system are Eagle Lake and Arlee strains. Eagle Lake is considered semi wild; Arlee is highly domesticated.

The average angler cannot consistently tell the two strains apart, but they are different, and that is important to the average angler.

For example, Eagle Lake rainbows make up a majority of the fish now cruising the Holter Lake shore. Come warmer weather they will head to deeper water searching out other fish, their favored food, making them a target for anglers in boats.


Walt Dalbey of Glasgow, left, hoists a stringer
of Holter Reservoir rainbows he caught
with Katie Hagengruber of Helena.

The Arlee strain eats more insects, or invertebrates, and so will spend the summer cruising the shore. That makes them more catchable for the shore angler.

Eagle Lake rainbows evolved in northeast California on the east slope of the Sierras. Though Montana’s Eagle Lake strain is hatchery raised, the eggs are gathered in the wild.

The Arlee strain was developed in the state’s Arlee hatchery where different strains were crossbred and over several generations certain characteristics were selected for, like fast growth and fall breeding.

That’s right, fall breeding. Rainbow trout in the wild breed in the spring, often the peak is mid-April. Easy enough to duplicate in a hatchery if the goal is stocking fingerlings in the early summer.

However, that doesn’t work well in the upper Missouri reservoirs as they have lots of hungry walleyes. Stocking 2- to 3-inch trout fingerlings merely turns them into forage, which is not the purpose of raising game fish in a hatchery.

Much better to stock 6- to 8-inch trout that stand a chance of survival. So fall spawning Arlees are stocked at that size in the spring and spring spawning Eagle Lakes in the fall.

The Eagle Lake rainbows now cruising the shoreline are looking for a place to spawn. Their genes are telling them to find suitable gravel not a hatchery truck. It’s just that the type of gravel they need usually occurs along a gradually sloping shoreline, like at a boat ramp.

And keeping a limit of 3- to 5-pound spawners from those reservoirs does not hurt the population. Those fish generally will not spawn because they won’t find the right spawning habitat. They have to be stocked to provide sport and food for anglers.

So have fun and hopefully catch a rainbow or two.