Time to Catch a Rainbow

By BRUCE AUCHLY for Fish Wildlife and Parks

Want a fishing tip?

Run, don’t walk, to your closest reservoir and start fishing near shore for a rainbow trout.

Rainbow trout are spawning. That’s not a secret to the men and women who know where and when to go fishing for them.

What most anglers don’t know is why. Why do rainbows cruise reservoir shorelines as they look for a place to spawn? And do those fish really congregate near boat ramps because they are trying to find the hatchery truck that brought them there?

Rainbow trout in the wild breed in the spring, often peaking from mid-April to late April, in rivers and streams with gravel bottoms. Those requirements are important for a couple of reasons.

First, the flowing water in a river or stream provides oxygen, which keeps the eggs alive.

Second, gravel bottoms help protect eggs from predators. Also a silt bottom could smother and kill the eggs.

Spring Rainbow Trout. Photo courtesy of Montana Fish Wildlife and Parks.

Rainbow trout will cruise reservoir shorelines in the spring, looking for a place to spawn. Photo by FWP.

When it’s time to spawn based on daylight length and warming water temperature (at least in the mid-40’s), a female rainbow will clear a slight depression, called a redd, in gravel. She does this by turning on her side and beating her tail up and down. Then she deposits 2,000 to 3,000 eggs in the redd. Next she begins digging at the upstream edge of the nest, covering the eggs with gravel.

As she releases her eggs, a male rainbow will move alongside her and release his milt over the eggs, fertilizing them.

And that’s it. The pair swim away and let nature take its course. In a month or two the eggs will hatch. Pretty simple, really.

Rainbow trout in lakes, then, must find a suitable tributary with flowing water and a gravel bottom. Right now, they are slowly swimming along the shorelines, searching.

Because many reservoirs in north central Montana lack suitable spawning tributaries for a self-sustaining rainbow population, they are stocked from the state’s hatcheries.

The only way those rainbows are going to get to a reservoir from a hatchery is by a stocking truck. And those trucks often use boat ramps to get close to the water’s edge.

However, when rainbows are seen swimming near a boat ramp this time of year, it doesn’t mean the fish have imprinted on the hatchery truck and returning to said ramp to spawn.

Rather, what usually lies on the lakes bottom near a boat ramp? Gravel.

As those rainbows are looking for spawning habitat, their genes are telling them to find suitable gravel not a hatchery truck. It’s just that the type of gravel they need often occurs along a gradually sloping shoreline, like at a boat ramp.

There is also the theory that sunshine on the concrete of the boat ramp warms up the nearby shallow water just a few degrees, making it attractive for the fish.

Oh yes, the old canard that keeping a limit of 3- to 5-pound spawners from reservoirs will hurt the population? Not true.

Those fish generally will not spawn because they won’t find the right spawning habitat.

So go catch a limit of rainbows or just a few rays of sunshine on a bright spring day.